How significant is the genetic component of homosexual behaviour?

From some basic googling, I found that nobody has ever proven that people are born gay and that environment plays a great part in homosexuality.

I wish to know if there is a genetic component to homosexual behaviour. If so, how significant is that component (in some measure such as percentage contribution to the behaviour). What are other factors that play a role in homosexuality i.e. what constitutes environmental factors?

This is not an area I know well, but I'm familiar with a couple of studies that have tried to estimate the heritable (genetic) component of homosexuality in humans. A review paper by Rice et al (2012) points out that:

Pedigree and twin studies indicate that homosexuality has substantial heritability in both sexes, yet concordance between identical twins is low and molecular studies have failed to find associated DNA makers.

Here is a selection of papers that should be of interest to you:

  • Pillard & Bailey. 1998. Human sexual orientation has a heritable component. Hum. Biol.
  • Bailey et al. 2000. Genetic and environmental influences on sexual orientation and its correlates in an Australian twin sample. J Pers Soc Psychol.
  • Kirk et al. 2000. Measurement models for sexual orientation in a community twin sample. Behav Genet
  • Rice et al. 2012. Homosexuality as a consequence of epigenetically canalized sexual development. Q Rev Biol.

Kirk et al (2000) find a rather strong heritable component (30-60%), which also differs between males and females (stronger in females).

Multivariate structural equation modeling techniques have been applied to examine the causes of individual differences in responses to several items concerning sexual orientation. To minimize potential ascertainment and response biases, the study sample involved a large (N = 4901) community-based cohort of Australian twins aged 18-52 who answered an anonymous questionnaire on sexual behavior and attitudes. The statistical power of the analysis was increased by the availability of multiple measures of sexual orientation (behaviors, attitudes and feelings), providing stronger evidence for the existence of additive genetic influences on this phenotype than in a previous analysis (Bailey et al., 2000). Estimates of the heritability of homosexuality in this sample ranged between 50 and 60% in females but were significantly lower (heritability of approximately 30%) in males.

Note however that is it often difficult to separate genetic and environmental components in these types of epidemiological data, especially when twins have grown up in the same environment. I'm also not in a position to judge or comment on the quality or methods of these studies.

I doubt that there is a gene or group thereof directly responsible for homosexuality.

Rather, if there is genetic correlation, it would be something like, a gene which makes it more likely for the mother to not produce enough sex hormones during pregnancy, or for the fetus's brain to react differently or not at all to them. Other causes of an unusual chemical composition of the womb are malnutrition in the mother or environmental causes.

And thus the tendency of the brain of a male to be aroused by female forms, or vice versa, is limited, and this makes it more likely, but not pre-determined, for homosexuality to develop.

(Educated guess)