In regards to the concept of a "gay gene", here's an excerpt from a very interesting discussion of the subject on EvoWiki:
How Bodies, Minds And Behaviour Are Built
If your knowledge of genetics comes from what you've read in the newspaper, heard on the radio or seen on television, you could be forgiven for thinking that genes are little machines which work alone to build a particular characteristic. There is a stream of stories about the discovery of the genes for IQ, wife beating, or love. Geneticists do indeed talk about the gene for this and that, but they use the term in a completely different way to journalists and their readers.
When geneticists talks about a gene for a particular trait, they are using a convenient short hand, most of the time meaning something more complex than just "this gene produces this characteristic". Journalists also use this language as a means of simplifying the science for communicating it, but unfortunately, if you don't know what the shorthand means, you're liable to misinterpret the science it's designed to convey.
The development of big multicellular organisms like humans, with our differentiated organs and tissues and our internally segregated cells, is far more complicated than this one-gene, one-characteristic idea. You could spend months reading about regulatory elements, polygenic traits, the epigenetic model of the genome and the complex interactions between the chemical and cellular environment and genes.
But this is what you need to know. One: genes do not act unilaterally. Any one characteristic will have many different genetic controls, and the sum of the genes involved is much greater and more complex than the individual effects. Two: nature is not opposed to nurture. There is absolutely no reason to think that because a characteristic has genetic influences it can not also have environmental, social and cultural influences. Behaviour and sexual desire is almost always an interaction between the two. A desire for simple explanations has led to a groundless dichotomy between nature and nurture.
Behaviour and mind are produced by the biological organ that is the brain. The brain requires cultural inputs to function as standard, but the brain, like the rest of the body, is built by a set of biological rules dictated by genes. The brain is powerful and malleable, but it is ultimately constrained by the structure laid down by our genes, and culture can only influence our behaviour in ways that the rules of the brain allows, and variation in brain structure may be related to variation in the way we behave and respond to cultural stimuli.
Biological development often requires environmental and social input to determine which path it takes, though each path follows the rules laid down by genetics. A simple example would be the development of the vision processing mechanism of the brain. If the eye is deprived of light early in development the optical nerve will not connect properly with the brain, and the eye will be useless. Normal development of the optical nerve is determined by genes and molecules, but requires the input of light to switch the mechanism on. An analogy would be a computer programme, in which the functions and algorithms take a variable input and calculate an output using a set of fixed rules.Heritability
Heritability is a technical term that describes how much of the variation in a phenotype (the physical and mental properties of the body) corresponds to variation in the genotype (the genes). Heritability can give us a clue as to whether a characteristic is a product of genetics or not. If a characteristic is shown to be highly heritable, that is, variation in the characteristic corresponds to variation in a particular gene or set of genes, that suggests that the characteristic is, to some extent, controlled by those genes. Heritability of behaviour is studied in twins as identical twins have identical sets of genes, while non-identical twins share around half their genes.
There are many limitations to what heritability can actually tell us about the genes that control development. Heritability only shows us the correlation between variation of genotype and phenotype, not a quantitative measurement, and can tell us nothing when there is no variation in a genotype (which rules out a significant proportion of genes). There are also limitations to what twin studies can tell us about heritability, as there is no guarantee that the increased heritability in a particular characteristic of twins is caused directly by genetics, or by them being treated differently to fraternal twins or having less variation between their environments. These are addressed by studies of biological offspring and foster children, which make the studies more reliable, but not immune to debate.
The evidence from studies shows that homosexuality is to some extent heritable, with 52% of identical twins of homosexuals also being gay, 22% for brothers and 11% in the general population. This contributes to the evidence that genetics is a factor, though the evidence of heritability could neither prove or disprove the hypothesis - after all, identical twin brothers also share more of their environment, including the womb, than non-twin brothers.
. . .Is homosexuality genetic?
Yes. Even without direct evidence of individual genes we could be confident in predicting that, like almost all aspects of behaviour, sexual orientation has multiple biological and cultural influences. As it is, there is growing evidence from multiple areas of research that give us an idea of exactly what the biological influences, both genetic and non-genetic, may be.
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So much of behaviour is an interaction of genetics and culture that the question "is homosexuality genetic" isn't even very interesting. Far more interesting, but possibly impossible to answer, is the question "is homosexuality an adaptation?"Adaptationism
Much of human and animal behaviour has evolved for a purpose. From the genes' point of view, the purpose of life is reproduction, not just the survival of the individual. But individual aspects of behaviour can not just be assumed to have evolved for a purpose, and even the implementation of genetics in the development of a characteristic does not prove that the characteristic is an adaptation. There are numerous complications to be considered when asking if a characteristic is an adaptation. Genes can have multiple effects, depending on the other genes with which they share their body, the environment in which they are expressed and the culture that also affects the characteristic. So a gene may be favoured by evolution for its effect on one characteristic, despite its non-adaptationary side-effects on other characteristics, or other situations.
I agree; desire rather than behavior really determines sexuality.
I think that the idea one is either straight or gay is probably a false dichotomy. That is, there can be various shadings between the two.