Information

Can you use a cell in meiosis to create a karyotype?


Why might it be problematic to use a cell undergoing meiosis to create a karyotype?


Yes, you can, but it's way more difficult.

First of all, in order to see meiotic chromosomes, you need to colect young floral bud in the right stage of development (not too young, since you don't want microspore mother cell, and not too old, since you don't want polen), and that's not that easy. Then, once you found the right stage and made a slide, you need good knowledge on all the phases of meiosis to understand what you are looking at. Bivalents and translocation rings can make it very difficult to really understand the chromosomes set you have in the cell, so you need a good sized sample and a lot of observation and analysis. If the species you are analyzing has small chromosomes, it's even more difficult to understand what you are looking at.


9.3: Errors in Meiosis

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  • Concepts of Biology at OpenStax CNX

Inherited disorders can arise when chromosomes behave abnormally during meiosis. Chromosome disorders can be divided into two categories: abnormalities in chromosome number and chromosome structural rearrangements. Because even small segments of chromosomes can span many genes, chromosomal disorders are characteristically dramatic and often fatal.


Nondisjunctions, Duplications, and Deletions

Of all the chromosomal disorders, abnormalities in chromosome number are the most easily identifiable from a karyogram. Disorders of chromosome number include the duplication or loss of entire chromosomes, as well as changes in the number of complete sets of chromosomes. They are caused by nondisjunction , which occurs when pairs of homologous chromosomes or sister chromatids fail to separate during meiosis. The risk of nondisjunction increases with the age of the parents.

Nondisjunction can occur during either meiosis I or II, with different results ([Figure 2]). If homologous chromosomes fail to separate during meiosis I, the result is two gametes that lack that chromosome and two gametes with two copies of the chromosome. If sister chromatids fail to separate during meiosis II, the result is one gamete that lacks that chromosome, two normal gametes with one copy of the chromosome, and one gamete with two copies of the chromosome.

Figure 2: Following meiosis, each gamete has one copy of each chromosome. Nondisjunction occurs when homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate during meiosis.

An individual with the appropriate number of chromosomes for their species is called euploid in humans, euploidy corresponds to 22 pairs of autosomes and one pair of sex chromosomes. An individual with an error in chromosome number is described as aneuploid , a term that includes monosomy (loss of one chromosome) or trisomy (gain of an extraneous chromosome). Monosomic human zygotes missing any one copy of an autosome invariably fail to develop to birth because they have only one copy of essential genes. Most autosomal trisomies also fail to develop to birth however, duplications of some of the smaller chromosomes (13, 15, 18, 21, or 22) can result in offspring that survive for several weeks to many years. Trisomic individuals suffer from a different type of genetic imbalance: an excess in gene dose. Cell functions are calibrated to the amount of gene product produced by two copies (doses) of each gene adding a third copy (dose) disrupts this balance. The most common trisomy is that of chromosome 21, which leads to Down syndrome. Individuals with this inherited disorder have characteristic physical features and developmental delays in growth and cognition. The incidence of Down syndrome is correlated with maternal age, such that older women are more likely to give birth to children with Down syndrome ([Figure 3]).

Figure 3: The incidence of having a fetus with trisomy 21 increases dramatically with maternal age.

Visualize the addition of a chromosome that leads to Down syndrome in this video simulation.

Humans display dramatic deleterious effects with autosomal trisomies and monosomies. Therefore, it may seem counterintuitive that human females and males can function normally, despite carrying different numbers of the X chromosome. In part, this occurs because of a process called X inactivation . Early in development, when female mammalian embryos consist of just a few thousand cells, one X chromosome in each cell inactivates by condensing into a structure called a Barr body. The genes on the inactive X chromosome are not expressed. The particular X chromosome (maternally or paternally derived) that is inactivated in each cell is random, but once the inactivation occurs, all cells descended from that cell will have the same inactive X chromosome. By this process, females compensate for their double genetic dose of X chromosome.

In so-called “tortoiseshell” cats, X inactivation is observed as coat-color variegation ([Figure 4]). Females heterozygous for an X-linked coat color gene will express one of two different coat colors over different regions of their body, corresponding to whichever X chromosome is inactivated in the embryonic cell progenitor of that region. When you see a tortoiseshell cat, you will know that it has to be a female.

Figure 4: Embryonic inactivation of one of two different X chromosomes encoding different coat colors gives rise to the tortoiseshell phenotype in cats. (credit: Michael Bodega)

In an individual carrying an abnormal number of X chromosomes, cellular mechanisms will inactivate all but one X in each of her cells. As a result, X-chromosomal abnormalities are typically associated with mild mental and physical defects, as well as sterility. If the X chromosome is absent altogether, the individual will not develop.

Several errors in sex chromosome number have been characterized. Individuals with three X chromosomes, called triplo-X, appear female but express developmental delays and reduced fertility. The XXY chromosome complement, corresponding to one type of Klinefelter syndrome, corresponds to male individuals with small testes, enlarged breasts, and reduced body hair. The extra X chromosome undergoes inactivation to compensate for the excess genetic dosage. Turner syndrome, characterized as an X0 chromosome complement (i.e., only a single sex chromosome), corresponds to a female individual with short stature, webbed skin in the neck region, hearing and cardiac impairments, and sterility.

An individual with more than the correct number of chromosome sets (two for diploid species) is called polyploid . For instance, fertilization of an abnormal diploid egg with a normal haploid sperm would yield a triploid zygote. Polyploid animals are extremely rare, with only a few examples among the flatworms, crustaceans, amphibians, fish, and lizards. Triploid animals are sterile because meiosis cannot proceed normally with an odd number of chromosome sets. In contrast, polyploidy is very common in the plant kingdom, and polyploid plants tend to be larger and more robust than euploids of their species.


Activity Preparation

This portion of the activity is rooted in the CLD element Groupings, in which students work together in small groups, help each other, and generate shared meaning (Gagnon & Collay, 2001, pp. 35–50). Students are divided into groups of three or four students and supplied with scissors and tape. They should generate one set of karyotypes per group. This activity requires 60–90 minutes to complete, depending on the amount of pre-activity and Reflection discussion that occurs.

The CLD element Bridge involves reviewing what your students should know and linking what they know with what they are going to learn (Gagnon & Collay, 2001, pp. 51–63). Students should review the structure of chromosomes and the steps of meiosis before the activity. The Bridge can consist of a pre-activity to confirm that they have the basic information they need to complete the activity and identify any misconceptions.

For example, you could ask students to draw a cell with two pairs of homologous chromosomes. Students should label each chromosome either A, a, B, or b, where A/a and B/b represent homologous pairs of chromsomes. The instructor should confirm that students have not drawn sister chromatids at this point, which students typically draw as an X.

Next, ask students to draw the same cell after it has completed S phase and the DNA has been replicated. This is an opportunity to confirm that all students recall that this stage consists of 4 chromosomes, each with sister chromatids that are attached at the centromere region, as well as an opportunity to reinforce the genetic difference between chromatids, homologous chromosomes, and nonhomologous chromosomes.

Students can then be asked to draw this cell at the end of meiosis I. At this point, the instructor can confirm that all students have drawn two cells, each containing only two chromosomes, one of each homologous pair, arranged as sister chromatids still attached at the centromeres. Students should then be asked to draw the cell at the end of meiosis II, and the instructor can confirm that each student has drawn four cells, each containing two chromosomes, one of each homologous pair.

Students can label each cell as “n” or “2n” according to the number of chromosomes in the original cell, and the instructor can confirm that the cells drawn at the end of meiosis I and II are all labeled “n”. Lastly, the instructor can have the students label each cell with the number of chromosomes you find in a human germ cell at each time point indicated. The instructor can confirm that the cells drawn before the start of meiosis are labeled “46” and that the cells drawn after the end of meiosis I are labeled “23”.


Sexual Reproduction

Strawberry plants, aspen trees, and coral can also reproduce sexually. Sexual selection is a slower process than asexual reproduction, but it leads to genetic diversity among species. Not every sexually produced organism will have higher fitness however, the increased diversity within the population increases the likelihood of high fitness offspring.

Homologous Chromosomes

Almost all nucleated cells in your body are genetic clones. Your body cells are diploid (2n) because they contain a pair of each chromosome. A human diploid cell has 46 chromosomes in 23 pairs.

A pair of chromosomes comprise one chromosome that came from mom and one chromosome that came from dad. Both chromosomes contain the same genes but have different allele combinations. Therefore, chromosome pairs are homologous to each other.

Paternal = Dad maternal = mom

Fertilization

Gametes are sperm and eggs. Gametes are the only haploid (n) cells because they contain a single set of chromosomes, not homologous pairs.

When a sperm fertilizes an egg, the two sets of chromosomes unite and form 23 homologous pairs. The united diploid cell is a zygote that has the same number of chromosomes as their parents. Let’s use some math to explain:

sperm (n) + egg (n) = zygote (2n)

23 chromosomes + 23 chromosome = 46 chromosomes

Gametes must be haploid to ensure that all human offspring have the same number of chromosomes and that they are in 23 homologous pairs.

Karyotype

A karyotype is the arrangement of chromosomes after they have been copied but before they separate during division.

Meiosis

The testes in males and the ovaries in females (collectively known as the gonads) process of meiosis to separate the homologous pairs and cut the chromosome number in half.

Here is how a diploid cell (2n) becomes four haploid cells (n) during meiosis:

  1. Interphase is not a part of meiosis however, it must precede meiosis so the DNA can make a copy of itself. Chromosomes look like lowercase l-shaped but become X-shaped after each chromosome is copied. The DNA copies will separate during meiosis, resulting in 4 genetically unique gametes.
  2. Meiosis I: The homologous pairs separate, and two new haploid cells form
  3. Meiosis II: The chromosome copies separate, resulting in four genetically different haploid cells (sperm and eggs).

Genetic Diversity

Meiosis increase genetic diversity by:

  • Crossing over is a process in which homologous chromosomes swap portions of their chromosomes with each other. The chromosomes swap the same genes however, the allele combinations are different. The result is two chromosomes with a new allele combination.

  • During meiosis I, the homologous chromosomes randomly line up at the center of the cell, analogous to shuffling a deck of cards. Each cell that undergoes meiosis randomly places the homologous chromosomes along the center of the cell just like each poker game, the deck is randomly shuffled.
  • After the random lineup of homologous chromosomes, the chromosomes are independently assorted into two cells, analogous to dealing the cards into two piles.


03 Genetics SL

This page lists the understandings and skills expected for topic three. Helpful for revision.
Detailed revison notes, activities and questions can be found on each of the sub-topic pages.

  • 3.1 Genes
  • 3.2 Chromosomes
  • 3.3 Meiosis
  • 3.4 Inheritance
  • 3.5 Genetic modification and biotechnology

3.1 Genes

  • Definition of a gene is, "a heritable factor that consists of a length of DNA and influences a specific characteristic."
  • A gene locus is, "the specific position of a gene on a chromosome."
  • Alleles are, "The various specific forms of a gene which differ from each other by one or only a few bases".
  • New alleles are formed by mutation.
  • A genome is, "the whole of the genetic information of an organism."
  • The entire base sequence of human genes was sequenced in the Human Genome Project.
  • The Genbank® database can be used to search for DNA base sequences.

Skills ( can you . )

  • Explain the causes of sickle cell anemia, including a base substitution mutation, subsequent change to the mRNA transcribed from it and a change to the sequence of amino acids in a polypeptide of hemoglobin.
  • Recall of one specific base substitution that causes glutamic acid to be substituted by valine as the sixth amino acid in the hemoglobin polypeptide is required. (Deletions, insertions and frame shift mutations are not needed.)
  • Compare the number of genes in humans with other species.
    At least one plant and one bacterium should be included in the comparison and at least one species with more genes and one with fewer genes than a human.
    (note: "genome size" is total amount of DNA, not the number of genes in a species)
  • Use of a database to determine differences in the base sequence of a gene in two species. Look up "GENBANK"

3.2 Chromosomes

  • Prokaryotes have one single circular DNA molecules as a chromosome.
  • Some prokaryotes also have plasmids but eukaryotes don't.
  • Eukaryote chromosomes are linear DNA molecules associated with histone proteins.
  • In a eukaryote species there are a characteristic number of different chromosomes each carrying different genes.
  • Pairs of chromosomes with the same sequence of genes (not necessarily the same alleles) are "Homologous chromosomes"
  • Diploid nuclei have pairs of homologous chromosomes.
  • Haploid nuclei have one chromosome of each pair.
  • Sister chromatids are the two DNA molecules formed by DNA replication before cell division
  • two separate chromosomes are formed at the splitting of the centromere at the start of anaphase.
  • A karyogram (a chart) shows the chromosomes of an organism in homologous pairs of decreasing length. (Karyotype is the number and type of chromosomes present in the nucleus)
  • Sex chromosomes determine the gender of an individual and autosomes are chromosomes that do not determine sex.

Skills (can you . )

  • Understand Cairns&rsquo technique for measuring the length of DNA molecules by autoradiography.
  • Compare genome size in T2 phage, Escherichia coli, Drosophila melanogaster, Homo sapiens and Paris japonica. (selected for points of interest.Genome size comparative activity
  • Use karyograms to compare diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum.
  • Use karyograms to deduce sex and diagnose Down syndrome in humans.
  • Use of databases to identify the locus of a human gene and its polypeptide product

3.3 Meiosis

  • Meiosis produces four haploid nuclei from one diploid nucleus.
  • Haploid nuclei allow a life cycle with fusion of gametes.
  • DNA is replicated before meiosis so that all chromosomes at start of meiosis are 'double stranded' with two sister chromatids.
  • The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.
  • Random orientation of pairs of homologous chromosomes.
  • Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number.
  • Genetic variation is the result of crossing over and random orientation.
  • Different parents providing gametes promotes genetic variation.

Skills ( can you . )

  • Describe how non-disjunction can cause Downs syndrome and other chromosome abnormalities.
  • Remember studies showing age of parents influences chances of non-disjunction.
  • Describe methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks.
  • Draw diagrams to show the stages of meiosis (possibly using prepared slides), and resulting in the formation of four haploid cells. (Chiasmata not required)

3.4 Inheritance

  • Mendel experiments with pea plants showing his rules of inheritance.
  • Gametes are haploid so contain only one allele of each gene.
  • The two alleles of each gene separate independently during meiosis.
  • Fusion of gametes results in diploid zygotes with two alleles of each gene.
  • Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint effects.
  • Many genetic diseases in humans are due to recessive, dominant or co-dominant alleles of autosomal genes.
  • Some genetic diseases are sex-linked, shown as superscript letters eg. X h . The pattern of inheritance is different due to their location on sex chromosomes.
  • Many genetic diseases have been identified in humans & most are very rare.
  • Radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and cancer.

Skills (can you . )

  • Explain the inheritance of ABO blood groups.using I A , I B or i allele notation.
  • Explain inheritance of red-green colour blindness and hemophilia as examples of sex-linked inheritance.
  • Explain the autosomal inheritance of cystic fibrosis and Huntington&rsquos disease.
  • Construct Punnett grids for monohybrid genetic crosses.
  • Compare predicted and actual outcomes of genetic crosses using real data.
  • Analyse pedigree charts and to deduce the pattern of inheritance of genetic diseases.

3.5 Genetic Modification

  • Gel electrophoresis is used for the separation of fragments of DNA (or proteins).
  • PCR (polymerase chain reaction) can be used to make many copies of small amounts of DNA. (called amplification)
  • DNA profiling uses PCR and gel electrophoresis to compare samples of DNA (eg. in paternity disputes)
  • Genetic modification is the transfer of genes from one species to another.
  • Clones are groups of genetically identical organisms, derived from a single original parent cell.
  • Many plant species and some animal species have natural methods of cloning.
  • Animals can be cloned at the embryo stage by breaking up the embryo into more than one group of cells. or by using differentiated cells in adults.
  • The cloned sheep, Dolly can be used as an example of the cloning method of somatic-cell nuclear transfer.

Skills (can you . )

  • Use images of DNA profiling to solve paternity disputes and other forensic examples.
  • Explain that Gene transfer using plasmids in bacteria makes use of the enzymes restriction endonucleases and DNA ligase.
  • Assess the potential risks and benefits of GMO crops, including data on risks to monarch butterflies of Bt crops.
  • Design of an experiment to assess one factor affecting the rooting of stem-cuttings of a plant which easily produces roots in soil.

Chromosomes 3.2

Chromosomes are circular in prokaryotes and linear in eukaryotes. The number of chromosomes is a characteristic of eukaryote species. The structure and shape of the chromosomes in an organism can also give information about the genetic diseases and gender.

Genes 3.1

Genes provide the instructions to build proteins and more besides. The human genome has been decoded by the human genome project and now biologists can search databases to find the location of specific genes.

Genetic Modification 3.5

The ability to find the sequence of the DNA code has provided new tools for biologists to investigate and manipulate DNA. These tools include PCR, gel electrophoresis, DNA profiling, genetic modification. This topic covers these techniques.

Inheritance 3.4

Theoretical genetics started with Gregor Mendel who established some simple rules of inheritance based on the idea that characteristics or "traits" are inherited independently. The gametes carry a single copy of any gene which becomes a pair of alleles.

Meiosis 3.3

Meiosis is the process which allows sexual reproduction to be possible. It produces four haploid cells which make gametes. In this topic you cover how the chromosomes move during meiosis, crossing over of chromatids and non-disjunction.


Sex Chromosome Nondisjunction

Humans display dramatic deleterious effects with autosomal trisomies and monosomies. Therefore, it may seem counterintuitive that human females and males can function normally, despite carrying different numbers of the X chromosome. In part, this occurs because of a process called X inactivation. Early in development, when female mammalian embryos consist of just a few thousand cells, one X chromosome in each cell inactivates by condensing into a structure called a Barr body. The genes on the inactive X chromosome are not expressed. The particular X chromosome (maternally or paternally derived) that is inactivated in each cell is random, but once the inactivation occurs, all cells descended from that cell will have the same inactive X chromosome. By this process, females compensate for their double genetic dose of X chromosome.

In so-called “tortoiseshell” cats, X inactivation is observed as coat-color variegation (Figure 6). Females heterozygous for an X-linked coat color gene will express one of two different coat colors over different regions of their body, corresponding to whichever X chromosome is inactivated in the embryonic cell progenitor of that region. When you see a tortoiseshell cat, you will know that it has to genetically be a female.

Figure 6 Embryonic inactivation of one of two different X chromosomes encoding different coat colors gives rise to the tortoiseshell phenotype in cats. (credit: Michael Bodega)

In an individual carrying an abnormal number of X chromosomes, cellular mechanisms will inactivate all but one X in each of her cells. As a result, X-chromosomal abnormalities are typically associated with mild mental and physical defects, as well as sterility. If the X chromosome is absent altogether, the individual will not develop.

Several errors in sex chromosome number have been characterized. Individuals with three X chromosomes, called triplo-X, appear female but express developmental delays and reduced fertility. The XXY chromosome complement, corresponding to one type of Klinefelter syndrome, corresponds to male individuals with small testes, enlarged breasts, and reduced body hair. The extra X chromosome undergoes inactivation to compensate for the excess genetic dosage. Turner syndrome, characterized as an X0 chromosome complement (i.e., only a single sex chromosome), corresponds to a female individual with short stature, webbed skin in the neck region, hearing and cardiac impairments, and sterility.


Section Summary

The number, size, shape, and banding pattern of chromosomes make them easily identifiable in a karyogram and allow for the assessment of many chromosomal abnormalities. Disorders in chromosome number, or aneuploidies, are typically lethal to the embryo, although a few trisomic genotypes are viable. Because of X inactivation, aberrations in sex chromosomes typically have milder effects on an individual. Aneuploidies also include instances in which segments of a chromosome are duplicated or deleted. Chromosome structures also may be rearranged, for example by inversion or translocation. Both of these aberrations can result in negative effects on development, or death. Because they force chromosomes to assume contorted pairings during meiosis I, inversions and translocations are often associated with reduced fertility because of the likelihood of nondisjunction.

Exercises

Glossary

aneuploid: an individual with an error in chromosome number includes deletions and duplications of chromosome segments

autosome: any of the non-sex chromosomes

chromosome inversion: the detachment, 180° rotation, and reinsertion of a chromosome arm

euploid: an individual with the appropriate number of chromosomes for their species

karyogram: the photographic image of a karyotype

karyotype: the number and appearance of an individuals chromosomes, including the size, banding patterns, and centromere position

monosomy: an otherwise diploid genotype in which one chromosome is missing

nondisjunction: the failure of synapsed homologs to completely separate and migrate to separate poles during the first cell division of meiosis

polyploid: an individual with an incorrect number of chromosome sets

translocation: the process by which one segment of a chromosome dissociates and reattaches to a different, nonhomologous chromosome

trisomy: an otherwise diploid genotype in which one entire chromosome is duplicated

X inactivation: the condensation of X chromosomes into Barr bodies during embryonic development in females to compensate for the double genetic dose


The Phases of Meiosis II

Meiosis II may begin with interkinesis or interphase II. This differs from interphase I in that no S phase occurs, as the DNA has already been replicated. Thus only a G phase occurs. Meiosis II is known as equational division, as the cells begin as haploid cells and end as haploid cells. There are again four phases in meiosis II: these differ slightly from those in meiosis I.

1. Prophase II

Chromatin condenses to form visible chromosomes again. The nuclear envelope and nucleolus disintegrate, and spindle fibers begin to appear. No crossing over occurs.

2. Metaphase II

Spindle fibers connect to the kinetochore of each sister chromatid. The chromosomes align at the equatorial plane, which is rotated 90° compared to the equatorial plane in meiosis I. One sister chromatid faces each pole, with the arms divergent.

3. Anaphase II

The spindle fibers connected to each sister chromatid shorten, pulling one sister chromatid to each pole. Sister chromatids are known as sister chromosomes from this point.

4. Telophase II

Meiosis II ends when the sister chromosomes have reached opposing poles. The spindle disintegrates, and the chromosomes recoil, forming chromatin. A nuclear envelope forms around each haploid chromosome set, before cytokinesis occurs, forming two daughter cells from each parent cell, or four haploid daughter cells in total.

Figure 1. The phases of meiosis I and meiosis II, showing the formation of four haploid cells from a single diploid cell.

Image Source: Wikimedia Commons


Protein synthesis

Proteins are the most common large molecules in cells. Bones, muscles, and red blood cells (among lots of other body parts) are made mostly of proteins. Therefore, the production of proteins is obviously a very important process in the body.

Proteins are made of smaller units called amino acids. There are 20 different amino acids, 9 of which are "essential amino acids", which means that they must be consumed through the diet, rather than being synthesized by the body. The sequence of amino acids in a protein determines its structure and function.

The base pair sequence of the DNA molecule is known as the genetic code. The genetic code consists of 3-base sequences called codons.(Remember our friends A, T, G, and C -- the nitrogenous bases? That's what we're talking about here.) Each codon either codes for an amino acid, or signals that the protein chain is starting (an initiation codon) or stopping (a termination codon). The table on the right shows which codons code for which amino acids.

Each DNA molecule contains the information to make up many different proteins. A portion of a DNA molecule responsible for making up a single protein (or sometimes just part of a protein, called a polypeptide) is a gene. Therefore, each DNA molecule consists of many genes, that code for many proteins.

Protein synthesis has two basic steps: (1) transcription and (2) translation.

1) The gene (DNA) is copied onto RNA. The RNA copy of the gene is called the messenger RNA (or mRNA).

2) The mRNA leaves the nucleus and goes to the rough endoplasmic reticulum. (Remember this organelle from the cell structure section above?)

1) The mRNA goes into the ribosomes, where tRNA (translation RNA) reads the mRNA.

2) As the tRNA reads the mRNA, it attaches complementary amino acids to the newly synthesized amino acid chain (AKA the growing protein chain).

Important note: The important thing to remember about protein synthesis is that when a protein is synthesized, sometimes accidents or mistakes happen. This is where some of the variation in protein synthesis comes from. Additionally, remember the possible mistakes in DNA replication? If the original DNA strand is changed so that a gene is altered, that can affect protein synthesis as well.

This video has fantastic digital animations illustrating DNA replication and protein synthesis.


Inheritance: Meiosis and Sexual Reproduction

Sexual reproduction is the union of male and female gametes to form a fertilized egg, or zygote. The resulting offspring inherit one half of their traits from each parent. Consequently they are not genetically identical to either parent or siblings, except in the case of identical twins. As hypothesized by Mendel, adults are diploid, signified as 2N, having two alleles available to code for one trait. The gametes must be haploid, signified by N, containing only one allele so that when two haploid gametes combine, they produce a normal diploid individual. The process where haploid sex cells are created from diploid parents is called meiosis, and it occurs only in the reproductive organs.

Bioterms

Gametogenesis occurs only in the ovaries and testes and represents the formation of haploid egg and sperm as a result of meiosis.

A diploid cell undergoing meiosis first duplicates itself and then divides two times, creating four haploid cells. Meiosis begins with the same G1, S, and G2 stages as mitosis and also ends with a duplicate set of chromosomes. In both processes, the cell divides to form two diploid (2N) offspring however, meiosis continues with another division, which creates the four haploid gametes, in a process called gametogenesis. In meiosis, several interesting events may happen along the way to provide genetic recombination, an unexpected change in the hereditary genetic material.

Bionote

A chromosome contains numerous genes and is made of two sister chromatids joined by a centromere. The centromere is the region of a chromosome where the two sister chromatids are joined. Homologous chromosomes are inherited from each parent and are the two chromosomes that make up a pair in a diploid cell. They are normally the same length, contain similar genes in the same location, and have a centromere at the same locus.

Mechanism of Meiosis I

The functional difference between mitosis and meiosis occurs in meiosis I. A synapsis occurs during prophase I, where homologous chromosomes align next to each other. Homologous chromosomes are the matched pair found in a diploid cell. The maternal and paternal homologous chromosomes are made of two sister chromatids that are duplicated copies so at synapsis, four chromatids are aligned together in a structure called a tetrad, which is a fundamental difference between mitosis and meiosis. In metaphase I, the chromosome tetrads are oriented along the metaphase plate, the equator between the two opposite ends of the cell. In anaphase I, the tetrads split, with the sister chromatids remaining joined at their centromere. When the chromosomes arrive at their respected sides of the cell, in telophase, cytokinesis begins and one cell becomes two. They remain duplicates, but they are still haploid. The two cells now enter meiosis II. Refer to the illustration Meiosis I for a pictorial representation.

Mechanism of Meiosis II

The overall result of meiosis II is to create four haploid sex cells from the two diploid cells that began the process refer to the illustration Meiosis II for a pictorial representation. Like mitosis, in metaphase II the chromosomes line up along the cell equator, and the paired chromatids separate in anaphase II. This final separation reduces the chromosome number by one-half, creating the haploid sperm and egg. Because of segregational and independent assortment, they may contain completely different alleles.

Abnormalities, Genetic Recombination, Variability

Spontaneous mistakes occur during meiosis that lead to gametes with unusual changes in their genetic structure (makeup). These gene changes lead to an unexpected genetic recombination that, if the organism survives, increases the genetic variability for the population. There are opportunities described next to increase genetic variability during meiosis.

The crossing over of sister chromatids sometimes occurs when they are aligned as tetrads in metaphase I. One chromatid or chromatid piece mistakenly lies on top of a neighboring chromatid. The neighboring nonsister chromatid absorbs the new piece of chromatid into the chromosome and releases the corresponding piece to be absorbed by the first chromosome. The net result is a new genetic recombination, because neighboring chromosomes have exchanged pieces of chromatid that will undergo meiosis as a new component of the chromosome. Refer to the illustration Crossing over.

The random alignment of chromosomes during metaphase I allows equal opportunity for a particular chromosome to migrate into a cell. This type of independent assortment gives rise to exponential gene combinations in the offspring.

Sometimes the spindle fibers fail to separate homologous chromosomes during anaphase I, which overloads one cell with chromosomes and short stocks the other. Likewise, in anaphase II of meiosis II, if a pair of sister chromatids fails to separate and migrates into the same cell, that cell now has too many chromosomes and the other, too few. These scenarios are examples of nondisjunction, which results in the production of gametes with an odd number of chromosomes. Because it often occurs in meiosis, the genetic recombination only affects the X and Y chromosomes, the chromosomes most noted for determining the sex of the offspring, giving rise to the following abnormalities:

  • XXy = the offspring is a male with Kleinfelter's syndrome (also includes XXXy, XXXXy, and XXyy the appearance of a single y chromosome apparently is enough to create a male). Individuals with Kleinfelter's syndrome usually display lanky builds with feminine characteristics such as breast development and poor facial and chest hair growth, and they are mentally retarded and sterile.
  • Xyy = the offspring is a normal male, often called a supermale because the presence of an extra y chromosome may contribute to characteristics of increased height, weight, muscular bulk, and aggressiveness
  • XXX = The offspring are female, called metafemales or superfemales, and appear normal.
  • XO = The offspring are female and have Turner's syndrome (only one chromosome present). Individuals with Turner's syndrome are sterile females that are short in stature, do not sexually mature, and have a thickened web of skin between the shoulders and neck.

Mutations are the primary source of genetic variability because a mutation creates a new gene. Variability is also increased in other ways, such as by the randomness of the union between sperm and egg leading to fertilization.

Mutation

A mutation is a unique type of abnormality and is the greatest source of genetic variability because it creates a change in the nucleotide sequence composing the DNA. Mutations can be either good or bad.

Assuming the daughter cells receive the correct number of chromosomes, problems may arise in the structure of the DNA itself. Mutations involving the rearrangement of the DNA nucleotides is caused in four distinct ways.

A translocation occurs when the DNA double helix is broken and a piece of the chromosome attaches to a neighboring nonhomologous chromosome, making it longer than its homologous chromosome. The donating chromosome is obviously now shorter that its homolog. Refer to the illustration Translocation.

Whenever a segment of a chromosome is broken off and lost, the resulting deletion has serious effects on the transmission of the original genetic material. Refer to the illustration Deletion.

If a deleted segment returns and joins with a homologous chromosome, a duplication of genes has occurred. Refer to the illustration Duplication.

Finally, if a segment breaks loose, reverses, and reattaches in reverse order, an inversion results. Refer to illustration Inversion.

Bionote

A karyotype is a display of an individual's chromosomes that have been stained for easier observation. In humans, the karyotype shows any alteration in the 22 autosomal (genes that code for the body) chromosome pairs or the one pair of sex chromosomes.

As a result of inversions, the genes are still present and the gene number is still the same in translocations and deletions, however, the resulting mutations may create serious problems, especially in the case of deletions because the ?reading? of the genetic code will be altered by an extra omitted gene.

On a smaller scale, a point mutation in a gene is the single exchange of one nucleotide for another. This type of genetic recombination may be too small to affect the overall function of the protein and may not be noticed by the individual, especially if it is an interon which are described in Regulation of Gene Expression in Prokaryotes and Eukaryotes. In other cases, a point mutation may improve the organism by making it more fit, or make it worse by decreasing the fitness thereby lowering its chance of survival.


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