Information

Why are parents of contrasting genotypes involved for a reciprocal cross?


In a reciprocal cross, we try to determine whether a trait is sex-linked Or not… One thing I couldn't understand was that, why are parents of contrasting genotypes involved? Does that mean that at one time, female parent has the particular trait we are that interested in and at the other time, the male parent has that particular trait?(please correct me if I am wrong) and if that's so… Then, why is that? I mean what is the purpose of doing that?


Reciprocal cross

In genetics, a reciprocal cross is a breeding experiment designed to test the role of parental sex on a given inheritance pattern. [1] All parent organisms must be true breeding to properly carry out such an experiment. In one cross, a male expressing the trait of interest will be crossed with a female not expressing the trait. In the other, a female expressing the trait of interest will be crossed with a male not expressing the trait. It is the cross that could be made either way or independent of the sex of the parents. For example, suppose a biologist wished to identify whether a hypothetical allele Z, a variant of some gene A, is on the male or female sex chromosome. She might first cross a Z-trait female with an A-trait male and observe the offspring. Next, she would cross an A-trait female with a Z-trait male and observe the offspring. Via principles of dominant and recessive alleles, she could then (perhaps after cross-breeding the offspring as well) make an inference as to which sex chromosome contains the gene Z, if either in fact did.


Mendel&rsquos Experiments and Heredity

Figure 1. Experimenting with thousands of garden peas, Mendel uncovered the fundamentals of genetics. (credit: modification of work by Jerry Kirkhart)

Genetics is the study of heredity. Johann Gregor Mendel set the framework for genetics long before chromosomes or genes had been identified, at a time when meiosis was not well understood. Mendel selected a simple biological system and conducted methodical, quantitative analyses using large sample sizes. Because of Mendel&rsquos work, the fundamental principles of heredity were revealed. We now know that genes, carried on chromosomes, are the basic functional units of heredity with the capability to be replicated, expressed, or mutated. Today, the postulates put forth by Mendel form the basis of classical, or Mendelian, genetics. Not all genes are transmitted from parents to offspring according to Mendelian genetics, but Mendel&rsquos experiments serve as an excellent starting point for thinking about inheritance.

Mendel&rsquos Experiments and the Laws of Probability

Figure 2. Johann Gregor Mendel is considered the father of genetics.

Johann Gregor Mendel (1822&ndash1884) (Figure 2) was a lifelong learner, teacher, scientist, and man of faith. As a young adult, he joined the Augustinian Abbey of St. Thomas in Brno in what is now the Czech Republic. Supported by the monastery, he taught physics, botany, and natural science courses at the secondary and university levels. In 1856, he began a decade-long research pursuit involving inheritance patterns in honeybees and plants, ultimately settling on pea plants as his primary model system (a system with convenient characteristics used to study a specific biological phenomenon to be applied to other systems). In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local Natural History Society. He demonstrated that traits are transmitted faithfully from parents to offspring independently of other traits and in dominant and recessive patterns. In 1866, he published his work, Experiments in Plant Hybridization, in the proceedings of the Natural History Society of Brünn.

Mendel&rsquos work went virtually unnoticed by the scientific community that believed, incorrectly, that the process of inheritance involved a blending of parental traits that produced an intermediate physical appearance in offspring this hypothetical process appeared to be correct because of what we know now as continuous variation. Continuous variation results from the action of many genes to determine a characteristic like human height. Offspring appear to be a &ldquoblend&rdquo of their parents&rsquo traits when we look at characteristics that exhibit continuous variation. The blending theory of inheritance asserted that the original parental traits were lost or absorbed by the blending in the offspring, but we now know that this is not the case. Mendel was the first researcher to see it. Instead of continuous characteristics, Mendel worked with traits that were inherited in distinct classes (specifically, violet versus white flowers) this is referred to as discontinuous variation. Mendel&rsquos choice of these kinds of traits allowed him to see experimentally that the traits were not blended in the offspring, nor were they absorbed, but rather that they kept their distinctness and could be passed on. In 1868, Mendel became abbot of the monastery and exchanged his scientific pursuits for his pastoral duties. He was not recognized for his extraordinary scientific contributions during his lifetime. In fact, it was not until 1900 that his work was rediscovered, reproduced, and revitalized by scientists on the brink of discovering the chromosomal basis of heredity.

Mendel&rsquos Model System

Mendel&rsquos seminal work was accomplished using the garden pea, Pisum sativum, to study inheritance. This species naturally self-fertilizes, such that pollen encounters ova within individual flowers. The flower petals remain sealed tightly until after pollination, preventing pollination from other plants. The result is highly inbred, or &ldquotrue-breeding,&rdquo pea plants. These are plants that always produce offspring that look like the parent. By experimenting with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring that might occur if the plants were not true breeding. The garden pea also grows to maturity within one season, meaning that several generations could be evaluated over a relatively short time. Finally, large quantities of garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not come about simply by chance.

Mendelian Crosses

Mendel performed hybridizations, which involve mating two true-breeding individuals that have different traits. In the pea, which is naturally self-pollinating, this is done by manually transferring pollen from the anther of a mature pea plant of one variety to the stigma of a separate mature pea plant of the second variety. In plants, pollen carries the male gametes (sperm) to the stigma, a sticky organ that traps pollen and allows the sperm to move down the pistil to the female gametes (ova) below. To prevent the pea plant that was receiving pollen from self-fertilizing and confounding his results, Mendel painstakingly removed all of the anthers from the plant&rsquos flowers before they had a chance to mature.

Plants used in first-generation crosses were called P0, or parental generation one, plants (Figure 3). Mendel collected the seeds belonging to the P0 plants that resulted from each cross and grew them the following season. These offspring were called the F1, or the first filial (filial = offspring, daughter or son), generation. Once Mendel examined the characteristics in the F1 generation of plants, he allowed them to self-fertilize naturally. He then collected and grew the seeds from the F1 plants to produce the F2, or second filial, generation. Mendel&rsquos experiments extended beyond the F2 generation to the F3 and F4generations, and so on, but it was the ratio of characteristics in the P0&minusF1&minusF2 generations that were the most intriguing and became the basis for Mendel&rsquos postulates.

Figure 3. In one of his experiments on inheritance patterns, Mendel crossed plants that were true-breeding for violet flower color with plants true-breeding for white flower color (the P0 generation). The resulting hybrids in the F1 generation all had violet flowers. In the F2 generation, approximately three quarters of the plants had violet flowers, and one quarter had white flowers.

Garden Pea Characteristics Revealed the Basics of Heredity

In his 1865 publication, Mendel reported the results of his crosses involving seven different characteristics, each with two contrasting traits. A trait is defined as a variation in the physical appearance of a heritable characteristic. The characteristics included plant height, seed texture, seed color, flower color, pea pod size, pea pod color, and flower position. For the characteristic of flower color, for example, the two contrasting traits were white versus violet. To fully examine each characteristic, Mendel generated large numbers of F1 and F2 plants, reporting results from 19,959 F2 plants alone. His findings were consistent.

What results did Mendel find in his crosses for flower color? First, Mendel confirmed that he had plants that bred true for white or violet flower color. Regardless of how many generations Mendel examined, all self-crossed offspring of parents with white flowers had white flowers, and all self-crossed offspring of parents with violet flowers had violet flowers. In addition, Mendel confirmed that, other than flower color, the pea plants were physically identical.

Once these validations were complete, Mendel applied the pollen from a plant with violet flowers to the stigma of a plant with white flowers. After gathering and sowing the seeds that resulted from this cross, Mendel found that 100 percent of the F1 hybrid generation had violet flowers. Conventional wisdom at that time would have predicted the hybrid flowers to be pale violet or for hybrid plants to have equal numbers of white and violet flowers. In other words, the contrasting parental traits were expected to blend in the offspring. Instead, Mendel&rsquos results demonstrated that the white flower trait in the F1 generation had completely disappeared.

Importantly, Mendel did not stop his experimentation there. He allowed the F1 plants to self-fertilize and found that, of F2-generation plants, 705 had violet flowers and 224 had white flowers. This was a ratio of 3.15 violet flowers per one white flower, or approximately 3:1. When Mendel transferred pollen from a plant with violet flowers to the stigma of a plant with white flowers and vice versa, he obtained about the same ratio regardless of which parent, male or female, contributed which trait. This is called a reciprocal cross&mdasha paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross. For the other six characteristics Mendel examined, the F1 and F2 generations behaved in the same way as they had for flower color. One of the two traits would disappear completely from the F1 generation only to reappear in the F2 generation at a ratio of approximately 3:1 (Table 1).

  • 705 violet
  • 224 white
  • 651 axial
  • 207 terminal
  • 787 tall
  • 277 dwarf
  • 5,474 round
  • 1,850 wrinkled
  • 6,022 yellow
  • 2,001 green
  • 882 inflated
  • 299 constricted
  • 428 green
  • 152 yellow

Upon compiling his results for many thousands of plants, Mendel concluded that the characteristics could be divided into expressed and latent traits. He called these, respectively, dominant and recessive traits. Dominant traits are those that are inherited unchanged in a hybridization. Recessive traits become latent, or disappear, in the offspring of a hybridization. The recessive trait does, however, reappear in the progeny of the hybrid offspring. An example of a dominant trait is the violet-flower trait. For this same characteristic (flower color), white-colored flowers are a recessive trait. The fact that the recessive trait reappeared in the F2 generation meant that the traits remained separate (not blended) in the plants of the F1 generation. Mendel also proposed that plants possessed two copies of the trait for the flower-color characteristic, and that each parent transmitted one of its two copies to its offspring, where they came together. Moreover, the physical observation of a dominant trait could mean that the genetic composition of the organism included two dominant versions of the characteristic or that it included one dominant and one recessive version. Conversely, the observation of a recessive trait meant that the organism lacked any dominant versions of this characteristic.

Working with garden pea plants, Mendel found that crosses between parents that differed by one trait produced F1 offspring that all expressed the traits of one parent. Observable traits are referred to as dominant, and non-expressed traits are described as recessive. When the offspring in Mendel&rsquos experiment were self-crossed, the F2 offspring exhibited the dominant trait or the recessive trait in a 3:1 ratio, confirming that the recessive trait had been transmitted faithfully from the original P0 parent. Reciprocal crosses generated identical F1 and F2 offspring ratios. By examining large sample sizes, Mendel showed that his crosses behaved reproducibly according to the laws of probability, and that the traits were inherited as independent events.


Use of translocations in producing duplications and deletions

Geneticists regularly need to make specific duplications or deletions to answer specific experimental questions. We have seen that they use inversions to do so, and now we shall see how translocations also can be used for the same purpose. Let’s take Drosophila as an example. For reasons that are still unclear, the densely staining chromosomal regions near the centromere—regions called heterochromatin 𠅊re physically extensive but contain few genes. In fact, for a long time, heterochromatin was considered useless and inert material. In any case, for our purposes, Drosophila can tolerate a loss or an excess of heterochromatin with little effect on viability or fertility.

Now let’s select two different reciprocal translocations of the same two chromosomes. Each translocation has a breakpoint somewhere in heterochromatin, and each has another breakpoint in euchromatin (nonheterochromatin) on opposite sides of the region we want to duplicate or delete (Figure 17-30). It can be seen that, if we have a large collection of translocations having one heterochromatic break and euchromatic breaks at many different sites, then duplications and deletions for many parts of the genome can be produced at will for a variety of experimental purposes. More generally, if one breakpoint of a translocation is near a dispensable tip, then duplication or deletion of this tip can be ignored, and the translocation can be used as a way of generating duplications or deficiencies for the other translocated segment.

Figure 17-30

Using translocations with one breakpoint in heterochromatin to produce a duplication and a deletion. If the upper product of translocation 1 is combined with the upper product of translocation 2 by means of an appropriate mating, a deletion of b results. (more. )


Test cross and backcross are two types of popular crosses in plant breeding. Test cross happens between a dominant phenotype with the recessive phenotype to determine the genotype of the dominant phenotype. Backcross helps to recover important characters of the parent population within hybrid populations.


Reciprocal cross

reciprocal cross
where A and B represent different genotypes, a cross involving A males xB females, and B males and xA females
Source: Jenkins, John B. 1990. Human Genetics, 2nd Edition. New York: Harper & Row .

Reciprocal crosses
Pairs of genetic crosses which in one case DNA from strain #1 is transferred into strain #2 and in the second case the strain #2 is used as a donor to transfer the same region into strain #1. For example
Cross A = donor (pro::Tn10) x recipient (pro+) .

reciprocal cross: a paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross
trait: a variation in an inherited characteristic
.

Using male and female gametes for two different traits, alternating the source of gametes. sex chromosomes Sex determination is based on sex chromosomes sex-linked. A gene coded on a sex chromosome, such as the X-chromosome linked genes of flies and man.

es, they are phenotypically different. This is because phenotype is due to the action of the mother's genotype.

But to do this right we should really do

es (i.e. cross a male from population A with a female from population B and a male from population B with a female from population A). This brings the total crosses we need to make up to 999,000. But don't we also need to make replicates?

Full diallel in which parents and

leopon The hybrid produced by crossing a lioness with a leopard. The

, produced with a lion father, is called a lipard. MORE INFORMATION
Lepidoptera /LEP-əd-DAWP-ter-ə/ The order of insects including moths and butterflies (lepidopterans).

Emsen, E., 2005. Testicular development and body weight gain from birth to 1 year of age of Awassi and Redkaraman sheep and their

es. Small Ruminant Research., 59:79-82.

The trait not shown in the F1 reappeared in the F2 in about 25% of the offspring.
Traits remained unchanged when passed to offspring: they did not blend in any offspring but behaved as separate units.


Dihybrid Cross Example

Look at the above illustration. The drawing on the left shows a monohybrid cross and the drawing on the right shows a dihybrid cross. The two different phenotypes being tested in this dihybrid cross are seed color and seed shape. One plant is homozygous for the dominant traits of yellow seed color (YY) and round seed shape (RR)—this genotype can be expressed as (YYRR)—and the other plant displays homozygous recessive traits of green seed color and wrinkled seed shape (yyrr).

F1 Generation

When a true-breeding plant (organism with identical alleles) that is yellow and round (YYRR) is cross-pollinated with a true-breeding plant with green and wrinkled seeds (yyrr), as in the example above, the resulting F1 generation will all be heterozygous for yellow seed color and round seed shape (YyRr). The single round, yellow seed in the illustration represents this F1 generation.

F2 Generation

Self-pollination of these F1 generation plants results in offspring, an F2 generation, that exhibit a 9:3:3:1 phenotypic ratio in variations of seed color and seed shape. See this represented in the diagram. This ratio can be predicted using a Punnett square to reveal possible outcomes of a genetic cross.

In the resulting F2 generation: About 9/16 of F2 plants will have round, yellow seeds 3/16 will have round, green seeds 3/16 will have wrinkled, yellow seeds and 1/16 will have wrinkled, green seeds. The F2 progeny exhibit four different phenotypes and nine different genotypes.

Genotypes and Phenotypes

Inherited genotypes determine the phenotype of an individual. Therefore, a plant exhibits a specific phenotype based on whether its alleles are dominant or recessive.

One dominant allele leads to a dominant phenotype being expressed, but two recessive genes lead to a recessive phenotype being expressed. The only way for a recessive phenotype to appear is for a genotype to possess two recessive alleles or be homozygous recessive. Both homozygous dominant and heterozygous dominant genotypes (one dominant and one recessive allele) are expressed as dominant.

In this example, yellow (Y) and round (R) are dominant alleles and green (y) and wrinkled (r) are recessive. The possible phenotypes of this example and all possible genotypes that may produce them are:

Yellow and round: YYRR, YYRr, YyRR, and YyRr

Yellow and wrinkled: YYrr and Yyrr

Green and round: yyRR and yyRr

Green and wrinkled: yyrr

Independent Assortment

Dihybrid cross-pollination experiments led Gregor Mendel to develop his law of independent assortment. This law states that alleles are transmitted to offspring independently of one another. Alleles separate during meiosis, leaving each gamete with one allele for a single trait. These alleles are randomly united upon fertilization.


Examples of Monohybrid Cross

Gregor Mendel’s Peas

Although he did not know it at the time, Gregor Mendel used monohybrid crosses to identify dominant and recessive traits in his landmark experiments with peas.

Gregor Mendel focused on several different genetic traits, but we will focus on one: stem length. Imagine that two types of pea plants grow in a garden. One type of pea plant has long stems, while the other has short stems. For the sake of this example, assume that both types of pea plant have a homozygous genotype (LL and ll), and that long stems (LL) are dominant over short stems (ll).

Huntington’s Disease

Huntington’s Disease is a progressive degenerative condition that occurs in 4 to 15 of every 100,000 people in the United States. Having no cure, it is a certain death sentence for those diagnosed. While little is known about this condition, geneticists are sure that it is inherited via a dominant gene.

At the simplest level, a monohybrid cross was used to determine the genetic nature of Huntington’s disease. Everyone carries the aptly-named Huntingtin gene, the gene responsible for the complication. With this information, scientists paired the Huntingtin genes of an individual who is homozygous dominant for the condition (HH) with the Huntingtin genes of an individual who is homozygous recessive for the condition (hh).

Although this example is highly abridged, the result remains that all offspring from the cross carried the dominant allele for Huntington’s disease. While this experiment, if conducted on humans, would bring sad news to both parent and child, it would also highlight the dominant nature of the disease.

Confirming Dominant Traits

We have already discussed how scientists use monohybrid crosses to determine the dominant allele of a genotype. However, monohybrid crosses between homozygous individuals is often only the first step. Heterozygous crosses, in which both parents carry a dominant allele and a recessive allele, helps confirm whether a trait is dominant or recessive.

The model for this second step greatly resembles the process Gregor Mendel followed, with peas. Using stem length as an example, scientists breed two parents that both have long stems, with genotype Ll. In an ideal scenario, one in every four of their offspring will carry the genotype ll, and thus have a short stem. Because long stems occur more often than short stems in this second iteration, scientists can reasonably determine long stems are a dominant trait.


Genetics, Class 12, Biology | EduRev Notes

INTRODUCTION

Genetics term was given by W. Bateson. W. Bateson is Father of Modern Genetics.

Genetics = Collective study of heredity & Variations.

Heredity = Transmission of genetic characters from parent to offsprings.

Variation = individuals of same species have  some differences, these are called variation.

Muller – Proposed the term "Cytogenetics" (Cytology + Genetics) Father of Actinobiology Actinobiology - study the effect of radiation on living organisms.

Morgan – Father of Experimental genetics He experiment on Drosophila & proposed various concepts.

Gene theory - According to gene theory genes are linearly located on chromosome.

Linkage term, Theory of sex linkage, Crossing over term, Criss - cross inheritance, Linkage map on Drosophila given by Morgan.

  • A. Garrod = Father of human genetics & Biochemical genetics. Garrod discovered first human Metabolic genetic disorder which is calledalkaptonuria(black urine disease). In this disease enzyme homogentisic acid oxidase is deficient. Gave the concept 'One mutant gene one metabolic block'  

PRE-MENDELISM

To explain the like begets like (offsprings are similar to their parents) several theories were given. They are collectively known as Theories of Blending Inheritance. Some of them are as follows –

1. Vapour fluid theory – Greek philosopher Pythagoras [500B.C.] proposed this theory.

According to this theory, at the time of coitus of male and female, moist vapour secretes from the brain and due to this offsprings are similar as their parents.

2. Semen theory :- This theory has given by Empedocles.

According to his view, the semen of male and female is mixed during coitus. Characters of parents appear into the offsprings due to the mixture.

According toAristotle - a semen of male is considered as "highly purified blood". Which has power of life and it is nourished by semen of female.

3. Preformation theory :- According to this theory germinal [reproductive] cell contains "a miniature figure of a man"

According to Swammerdam  preformation of a miniature of man is found inside the egg is called Mankin.

Those scientists  who believed in the hypothesis of Swammerdam are known as Ovists. On contrary, according to Hartsoeker preformed miniature of man is present in sperm.

A miniature of man is present in sperm, he called - Homunculus Scientists, who believed in  Hart soeker view, are called "spermist."

4. Encasement theory :- This theory was proposed by Charles Bonnet.

According to his view, body of female is just like a chinese box. The all future progenies are packed in the body of female like chinese box.

5. Epigenesis theory :- This theory was proposed by K.Wolf.

According to his view, germinal cells possess an undifferentiate material. This material develops step by step [gradually] after the fertilization. Such type development is known as Epigenesis.

6. Pangenesis theory :- The theory of pangenesis was described by C.Darwin.

This theory postulated that all part of a living body [tissues] synthesize "micro molecules." these micro molecules are known as Pangene or Gemmules.

The male and female pangenes fuse together during the fertilization  these are, further again distributed in the various organs of the body at the time of development.

7. Germplasm theory :- The view was proposed by A.Weisman (1886).

According to him living body of an individual possess two different types of fluid material - Somatoplasm and Germplasm.

Somatoplasm does not participate in the formation of germinal cells.  Therefore, variations are not transferred into the progeny. somatoplasm is mortal because eventually it dies.  

Experiments performed by Mendel on genetics and description of mechanisms of hereditory processes and formulation of principles are known as Mendelism.

Mendel postulated various experimental laws in relation of genetics.

Gregor Johann Mendel (1822 - 1884) :- Mendel was born on July 22,1822 at Heinzendorf  in Austria at Silesia village. Mendel worked in Augustinian Monastery as monk at Brunn city, Austria.

In 1856-57, he started his historical experiments of heredity on pea(Pisum sativum) plant. His experimental work continued  on pea plant till 1865 (19 th century).

The results of his experiments were published in the science journal, "Nature For schender varein" in 1866.

This  journal was in Germen language. Title was "verschue uber Pflangen Hybridan".

This journal was published by 'Natural History society of Bruno'.

A paper of Mendel by the name of "Experiment in plant Hybridization" published in this journal.

Mendel was unable to get any popularity. No one understood of him. He died in 1884 without getting any credit of his work (due to kidney disease (Bright disease) After 16 years of Mendel's death in 1900, Mendel's postulates were rediscovered.

Rediscovery by three scientists independently.

1. Carl Correns - Germany - (Experiment on Maize)

2. Hugo deVries (Holland) (Experiment on Evening Primerose) He republished  the Mendel's results in 1901 in Flora magazine

3. Erich von Tschermak Seysenegg - (Austria) (Experiment on different flowering plants)

The credit of rediscovery of Mendelism goes to three scientists.

Correns gave two laws of Mendelism.

Law of Heredity/Inheritance/Mendelism

I st Law - Law of segregation.

II nd Law - Law of independent assortment.

Mendel experiments remain hidden for 34 years.


Mendel results remain hidden due to :

1. At that time Darwin's book "Origin of Species" published. Scientists  were busy in discussion with this book.

2. Mendel's ideas were ahead of that time.

3. Mendel used higher statistical calculation in his experiments so the results were complicated to understand.

4. Mendel also performed his experiments on Hieraceum plant on suggestion of Karl Nageli but Mendel did not get succeed because in Heiracium, Parthenogenesis is present.


Reasons for Mendel's success :

1. Mendel studied the inheretance of one or two characters at a time unlike his predecessors who had considered many characters at a time. (Kolreuter-Tobacco plant, John Goss & Knight -Pea plant).

Selection of garden Pea plant is suitable for studies which have the following advantages :
(i) Pea plant is annual plant with short life cycle of 2-3 months so large no. of offsprings can be analysed within a
short period of time.
(ii) It has many contrasting traits.
(iii) Natural self pollination is present in pea plant.
(iv) Cross pollination can be performed in it artificially so hybridization can be made possible.
(v) Pea plant easy to cultivate.
(vi) Pea seeds are large. In addition to pea, Mendel worked on rajama and honey bee.
3. Mendel quantitatively analyse the inheritance of qualitative characters.
4. He maintained the statistical records of all the experiments.
Mendel's work : Mendel studied 7 characters or 7 pairs of contrasting traits.

Average of all traits studied 2.98: (= 3:1)

Gene which controls more than one character is called as pleiotropic gene.

In Wrinkled seed free sugar is more in place of starch.


Special Point :
S. Blixt concluded that the genes studied by Mendel are located on four different pairs of chromosomes.

Pod colour --------------  Ch. no. 5 th
Seed form ----------------- Ch. no. 7 th
Two of the genes are on chromosome 1 st and three are on chromosome 4, genes are located far apart on the chromosome except genes controlling plant height and pod shape.
Mendel did not study the gene controlling plant height and pod shape so Mendel did not detect linkage.


Technique of Mendel

He developed a technique Emasculation and Bagging for hybridization in plants.

Flowers of pea plant are bisexual. In this method one considered as male and another as female.

Stamens of the plant which is used as female, are removed at juvenile stage, this is called Emasculation.

Emasculation is done to prevent self pollination.

Emasculated flowers covered by bags, this is called bagging.

Bagging is only used to prevent undesirable cross pollination.

Mature pollen grains are collected from male plants and spread over emasculated flower.

Seeds are formed in the female flower after pollination.

The plants that are obtained from these seeds are called First Filial generation or F1 generation according to Mendel.

Mendel was great plant breader(true breader).


SOME GENETICAL TERMS
1. Factors :- Unit of heredity which is responsible for inheritance and appearance of characters.

These factors were referred as genes by Johannsen(1909). Mendel used term "element" for factor.

Morgan first used symbol to represent the factor. Dominant factors are represented by capital letter while recessive factor by small letter.

2 Allele :- Alternative forms of a gene which are located on same position [loci] on the homologous chromosome is called Allele. Term allele was coined by Bateson.

3. Homozygous :- A zygote is formed by fusion of two gametes having identicle factors is called homozygote and organism developed from this zygote is called homozygous.

4. Heterozygous :- A zygote is formed by fusion of two different types of gamete carrying different factors is called heterozygote (Tt, Rr) and individual developed from such zygote is called heterozygous.

The term homozygous and heterozygous are coined by Bateson.

5. Hemizygous :- If individual contains only one gene of a pair then individual said to be Hemizygous. Male individual is always Hemizygous for sex linked gene.

6. Phenotype :- It is the external and morphological appearance of an organism for a particular character.

7. Genotype :- The genetic constitution or genetic make-up of an organism for a particular character.

Genotype & phenotype terms were coined by Johannsen.

8. Phenocopy :- If different genotypes are placed in different environmental conditions then they produce same
phenotype. Then these genotypes are said to be Phenocopy of each other.

MONOHYBRID CROSS
When we consider the inheritance of one character at a time in a cross this is called monohybrid cross. First of all, Mendel selected tall and dwarf plants.


Checker Board Method :

First time, it was used by Reginald. C. Punnett (1875 - 1967)
The representation of generations to analyse in the form of symbols of squares. Male gamets lie horizontally and female gametes lie vertically.

T T = Tall (dominant homozygous),

T t = Tall (dominant heterozygous),

t t = Dwarf (recessive homozygous).

The ratio of characters (traits) appear/ visible morphologically is phenotypicratio. It is 3: 1. Genetic constitution is called Genotype [using symbols for genes] it is 1 : 2 :1

Conclusions (results) of Monohybrid Cross


I st Conclusion (Postulate of paired factors) :

According to Mendel each genetic character is controlled by a pair of unit factor. It is known as conclusion of
paired factoror unit factor.

II nd Conclusion (Postulate of Dominance):
This conclusion is based on F1 - generation. When two different unit factors are present in single individual, only one unit factor is able to express itself and known as dominant unit factor. Another unit factor fails to express is the recessive factor. In the presence of dominant unit factor recessive unit factor can not express and it is known as conclusion of dominance.

III rd Conclusion (Law of segregation):

During gamete formation the unit factors of a pair segregate randomly and transfer inside different gamete.

Each gamete receives only one factor of a pair so gametes are pure for a particular trait. It is known  as conclusion of purity of gametes or segregation.

  • There is no exception of Law of segregation. The segregation is essential during the meiotic division in all sexually reproducing organisms. (Nondisjunction may be exception of this law).


DIHYBRID CROSS

A cross in which study of inheritance of two pairs of contrasting traits.

Mendel wanted to observe the effect of one pair of heterozygous on other pair.

Mendel selected traits for dihybrid cross for his experiment as follows :-

[1] Colour of cotyledons→ Yellow (Y)  & Green (y)

[2] Seed form → Round (R) and Wrinkled (r) yellow and round characters are dominant and green and wrinkled are recessive characters.

Mendel crossed, yellow and round seeded plants with green and wrinkled seeded plants.

All the plants in F1–generation had yellow and round seeds.

When F1 plants were self pollinated to produce four kinds of plants in F2 generation such as yellow round, yellow–wrinkled, green round and green wrinkled,  there were in the ratio of 9 : 3 : 3 : 1. This ratio is known as dihybrid ratio.

Expression of yellow round (9) and green wrinkled (1)  traits shows as their parental combination.

Green Round and yellow wrinkled type of plants are produced by the results of new combination.


Demonstration by checker board method :-

F2 - Generation 

Thus, Phenotypic Ratio = 9 : 3 : 3 : 1

Homozygous yellow & Homozygous Round – YY RR = 1

Homozygous yellow & Heterozygous Round – YY Rr = 2

Heterozygous yellow & Homozygous Round – Yy RR = 2

Heterozygous yellow & Heterozygous Round – Yy Rr = 4

Homozygous yellow & Homozygous wrinkled – YY rr = 1

Heterozygous yellow & Homozygous wrinkled – Yy rr = 2

Homozygous green & Homozygous Round – yy RR = 1

Homozygous green & Heterozygous Round – yy Rr = 2

Homozygous green & Homozygous wrinkled – yy rr = 1

Thus, Genotypic Ratio = 1:2:2:4:1:2:1:2:1


Fork line method -To find out the composition of factors inside the gamete, we use fork line method.

Type of gamete / phenotypic category = 2 n

 n = No of hybrid character or heterozygous pair.

eg in dihybrid cross = 32 = 9 genotype

No. of zygote produced by selfing of a gen otype = 4 n

Conclusion (Law of Independent Assortment): The F2 generation plant produce two new phenotypes, so inheritance of seed colour is independent from the inheritance of shape of seed. Otherwise it can not possible to obtain yellow wrinkled and green round type of seeds.

This observation leads to the Mendel's conclusion that different type of characters present in plants assorted independently during inheritance.

This is known as Conclusion of Independent Assortment. It is based on F2 - generation of dihybrid cross.

The nonhomologous chromosome show random distribution during anaphase-I of meiosis.


Explaination :-

A pure yellow and round seeded plant crossed with green and wrinkled seeded plant which are having genotype YYRR and yyrr to produced F1 generation having YyRr genotype.

Both the characters recombine independently from  each other during gamete formation in F1 generation .

Factor (R) of pair factor (Rr) is having equal chance to (Y) factor or (y) factor of gametes during recombination to form two type of gametes (YR) and (yr).

Similarly (r) factor also having equal chance with (Y) factor or (y) factor of gametes to form a two type gametes - (Yr) and (yr).

Thus, total four types of gametes - (YR), (yR), (Yr), and (yr) are formed.

Therefore, during the gametes formation in F1 generation , independent recombination is possible.

– The law of independent assortment is most criticised. Linkage is the exception of this.

A back cross is a cross in which F1 individuals are crossed with any of their parents.

(1) Out Cross : When F1 individual is crossed with dominant parent then it is termed out cross. The generations obtained from this cross, all possess dominant character. so the any analysis can not possible in F1 generation.

[2] Test Cross : When F1 progeny is crossed with recessive parent then it is called test cross. The total generations obtained from this cross, 50% having dominant character and 50% having recessive character. [Monohybrid test cross]. Test cross helps to find out the  genotype of dominant individual.

[a] Monohybrid Test Cross :- The progeny obtained from the monohybrid test cross are in equal proportion , means 50%  is dominant phenotypes and 50% is recessive phenotypes.

It can be represented in symbolic forms as follows.

[b] Dihybrid Test Cross:- The progeny is obtained from dihybrid test cross are four types and each of them is 25%.

   The ratio of Dihybrid test cross = 1:1:1:1

Conclusion:-  In test cross phenotypes and genotypes ratio are same.  


RECIPROCAL CROSS
When two parents are used in two experiments in such a way that in one experiment "A" is used as the female parent and "B" is used as the male parent, in the other experiment "A" will be used as the male parent and "B" as the female parent. such type of a set of two experiments is called Reciprocal cross.
Characters which are controlled by karyogene are not affected by Reciprocal cross. In case of cytoplasmic inheritance result change by Reciprocal cross.

    


GENE INTERACTION

Gene interaction is two types :

(i) Allelic interaction/Intragenic interaction

(ii) Non allelic interaction/Intergenic interaction

(i) Allelic interaction/Intragenic interaction: Allelic interaction takes place between allele of same gene which are present at same locus.

Example of Allelic interaction are as follows :–

[1] Incomplete dominance :- According to Mendel's law of dominance, dominant character must be present in F1 generation. But in some organisms, F1 generation is different from the both parents.

Both factors such as dominant and recessive are present in incomplete dominance but dominant factors is unable to express its character completely, resulting Intermediate type of generation is formed  which is different from the both parents. Some examples are –

  • (a) Flower colour in Mirabilis jalapa : Incomplete dominance was first discovered by Correns in Mirabilis jalapa. This plant is called as Ɗ O' clock plant 'or'Gul-e-Bans'. Three different types of plant are found in Mirabilis on the basis of flower colour, such as red , white and pink.

& When plants with red flowers is crossed with white flower, plants with pink flower obtained in F1 generation. The reason of this is that the genes of red colour is incompletely  dominant over the genes of white colour.

& When, F1 generation of pink flower is self pollinated then the phenotypic  ratio of F2 generation  is red, pink, white is 1:2:1 ratio in place of  normal monohybrid cross ratio 3:1.

& The ratio of phenotype and genotype of F2 generation in incomplete dominance is always same.

(b) Flower colour in Antirrhinum majus :- Incomplete dominance is also seen in flower colour of this plant.This plant is also known as 'Snapdragon ' or 'Dog flower'. Incomplete dominance is found in this plant which is the same as Mirabilis.

(c) Feather colour in Andalusian Fowls :- Incomplete dominance is present for their feather colour.

When a black colour fowl is crossed with a white colour fowl, the colour of F1 generation  is blue.

[2] Co-dominance :- In this phenomenon, both the gene expressed for a particular character in F1 hybrid progeny. There is no blending of characters, wherease both the characters expressed equally.

Examples :- Co-dominance is seen in animals for coat colour. when a black parent is crossed with white parent, a roan colour F1 progeny is produced.

When we obtain F2 generation from the F1 generation, the ratio of black black-white (Roan) white animals is  1 : 2 : 1

Note :-  F2 generation is obtained in animals by sib-mating cross.

 

It is obvious by above analysis that the ratio of phenotype as well as genotype is 1:2:1 in co-dominance.

Sp. Note :- In incomplete dominance, characters are blended phenotypically, while in co-dominace, both the genes of a pair exhibit both the characters side by side and effect of both the character is independent from each other.

Other Examples of Co-dominance :
(ii) AB blood group inheritance (I A I B )
(iii) Carrier of Sickle cell anaemia (Hb A Hb S )

[3] Multiple allele :– More than 2 alternative forms of same gene called as multiple allele. Multiple allele is formed due to mutation. Multile allele located on same locus of homologous chromosome.

A diploid individual contains two alleles and gamete contains one allele for a character.

Ex. Blood group - 3 alleles Coat colour in rabbit - 4 alleles

If n is the number of allele of a gene then number of different possible genotype = 

Example  of multiple allele :

1. ABO blood group → ABO blood groups are determined by allele I A , allele I B , allele I O

I O = recessive Possible phenotypes - A, B, AB, O

2. Coat colour in rabbit → Four alleles for coat colour in rabbit

Wild type = Full coloured = agouti = C +

Himalayan [white with black tip on extremities (like nose, tail and feet)] = c h

 Chinchilla [mixed coloured and white hairs] = c ch

These alleles show a gradient in dominance  C + > c ch > c h > c a

Coloured = C+C+, C+c ch , C+c h C+c a

Chinchilla = c ch c ch , c ch c h , c ch c a

Himalayan = c h c h , c h c a

Possible genotype  =    = 10 genotypes

Eye colour in Drosophila  and self incompatibility genes in plants are also the example of multiple allelism.

[4] Lethal gene :– Gene which causes death of individual in early stage when it comes in homozygous condition called lethal gene. Lethal gene may be dominant or recessive both, but mostly recessive for lethality. Many of these genes which do not cause definite lethality are called semilethals. In semilethal gene death occurs in late stage.

1. Lethal gene was discovered by L. Cuenot in coat colour of mice.
Yellow body colour(Y) was dominant over normal brown colour(y).
Gene of yellow body colour is lethal.
So homozygous yellow mice are never obtained in population. It dies in embryonal stage.
When yellow mice were crossed among themselves segregation for yellow and brown body colour was obtained in 2 : 1 ratio.

YY - death in embryonal stage modified ratio = 2 : 1

2. In plant lethal gene was first discovered by E. Baur in Snapdragon (Antirrhinum majus)

Homozygous golden leaves are never obtained.

3. Sickle cell anaemia in human. In human, gene of sickle cell anaemia HbS is the example of lethal gene.
When two carrier indivudials of sickle cell anaemia are crosed then offsprings are obtained in 2 : 1 ratio.

Sublethal gene but ratio 2 : 1

[5] Pleiotropic gene :– Gene which controls more than one character is called pleiotropic gene. This gene shows multiple phenotypic effect.
For example :

(1) In Pea plant : Single gene influences 
                                                                 

2) In Drosophila recessive gene of vestigial wings also influence the some another characters–

  • Structure of reproductive organs
  • Longevity (Length of Body)
  •  Bristles on wings.
  • Reduction in egg production.

(3) Examples of  pleiotropic gene in human.

(a) Sickle cell anaemia - Gene Hb S β provide a classical example of pleiotrophy. It not only causes haemolytic anaemia but also results increased resistance to one type of malaria that caused by the parasite Plasmodium falciparum. The sickle cell Hb S β allele also has pleiotropic effect on the development of many tissues and organs such as bone, lungs, kidney, spleen, heart.

(b) Cystic fibrosis – Hereditary metabolic disorder that is controlled by a single aoutosomal recessive gene.
The gene specifies an enzyme that produces a unique glycoprotein.
This glycoprotein results in the production of mucous.
More mucous interfere with the normal functioning of several exocrine glands including those in the skin, lungs, liver and pancreas.

(ii) Non allelic interaction/Intergenic interaction When interaction takes place between non allele is called non allelic gene interaction. It changes or modifies other non allelic gene.
Examples of nonallelic interaction.

1. Epistasis :- When, a gene prevents the expression of another non-allelic gene, then it is known as epistatic gene and this phenomenon is known as Epistasis.
Gene which inhibit the expression of another non alleleic gene is called epistatic gene and expression of gene which is suppressed  by epistatic gene called hypostatic gene.
 

Hair Colour in Dog :-

B = Dominant allele for black colour of hairs.
b = Recessive allele for brown colour of hairs.
I = Epistatic gene.
If the genotype bbii for brown colour and BBII for white colour.
Following types of generation will be obtained by following crosses.

It is obviously clear by above analysis, the phenotypic ratio of F2 - generation in epistasis is  - 12:3:1

2. Inhibitory gene - Inhibitory gene itself have no phenotype but inhibits the effect of other non allelic gene. Non allelic gene behaves as  recessive. * Inhibitory gene must be in dominant stage & inhibit the effect of only dominant gene.
Ex., Leaf colour in Rice
R – Purple
r – Green
I – Inhibitory gene
R – I – Green – 9
R – ii – Purple – 3
rr – I – Green – 3
rr – ii – Green – 1
13 (Green) : 3(Purple)

3. Complementary Gene :- Two pair of non allelic genes are essential in doninant form to produce a particular character.
Such genes that act together to produce an effect that neither can produce, it's  effect separately are called complementary genes.
Both types of gene must be present in dominant form.
 

Example :- Colour of flowers in Lathyrus odoratus :-

Thus phenotypic ratio of complementary genes = Coloured : Colourless   9  : ه

4. Duplicate Genes :-

Two pairs of non-allelic genes require  are for a character . If any one of them gene is dominant, then this character is expressed such type of gene is called duplicate gene.

Example :- Fruit shape in Capsella. Two pair of non-allelic genes are present in Capsella for triangular shape of fruits.

If any one gene out of them is dominant, the shape of fruit is triangular and no one gene is dominant than fruits will be elongated.

ttdd = For top shape of fruits

Phenotypic ratio of F2 -> Triangular : Top shaped 15    :   1

5. Additive Gene effect : In additive gene effect if non allelic gene seperately in dominant stage phenotype is same but both gene come dominant stage together phenotype is change due to additive effect. eg. Fruit shape in cucumber

A – B – discoid (new phenotype)

6. Collaboratory Gene :- Two pairs of non-allelic gene interacting together to produce a new phenotypic character.

Example :- Comb - shape in Chickens -

Both R & P = For walnut comb

A new type of phenotype walnut - (Rr Pp) comb is produced by the cross in between Rose comb (RR pp) and Pea comb (rr PP)

 

Thus, phenotypic ratio of collaboratory gene = Walnut : Rose : Pea : Single   = 9 : 3 : 3 : ف

7. Supplementary gene or Recessive Epistasis :- A pair of gene change the effect of another non allelicgene, is called supplementary gene.
Example :- Coat colour in Mice.
If alleles,   C = Black coat colour                  

c = Albino (Colourless coat) or (It has no effect)                  
A = Supplementary geneWhen black coat mice crossed with albino mice, the F1 generation is Agouti.
It means, here the effect of non allelic gene is changed.

Thus, Recessive epistasis or supplementary gene ratio in F2 -  Agouti : Black : Albino

POLYGENIC INHERITANCE
Inheritance of characters in which one character is controlled by many genes and intensity of character depends upon the number of dominant allele.
Polygenic inheritance first described byNilsson - Ehlein kernal colour of wheat.
Nilsson - Ehle said that kernal colour of wheat is regulated by two pairs of gene.

Example-2. :- Colour of the skin in Human.
The inheritance of colour of skin in human studied by Devenport.
Five types of phenotype of colour of skin are found in human.
When a Negro (AA BB) phenotype crossed with white (aa bb) phenotype, intermediate phenotype produced in F1 generation . Phenotypes of F2 generation as follows.

Phenotypic ratioof F2 generation of quantitative inheritance as

  • In new discovery human skin colour and kernal colour in wheat is regulated by 3 pairs of alleles so phenotypic ration of F2 generation.

CYTOPLASMIC INHERITANCE
Inheritance of characters which are controlled by cytogene or cytoplasm is called cytoplosmic inheritance. Genes which are present in cytoplasm called 'cytogene' or 'plasmagene' or extra nuclear gene.
Total cytogene present in cytoplasm is called 'Plasmon'.
A gene which is located in the nucleus is called 'karyogene'.

  • Inheritance of cytogene in organisms occurs only through the female. Because female gamete has karyoplasm, simultaneously it has cytogene because of more cytoplasm.
  • The male gamete of higher plant is called male nucleus. It has very minute [equivelent to nil] cytoplasm. so male gamete only inherited karyogene.
  • Thus, inheritance of cytogene occurs only through female. (also called maternal inheritance)
  • If there is a reciprocal cross in this condition, then results may be effected.

Cytoplasmic inheretance are of three types :

1. Cytoplasmic inheritance involving essential organelles like, Chloroplast and mitochondria called as organellar genetics.

2. Maternal effect depending indirectly on nuclear genes and involving no known cytoplasmic hereditary unit called aspredetermination. In this maternal effect is determined before fertilization.

3. Cytoplasmic inheritance involving dispensable and infective hereditary particle in cytoplasm which may or may not depend on nuclear genes called as Dauermodification.

Example of Organellar Genetics : (True examles of cytoplasmic inheritance)
(a) Plastid inheritance in Mirabilis jalapa – cytoplasmic inheritance first discovered by Correns in Mirabilis jalapa. InMirabilis jalapa branch (leaf) colour is decided by type of plastid present in leaf cells. So it is an example of cytoplasmic inheritance.
Branch colour


(b) Male sterility in maize plant : Gene of male sterelity present in mitochondria. If a normal male plant crossed with a female plant which has genes of male sterility then all the generation of male become sterile because a particular gene was present with female which inherited by female.

(c) Albinism in plant : Gene of albinism found in chloroplast. Gene of albinism in Maize is lethal.

(d) Inheritance of Bacterial plasmid : In bacteria plasmid inheritance is due to conjugation.

(e) Petite form in yeast (mitochondrial gene) : Petite is mutant form of yeast. This mutant form  is slow growing on culture medium.

(f) Iojap inheritance in Maize : Iojap is characterized by constrasting strip of green and white colour of leaves.

(g) Poky Neurospora (mitochondrial gene) : Poky is mutant form of Neurospora. It is slow growing on culture medium.
Example of predetermination

Shell coiling in snail (Limnaea peregra) In snail shell coiling can be of dextral (Coiling to the right) or sinistral (coiling to the Left). This direction of coiling is genetically controlled. The dextral coiling  depends upon dominant allele 'D' and sinistral coiling depends upon recessive allele 'd'. So the dextral is DD, Dd and sinistral is dd.

                       
                 
    

Above reciprocal cross indicates that phenotype of offspring is decided by genotype of female parent not the phenotype of female parent. Even if female parent contains only one dominant gene 'D' then phenotype of all offsprings is dextral.
Example : 

Example of Dauermodification -

(a) Sigma particle in Drosophila:- These particles are virus like particles which are present in Drosophila and related to CO2 sensitivity. Inheritance of sigma particle takes place through the egg cytoplasm.

(b) Kappa particle in Paramecium:- Kappa particles are found in certain "Killer strains" of Paramecium and are responsible for production of substance paramecin which is toxic to strain not prossessing Kappa. (Sensitive Strain) The minimum number of kappa particles is  required 400 to secrete paramecin. Kappa particles are symbiotic bacteria named "Caedobacter taeniospiralis".

CHROMOSOMAL THEORY OF INHERITANCE

This theory was proposed by Walter Sutton and Theodor Boveri (1902). Following are the main points of this theory

1. Gametes serve as the bridge between two successive generations.

2. Male and Female gametes play an equal role in contributing hereditary components of future generation.

3. Only the nucleus of sperm combines with ovum. Thus, the hereditary information is contained in the nucleus.

4. Chromatin in the nucleus is associated with the cell division in the form of chromosomes.

5. Any type of deletion or addition in the chromosomes can cause structural and functional changes in living beings.

6. A sort of parallelism is observed between Mendelian factors and chromosomes.

7. A number of genes or Mendelian factors are found in each chromosome.

8. Determination of sex in most of the animals and plants is affected by specific chromosomes. These chromosomes are called sex chromosomes.

Parallelism Between Gene and Chromosomes

1. Chromosomes are also transferred from one generation to the next as in the case of genes (Mendelian factors).

2. The number of chromosomes is fixed in each living species.These are found as homologous pairs in diploid cells.

One chromosome from father and the other contributed by the mother constitute a homologous pair.

3. Before cell division, each chromosome as a whole and the alleles of genes get replicated and are separated during mitotic division.

4. Meiosis takes place during gamete formation. Homologous chromosomes form synapses during prophase-I stage which in later course get separated and transferred to daughter cells. Each gamete or a haploid cell has only one allele of each gene present in the chromosome.

5. A characteristic diploid number is again established by the union of the two haploid gametes.

6. Both chromosomes and the alleles (Mendelian factors) behave in accordance to Mendel's law of segregation.

In the homologus chromosomes of a pure tall plant, allele (T) is found for tallness in each chromosome. Likewise, in a pure dwarf plant (tt), allele (t) is present in each chromosome.

These homologous chromosomes get separated during meiotic divisoin. Hence, each gamete possesses only one chromosome of an each pair. Accordingly, all the gametes of tall plants possess a chromosome with an allele of tallness (T), while the gametes of dwarf plants possess a chromosome with an allele for dwarfness (t). Their cross to produce F1 generation will yield tall hybrid plants with homologous chromosomal pair containing Tt allelic pair. In this generation two kinds of gamete will be formed during gametogenesis, 50% with the allele (T) for tallness and 50% with the allele for dwarfness (t).Random combination of these gametes will produce offsprings in F2 generation in the ratio of 25% pure tall (TT), 50% hybrid tall (Tt) and 25% dwarf (tt)

Collective inheritance of character is called linkage. Linkage first time seen byBatesonandPunnett inLathyrus odoratus and gave coupling and repulsion phenomenon. But they did not explain the phenomenon of linkage. Sex linkage was  first discoverd by Morgan in Drosophila & coined the term linkage. He proposed the theory of linkage.

Linkage and independent assortment can be represented in dihybrid plant, as –

In case of linkage in dihybrid AaBb

In case of independent assortment in dihybrid AaBb

Theory of linkage

1. Linked genes are linearly located on same chromosome. They get separated if exchange (crossing over), takes place between them.

2. Strength of linkage ⓫/ distance between the genes . It means, if the distance between two genes is increased then strength of linkage is reduced and it proves that greater is the distance between genes, the greater is the probability of their crossing over.

Crossing over obviously disturbs or degenerates linkage. Linked genes can be separated by crossing over.


Factors affecting crossing over (C.O) :-

(1) Distance ­ />= C.O.­  />

(2) Temperature ­ />= C.O.­  />

(3) X-Ray  />­ =  C.O.­  />

(4) Age ­ =  C.O.

(5) Sex - Male C.O.  (Crossing over totally absent in male Drosophila.)

Arrangement of linked Genes on Chromosomes :-The arrangement of linked genes in any dihybrid plant is two types.

[a] Cis - Arrangement :- When, two dominant genes located on one chromosome and both recessive genes located on another chromosome, such type of arrangement is termed as cis-arrangement. Cis-arrangement is  an original arrangement.

[b] Trans-arrangement :- When a chromosome bears one dominant and  one recessive gene, and another chromosome also possess one dominant and one recessive gene, such type of arrangement is called trans-arrangement.Trans-arrangement is not an original form. It is due to crossing over. Two types of gamete also formed in trans-arrangement but it is different from cis-arrangement (Ab) and (aB).

Types  of Linkage :- There are two types of linkage –

1 COMPLETE LINKAGE :- Linkage in which genes always show parental combination. It never forms new combination.

Crossing over is absent in it. Such genes are located very close on the chromosomes. Such type of  linkage very rare in nature. e.g. male Drosophila, female silk moth.

2. INCOMPLETE LINKAGE :- When new combinations also appear along with parental combination in offsprings, this type of linkage is called incomplete linkage, the new combinations form due to crossing over. The percentage of new combination is equal to the percentage of crossing over.(អ%)

Linkage group :- All the genes which are located on one pair of homologous chromosome form one linkage group. Genes which are located on homologous chromosomes inherit together so we consider one linkage group.


Application of Linkage :-
 

Distance can be identified by the incomplete linkage. It's unit is centi Morgan.  

Genetic map/Linkage map/chromosome map - In genetic map different genes are linearly arranged according to % of crossing over (μ Distance) between them. With the help of genetic map we can find out the position of a particular gene on chromosome. Genetic map is helpful in the study of genome.

SEX LINKAGE

When the genes are present on sex-chromosome is termed as sex linked gene and such phenomenon is known as sex-linkage. Two - types of sex linkage :
 

1. X-linkage.
Genes of somatic characters are found on x-chromosome. The inheritance of x-linked character may be through the males and females. e.g. Haemophilia, Colour blindness

2. Y- linkage - The genes of somatic characters are located on Y- chromosome. The inheritance of such type of character is only through the males. Such type of character is called Holandric character. These characters
found only in male.

e.g. (1) Gene which forms TDF /sry-gene                   (3) Webbed toes
(2) Hypertrichosis (excessive hair on ear pinna.)       (4) Porcupine skin
Gene which is located on differential region of Y - chromosome is known as Holandric gene.

Example of X-Sex linkage :-
[i] Eye colour in Drosophila
:- Eye colour in Drosophila is controlled by a X–linked gene.

If a red eyed colour gene is represented as '+' and white eyed colour represented as 'w', then on basis of this
different type of genotypes are found in Drosophila.

Gene for red eye is dominant (+) and white colour of eye is recessive (w)

Homozygous red eyed female = X + X +
Heterozygous red eyed female = X + X w
Homozygous white eyed female = X w X w
Hemizygous red eyed male = X + Y
Hemizygous white eyed male = X w Y

It is clear by above different types of genotype that female either homozygous or heterozygous for eye colour.
But, for the male eye colour, it is always hemizygous.


[ii] Haemophilia :-

Haemophilia is also called "bleeder's disease" and first discovered by John Otto (1803). The gene of
haemophilia is recessive and x-linked lethal gene.

On the basis of x-linked, following types of genotype are found.

X h X = Carrier female
X h X h = Affected female
X h Y = Affected male.

But, X h X h type of female dies during embryo stage because in homozygous condition, this gene becomes lethal and causes death.

Haemophilia -A → due to lack of factor -VIII (Antihaemophilic globulin AHG)

Haemophilia B or Christmas disease - due to lack of factor - IX (Plasma thromboplastin component)

Haemophilia - C (Antosomal disorder) → due to lack of factor - XI (Plasma Thromboplastin antecedent)

[iii] Colour Blindness :- The inheritance of colour-blindness is alike as haemophilia, but it is not a lethal
disease so it is found in male and female.(discovered by Horner)

Three types of colour blindness are-
[a] Protanopia
:- It is for red colour.
[b] Deuteranopia :- It is for green colour
[c] Tritanopia :- For blue colour blindness. Colour blindness is cheked by ishihara - chart.

Other examples of X - sex linkage
[iv] Diabetesinsipidus (recessive).
[v] Duchenne muscular dystrophy (recessive).
[vi] Fragile xsyndrome(recessive).
[vii] Pesudoricketes (Dominant)
[viii] Defective enamel of teeth (Dominant)

Examples of X-Y linkage
(i) Xeroderma pigmentosum
(ii) Epidermolysis bullosa

Types of Inheritance of sex linked characters :-
1. Criss cross inheritance (Morgan)
:- In criss-cross inheritance male or female parent transfer a X- linked character to grandson or grand daughter through the offspring of opposite sex.

(a) Diagenic (Diagynic) :- Inheritance in which characters are inherited from father to the daughter and from daughter to grandson.

Father → daughter → grand son.

(b) Diandric :- Inheritance in which characters are inherited from mother to the son and from son to grand
daughter.

Mother → Son → Grand-daughter.

(2) Non criss-cross inheritance : In this inheritance male or female parent transfer sex linked character to grand son or grand daughter through the offspring of same sex.
(a) Hologenic (Hologynic) :- Mother → Daughter → Grand-daughter (female to female)
(b) Holandric :- Father → Son → Grand-son (male to male)

Sex-Limited Character :- These characters are present in one sex and absent in another sex. But their genes are present in both the sexes and their expression is depend on sex hormone.

Example :- Secondary sexual characters → these genes located on the autosomes and these genes are present in both male and female, but effect of these are depend upon presence or absence of sex-hormones.

For example - genes of beard-moustache express their effects only in the presence of male hormone - testosterone.

Sex Influenced Characters : - Genes of these characters are also present on autosomes but they are influenced differently in male and female. In heterozygous condition their effect is different in both the sexes.

Example :- Baldness :- Gene of baldness is dominant (B).

Gene Bb shows partiality in male and female, Baldness is found in male due to effect of this gene, but baldness is absent in female with this genotype.

SEX DETERMINATION

Establishment of sex through differential development in an individual at an early stage of life, is called sex
determination. There are different methods for sex determination in organisms like environmental, non-allosomic genetic determination, allosomic sex determination and haplodiploidy.

Sex Determination on the basis of fertilization.
Three types –
1. Progamic
– Sex is determined before fertilization.
eg. - drone in honey bee

2. Syngamic - Sex is determined during fertilization.
eg. - most of plants & animals

3. Epigamic - Sex is determined after fertilization.
eg. - Female in honey bee.

Environmental Determination of Sex. It is non-genetic determination of sex which is based purely on environmental conditions. The organisms are potentially hermaphrodite and capable of expressing any of the
two sexes.

1. In marine worm Bonellia, larva develops into female if it settles down alone in an isolated place. Any larva
coming in contact with the already grown female, it changes into male, and lives as a parasite in the uterus of
female.

2. Crepidula (marine mollusca) where larva develops into male in the company of female and develops into
femaleifleftalone.

3. In crocodiles low temperature induces femaleness and high temperature maleness.

4. ln turtles temperature below 28°C induces maleness, above 33°C femaleness while between 28 - 33°C
equal number of male and female animals are formed.

5. In marine fish Medusa sex changes according to environmental condition, becoming male in cold water and
female in warm water.

Allosomic determination of sex –
Chromosomes are of two types -

(a) Autosomes or somatic chromosomes -
These regulate somatic characters.

(b) Allosomes or Heterosomes or Sex chromosomes -

These chromosomes are associated with sex determination. Term "Allosome" & "Heterosome" were given
by Montgomery.

Sex chromosomes first discovered by "Mc Clung" in grass hopper
X- Chromosome discovered by "Henking" and called 'x-body'.

Wilson & Stevens proposed chromosomal theory for sex determination.

(1) XX - XY type or Lygaeus type :- This type of sex determination first observed by Wilson & Stevens in
Lygaeus insect. Two types–

(a) XX female and XY male :- In this type of sex determination female is Homogametic i.e produces only
one type of gamete

In male X-chromosome containing gametes is called "Gynosperm" and Y- chromosome containing gamete is called "Androsperm".
eg. Man and dioecious plants like Coccinea, Melandrium

(b) XY female and XX male or ZW female and ZZ male :- In this type of sex determination female is
Heterogametic i.e produces two types of gamete and male individual is homogametic i.e produces one
type of gamete.
It is found in some insects like butter flies, moths and vertebrates like birds, fishes and reptiles.
In plant kingdom this type of sex determination is found in Fragaria elatior.

(2) XX female and XO male :- or "Protenor type" :- In this type of sex deternination deficiency of one chromosome in male. In this type, female is homogametic and male is heterogametic.

Example :–
– Grass hopper
– Squash bug Anasa
– Cockroach
– Ascaris and in plants like - Dioscorea sinuta & Vallisneria spiralis

Genic balance theory :- C.B. Bridges proposed genic balance theory for sex determination in Drosophila.
– According to Bridges in Drosophila Y-chromosome is heterochromatic so it is not active in sex determination
In Drosophila sex determination takes place by sex index ratio.

In Drosophila gene of femaleness (Sxl- gene) (Sxl=Sex lethal gene) is located on x-chromosome and gene of
maleness is located on autosome
Gene of male fertility is located on y-chromosome and in Drosophila, y-chromosome plays additional role in
spermatogenesis and development of male reproductive organ, so y-chromosome is essential for the production offertilemale.

(c) X/A = 1.5 → Super female or meta female (sterile) (2A + XXX)

(d) X/A = less than 0.5 → Super male or meta male (Sterile) (3A + XY)

(e) X/A = = In between 0.5 and 1 → Intersex (Sterile) (3A+XX)

Gynandromorph –
Body of some Drosophila has some cells with male genotype (X0) and some cells with female genotype (XX).
Body of such type of Drosophila has half lateral part of male and half lateral part of female and it is called bilateral gynandromorph. It is formed due to loss of one x-chromosome at metaphase plate during first zygotic division. Formation of gynandromorph is the best evidence that y-chromosome does not play any role in sex differentiation. 

 Haploid - diploid mechanism –
In insects of order Hymenoptera which includes ants,honey bees, wasps etc.
Sex determination takes place by sets of chromosomes.
Diploid (two sets) → Female
Haploid (One set) → Male
In honey bee, male individual (Drone) develops from unfertilized eggs (Haploid). Male is always parthenote.
Queen and worker bees develop from diploid eggs i.e. fertilized egg.

Sex determination by Hormone –
Dizygotic twins are common in cattle like cow, sheep, goat etc. Some times the placentae of the two dizygotic
twins fuse forming blood vascular connections between two developing foetus. If twins are dizygotic, one
foetus may be male and the other female.

  •  Male hormone produced before female hormone by male twins which suppresses the differentiation of female internal sex organ. Such a sterile female with Under developed ovaries, oviducts, Uterous etc. is called free martin.  In free martin conditions, female is sterile & male is normal.

Cytological basis of sex determination –
Barr body technique or Lyon's hypothesis -

Interphasic nucleus of human female contains two X- chromosomes. Out of two, one X- chromosome becomes
heterochromatin and other X- chromosome is euchromatin. By staining X- heterochromatin, it appears as a
dense body which is called Barr body. (Facultative hetrochromatin)
No. of Barr body ⇒ (No. of X chromosomes – 1)
So in a Normal female (2A + XX) → One Barr body
Normal male (2A + XY) → Barr body absent
Turner syndrome (Sterile female) (2A + XO) → No. Barr body
Klinefelter syndrom (Sterile male)(2A + XXY) → One Barr body

Drum stick which occurs in blood of female of mammals, is also a type of barr body. Drum stick is absent in
neutrophils of Male.

Sex determination in human –
There occur a special gene on differential region of Y-chromosome of human, called Sry - gene (Sex determine
region on y chromosome ). This gene forms a proteinaceous factor called TDF (testes determining factor). TDF
responsible for the development of male reproductive organs. So presence and absence of Y- chromosome
determines sex.

Sex determination in plant –
H.E. Warmke
discovered sex determination in Melandrium plant.
In Melandrium Y- chromosome is long as compare to X- chromosome.
In plant sex chromosomes are found only in unisexual plant.
Pro. R.P. Roy gave the importance of Y-chromosome in plant.
He discovered sex determination in Coccinea indica (Family- cucurbitaceae)
Y- chromosome contains four regions and X- chromosome contains two regions. Different functions of these
regions-

  1. I st region - (Female suppressor region) :- This region suppresses the development of female reproductive structures.
  2. II nd region (Male promotor region) :- This region initiates or start the development of Anther
  3. III rd region (Male fertility region) :- This region induces the further development of Anther.
  4. IV th region (Homologous region) :- This region helps in the disjunction & Pairing of X and Y chromosome during meiosis.
  5. V th region (Differential region of X-chromosome) :- This region induces the development of female gonads

So when one or more than one Y- chromosome present then plant is male and in female plant Ychromosome
is absent.

Special Case :
If I st region of Y chromosome is removed then plant becomes bisexual (XY).

If II nd region of Y chromosome is removed then plant becomes female due to absence of II nd region, I st region of Y chromosome does not suppress the V th region of X-chromosome.

If III rd region of Y chromosome is removed then plant become sterile male due to absence of III rd region so further development of anther does not take place.

PHENOTYPIC EXPRESSION IN HAPLOID ORGANISMS (Neurospora Genetics)

Diploid organisms such as pea and Drosophila, have two alleles for each gene on each chromosome (the exceptions are for the X linked genes in XY or XO males). With the result, the recessive allele is not expressed
in the phenotype in presence of the dominant one. However, this is not so in the case of haploid organisms. Contrary to diploid organisms, the genetics of haploid organisms exhibit the following features:

1. Haploid organisms contain only one allele of a gene, so there is no complication of dominance. All the
genes, whether dominant or recessive, expresses itself in the offsprings.

2. In absence of dominance, any new mutation is immediately expressed in the phenotype, in haploid
organisms.

3. Study of inheritance of the mutated gene, its linkage, crossing over and biochemical consequence of a
mutation can easily be studied in haploid.

Linkage And Recombination in Neurospora (Drosophila of plant kindgom)
Detection of linkage and recombination of genes in haploid organisms as in fungi, bacteria etc. is comparatively
simple. Fungus Neurospora is one of the favourite material with geneticists, because :-
1. The life cycle of Neurospora is the product of a single meiosis.
2. The life cycle is of a short duration.
3. The meiotic products are linearly arranged in ascus as 8 ascospores as ordered tetrads (i.e, the eight
ascospores are arranged in the same order in which chromatids were on the meiotic metaphase plate).

Tetrad Analysis in Ordered Tetrads –
In Neurospora, the nuclei from hyphae of opposite mating type (+) and (–) fuse to form a diploid zygote. The
zygote is the only diploid stage in the life cycle of Neurospora. The zygote nucleus divides meiotically producing four haploid nuclei, each of which then undergoes mitosis. The eight cells produced this way, form 8 haploid ascospores enclosed in the ascus. The three divisions proceed along the longitudinal axis, so the ascospores are arranged in a line in a specific order that indicates the order of arrangement of chromatids on the meiotic metaphase plate. This is called linear or ordered tetrad. Each of the four products of meiosis can be cultured separately to study their phenotypes and genotypes. This is called tetrad analysis.

1. First Division Segregation Between Centromere and gene-a.
A cross between two strain of Neurospora, one normal (a + ) and other mutant (a) strain produces 8-ascospores, out of which four are normal (a + ) and other four mutants (a). The linear arrangement of ascospores in ascus is 4a + : 4a. It indicates the absence of crossing over between locus-a and centromere. This is described as first divisionsegregation.

2. Second Division Segregation Between Centromere and Gene-a.
In a similar cross if crossing over takes place leading to paired arrangement of ascospores with a particular
gene, it is described as second division crossing over. The arrangement of ascospores in the sequence ( 2 :
2: 2 : 2) is as follows:

(i) a + : a + : a : a : a : a : a + : a +
(ii) a : a : a + : a + : a + : a + : a : a
(iii) a + : a + : a : a : a + : a + : a : a

Single Gene Mapping in Neurospora
In Neurospora centromere behaves as a gene for mapping gene pair. In such a case distance of gene from the
centromere is calculated by calculating the percentage of cross overs between centromere and gene.
Que. If 10% asci show crossing over in ascocarp what will be distance between gene and centromere.
If total 100 asci are present in a Neurospora

asci is derivative of 4 chromatids
100 asci are derivative of 400 chromatids = total chromatids
10 asci are derivative of 40 chromatids
(Out of 40 only 20 will be the recombinant type)

 


Why are parents of contrasting genotypes involved for a reciprocal cross? - Biology

MENDEL'S GENETIC LAWS

Hail to the "Father of Genetics" !

My name is ma-ma-ma-ma-ma-Mendel. There are a few important vocabulary terms we should iron-out before diving into Mendel's Laws .

    Now, turns out there are three possible GENOTYPES - two big letters (like "TT"), one of each ("Tt"), or two lowercase letters ("tt"). Since WE LOVE VOCABULARY, each possible combo has a term for it.

    When we have two capital or two lowercase letters in the GENOTYPE (ex: TT or tt ) it's called HOMOZYGOUS ("homo" means "the same"). Sometimes the term " PURE " is used instead of homozygous.

    When the GENOTYPE is made up of one capital letter & one lowercase letter (ex: Tt ) it's called HETEROZYGOUS ("hetero" means "other"). Just to confuse you, a heterozygous genotype can also be referred to as HYBRID . OK?

    Let's Summarize:

    Genotype = genes present in an organism (usually abbreviated as two letters)
    TT = homozygous = pureTt = heterozygous = hybridtt = homozygous = pure
    For example, there is a gene for hair texture (whether hair is curly or straight). One form of the hair texture gene codes for curly hair. A different code for of the same gene makes hair straight. So the gene for hair texture exists as two alleles --- one curly code, and one straight code.

    Let's try & illustrate with a diagram.
    In this picture the two "hot dog" shapes represent a pair of homologous chromosomes. Homologous chromosomes are the same size & have the same genetic info (genes). Each letter in the diagram stands for an allele (form of a gene). What's important to notice is that the letters can be in different forms (capital or lowercase) --- that is what we mean by allele --- and that the letters are lined-up in the same order along each hot dog --- I mean homologous chromosome. The "a-forms" are in corresponding positions, so are the "B-forms", the "c" alleles, the "d" alleles, etc. etc. OK?
    Reread that "allele" definition again & study the picture.

    Getting back to our abbreviations, we could use a "C" for the curly allele, and a "c" for the straight allele. A person's genotype with respect to hair texture has three possiblilties: CC, Cc, or cc. So to review some vocab, homozygous means having two of the same allele in the genotype (2 big or 2 little letters --- CC or cc). Heterozygous means one of each allele in the genotype (ex: Cc).

    Now I could tell you which genotypes create curls & which do not, but then I'd be stealing some of Mr. Mendel's thunder. More on that in a minute .

Vocabulary Review Questions

1. Which of the following is a possible abbreviation for a genotype?

Ma-Ma-Ma-Ma-Mendel's First Law

The Law of Dominance
Stated "simply" it goes like so:
In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.

While Mendel was crossing (reproducing) his pea plants (over & over & over again), he noticed something interesting. When he crossed pure tall plants with pure short plants, all the new pea plants (referred to as the F1 generation) were tall. Similarly, crossing pure yellow seeded pea plants and pure green seeded pea plants produced an F1 generation of all yellow seeded pea plants. The same was true for other pea traits:

Parent Pea Plants F1 Pea Plants
tall stem x short stem all tall stems
yellow seeds x green seeds all yellow seeds
green pea pods x yellow pea pods all green pea pods
round seeds x wrinkled seeds all round seeds
axial flowers x terminal flowers all axial flowers

So, what he noticed was that when the parent plants had contrasting forms of a trait (tall vs short, green vs yellow, etc.) the phenotypes of the offspring resembled only one of the parent plants with respect to that trait. So, he said to himself, "Greg, there is a factor that makes pea plants tall, and another factor that makes pea plants short. Furthermore Greg ol' boy, when the factors are mixed, the tall factor seems to DOMINATE the short factor".

Let's revisit the three possible genotypes for pea plant height & add some MORE VOCABULARY.

Genotype Symbol Genotype Vocab Phenotype
TT homozygous DOMINANT
or
pure tall
tall
Tt heterozygous
or
hybrid
tall
tt homozygous RECESSIVE
or
pure short
short

Note: the only way the recessive trait shows-up in the phenotype is if the geneotype has 2 lowercase letters (i.e. is homozygous recessive).
Also note: hybrids always show the dominant trait in their phenotype (that, by the way, is Mendel's Law of Dominance in a nutshell).

The PUNNETT SQUARE (P-Square for short)

OK, now is as good of time as any to introduce you to a new friend, the Punnett Square. This little thing helps us illustrate the crosses Mendel did, and will assist you in figuring out a multitude of genetics problems.

We will start by using a P-Square to illustrate Mendels Law of Dominance. Recall that he "discovered" this law by crossing a pure tall pea plant & a pure short pea plant. In symbols, that cross looks like this:

where T = the dominant allele for tall stems
& t = recessive allele for short stems

The P-Square for such a cross looks like this:
Inside the 4 boxes are the possible genotypes (with respect to plant height) of the offspring from these parent pea plants. In this case, the only possible genotype is Tt (heterozygous). In hybrids, the dominant trait (whatever the capital letter stands for) is the one that appears in the phenotype, so all the offspring from this cross will have tall stems.

To "fill in the boxes" of the Punnett Square, say to yourself "letter from the left & letter from the top". The "t" from the left is partnered with the "T" from the top to complete each of the four squares.

A summary of this cross would be:

Parent Pea Plants
(P Generation)
Offspring
(F1 Generation)
Genotypes:
TT x tt
Phenotypes:
tall x short
Genotypes:
100% Tt
Phenotypes:
100% tall

Now, a helpful thing to recognize is this:

ANY TIME TWO PARENT ORGANISMS LOOK DIFFERENT FOR A TRAIT,
AND ALL THEIR OFFSPRING RESEMBLE ONLY ONE OF THE PARENTS,
YOU ARE DEALING WITH MEDEL'S LAW OF DOMINANCE.

Does setting up & using the Punnett Square confuse you? Would you like to see a step-by-step "how to" about the good ol' p-square?
If you said "yes", then check this out: "The Punnet Square (in baby steps)".

For some practice Punnett Square problems visit my very own: "P-Square Practice Page".

Don't forget to come back & learn more about Mendel!

Ma-Ma-Ma-Ma-Mendel's Second Law

The Law of Segregation
Goes like so: During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

The way I figure it, Mendel probably got really bored crossing pure dominant trait pea plants with pure recessive trait pea plants (over & over & over again) & getting nothing but pea plants with the dominant trait as a result. Except for gaining more & more evidence for his Law of Dominance, this probably grew tiresome. So, at one point he takes the offspring of a previous cross & crosses them. Ooooooooh .

Recall that his original cross for the tall & short pea plants was:

Parents F1 Offspring
Genotype(s) TT x tt 100% Tt
Phenotype(s) tall x short 100% tall

So, he takes two of the "F1" generation (which are tall) & crosses them. I would think that he is figuring that he's gonna get all tall again (since tall is dominant). But no! Low & behold he gets some short plants from this cross! His new batch of pea plants (the "F2" generation) is about 3/4 tall & 1/4 short. So he says to himself, "Greg ol' boy, the parent plants for this cross each have one tall factor that dominates the short factor & causes them to grow tall. To get short plants from these parents, the tall & short factors must separate, otherwise a plant with just short factors couldn't be produced. The factors must SEGREGATE themselves somewhere between the production of sex cells & fertilization."

I think it's easier to picture this law by using a p-square. Our cross is two hybrid parents, Tt x Tt.
The punnet square would look like this:
Now, when completing a Punnet Square, we model this "Law of Segregation" every time. When you "split" the genotype letters & put one above each column & one in front of each row, you have SEGREGATED the alleles for a specific trait. In real life this happens during a process of cell division called " MEIOSIS ". Meiosis leads to the production of gametes (sex cells), which are either eggs or sperm. Sometimes the term " GAMETOGENESIS " is used instead of meiosis. Scientists love vocabulary (sorry).

You can see from the p-square that any time you cross two hybrids, 3 of the 4 boxes will produce an organism with the dominant trait (in this example "TT", "Tt", & "Tt"), and 1 of the 4 boxes ends up homozygous recessive, producing an organism with the recessive phenotype ("tt" in this example).

Ma-Ma-Ma-Ma-Mendel's Third Law

The Law of Independent Assortment
Alleles for different traits are distributed to sex cells (& offspring) independently of one another.

OK. So far we've been dealing with one trait at a time. For example, height (tall or short), seed shape (round or wrinkled), pod color (green or yellow), etc. Mendel noticed during all his work that the height of the plant and the shape of the seeds and the color of the pods had no impact on one another. In other words, being tall didn't automatically mean the plants had to have green pods, nor did green pods have to be filled only with wrinkled seeds, the different traits seem to be inherited INDEPENDENTLY.

Please note my emphasis on the word "different". Nine times out of ten, in a question involving two different traits, your answer will be "independent assortment". There is a big ugly punnet square that illustrates this law so I guess we should take a look at it. It involves what's known as a "dihybrid cross", meaning that the parents are hybrid for two different traits.

The genotypes of our parent pea plants will be: RrGg x RrGg where
"R" = dominant allele for round seeds
"r" = recessive allele for wrinkled seeds
"G" = dominant allele for green pods
"g" = recessive allele for yellow pods

Notice that we are dealing with two different traits: (1) seed texture (round or wrinkled) & (2) pod color (green or yellow). Notice also that each parent is hybrid for each trait (one dominant & one recessive allele for each trait).

We need to "split" the genotype letters & come up with the possible gametes for each parent. Keep in mind that a gamete (sex cell) should get half as many total letters (alleles) as the parent and only one of each letter. So each gamete should have one "are" and one "gee" for a total of two letters. There are four possible letter combinations: RG, Rg, rG, and rg. These gametes are going "outside" the p-square, above 4 columns & in front of 4 rows. We fill things in just like before --- "letters from the left, letters from the top". When we finish each box gets four letters total (two "are's" & two "gees").

This is what it looks like:
RG Rg rG rg
RG RRGG
round
RRGg
round
RrGG
round
RrGg
round
Rg RRGg
round
RRgg
round
RrGg
round
Rrgg
round
rG RrGG
round
RrGg
round
rrGG
wrinkled
rrGr
wrinkled
rg RrGg
round
Rrgg
round
rrGg
wrinkled
rrgg
wrinkled

The results from a dihybrid cross are always the same:
9/16 boxes (offspring) show dominant phenotype for both traits (round & green),
3/16 show dominant phenotype for first trait & recessive for second (round & yellow),
3/16 show recessive phenotype for first trait & dominant form for second (wrinkled & green), &
1/16 show recessive form of both traits (wrinled & yellow).

So, as you can see from the results, a green pod can have round or wrinkled seeds, and the same is true of a yellow pod. The different traits do not influence the inheritance of each other. They are inherited INDEPENDENTLY.

Interesting to note is that if you consider one trait at a time, we get "the usual" 3:1 ratio of a single hybrid cross (like we did for the LAw of Segregation). For example, just compare the color trait in the offspring 12 green & 4 yellow (3:1 dominant:recessive). Same deal with the seed texture 12 round & 4 wrinkled (3:1 ratio). The traits are inherited INDEPENDENTLY of eachother --- Mendel's 3rd Law.

I would like to summarize Mendel's Laws by listing the cross that illustrates each.
LAW PARENT CROSS OFFSPRING
DOMINANCE TT x tt
tall x short
100% Tt
tall
SEGREGATION Tt x Tt
tall x tall
75% tall
25% short
INDEPENDENT ASSORTMENT RrGg x RrGg
round & green x round & green
9/16 round seeds & green pods
3/16 round seeds & yellow pods
3/16 wrinkled seeds & green pods
1/16 wrinkled seeds & yellow pods

Review Questions

1. Which cross would best illustrate Mendel's Law of Segregation?


Base questions #4-8 on the following information:

9. Crossing two dihybrid organisms results in which phenotypic ratio?

A. cD
B. Ee
C. AB
D. ee

TTFN !
(TA-TA FOR NOW)

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Vocabulary Term Review Questions - CORRECT ANSWERS ARE UNDERLINED

1. Which of the following is a possible abbreviation for a genotype?



Review Questions - ANSWERED & EXPLAINED

1. Which cross would best illustrate Mendel's Law of Segregation?


Base questions #4-8 on the following information:

4. Which phenotype is dominant? white
5. What are the genotypes of the original parent plants? WW (pure white) x ww (pink)
6. What is the genotype of all the F1 offspring? Ww (white)
7. What would be the percentages of genotypes & phenotypes if one of the white F1 plants is crossed with a pink-flowered plant?

50% heterozygous white & 50% homozygous recessive pink.

The cross for this question would be "Ww (white F1) x ww (pink)".
The alleles of the white parent are above the columns & those of the pink parent are in front of the rows. 2 of 4 boxes (50%) are "Ww", which is heterozygous & would have the dominant trait (white). The other 2 of 4 boxes (50%) are "ww", which is homozygous recessive & would have the recessive trait (pink).

8. Which of Mendel's Laws is/are illustrated in this question? Dominance is illustrated by the original cross (WW x ww).

9. Crossing two dihybrid organisms results in which phenotypic ratio?

A. cD
B. Ee- a possible allelic pair but NOT SHOWN IN THE DIAGRAM, so this CAN'T be an answer
C. AB
D. ee - an "allelic pair" is always two forms of the same letter. In this example they are two lowercase "e's".


Watch the video: Test Cross Determining Genotype (January 2022).