Sergey Gavrilets and William R. Rice have a new population genetic model for homosexuality out. You can read the full paper over at Gavrilets' website, while Ars Technica and Matt both have some nice commentary. I don't have much to add, and generally share Matt's skepticism of the utility of a one locus model, but at least it is somewhere to start. I will reiterate that the "problem" is obligate homosexuality. I don't find facultative homosexuality as surprising or evolutionarily mysterious. Also, I am intrigued by the the older brother effect because of its implications of genomic conflict.
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I wonder if the anti-gay-gene crowd will use this paper as an arguement against the existance of a gay gene.
i.e.,
1. models predict homosexuality should be very common
2. Homosexuality in reality is quite rare.
3. Therfore, there is not likely a strong genetic effect.
Well, I should hope so, considering that the paper discusses compelling evidence to reject the gay gene hypothesis.
Now, the gay genes hypothesis is another story... and even if no particular sets of genes can be held responsible, there's still the matter of inborn biological disposition/orientation to deal with.
"there's still the matter of inborn biological disposition/orientation"
I guess I'm just not a huge fan of this hypothesis. Are there any real studies that estimate the heritability of homosexuality?
twin studies have offered some estimates.
Several of the comments belie a lack of understanding between qualitative and quantitative genetic mechanisms, which Mendel discovered. Most of us were only trained in qualitative genetic mechanism; that is, recessive and dominant genes. Quantitative genetics, which is mostly what this paper is about, are quite different. There is no single gene for being gay, just as there is no single gene for many disorders or diseases that have genetic components, such as cancer, diabetes, heart disease, many mental illnesses and substance abuse. Quantitative genetics fundamentally means, in a simplistic way, that genes can be added, subtracted, multiplied and divided. It is is an elegant way of conserving genetic diversity for selective advantage without risk of dangerous mutations. Skin color, by the way, is a classic way of illustrating a quantitative mechanism, as there are many shades of skin color--not just "black" or "white." The later would be true if there were just qualitative genes for skin color. What this paper does very nicely is give the maths for testing some of the non-qualitative genetic mechanism that might maintain "gay" genes.
The twin studies (identical versus fraternal) provide a nice illustration of the something genetic going with gay behavior, just as those studies show heritability of many other dsipostional traits such as oppositional defiance or aggression. What is also interesting is to comtemplate what types of environments that might "evoke" or "turn on" such genes. Such environment X polygene (quantitative) intereactions is pretty well understood for such things as antisocial behavior is some briliant studies in the Scandanavian countries with great birth registry records. For example, we know that that a child born of a biological father with a criminal record if placed in an adoptive home with no criminal behaviors will be slightly more likely to have criminal record as an adult versus a baby born of a non-criminal record father placed in to similar non-criminal adoptive families. We also know that if the adoptive children (with a biological history) is placed in an adoptive family with a crminal history, more crime happens by that child, and it also increases the risk of baby with no biological history of crime. Now, place babies (with bio history) in an adoptive family (with criminal history) and in neighborhood (with criminal activity) and the expressed rates of criminal behavior increase dramatically. This is a gene X environment interaction.
I doubt seriously that the expression of "gay" genes depends on lviing in "gay" neighborhood or with adoptive "gay" parents. There is no more expressed gay behavior in children adopted by gay people than straight people. The environmental factors must be something of evolutionary significance (wherein the behavior is adaptive), if there is a gene x environment interaction.
Yeah, I decided to answer my own quesiton about twin studies- more or less, there is some evidence for the a genetic component to homosexuality- I really think that this is an area of human biological evidence that needs more atention.
An excerpt comtaining the critical data is posted here
Quantitative genetics, which is mostly what this paper is about, are quite different.
this paper is not quantitative genetics, it is population genetics. it uses a single locus model, not a polygenic model which converges upon a normal distribution. i discuss issues re: quantitative genetic models of homosexuality here.
RE: homosexuality (and now for something completely different)
Homosexuality Is Natural Part 2
http://beepbeepitsme.blogspot.com/2007/01/homosexuality-is-natural-part…
Is this fatal to Cochran's gay germ theory?