I'm going to take this opportunity to respond to this article:
Dr. Dobson's Newsletter: June, 2002
Can Homosexuality Be Treated and Prevented?
http://www.family.org/docstudy/newsletters/a0021043.cfm
I mainly want to say that the guy is spouting bad genetics.
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According to a Harris Poll in February 2000, 35 percent of the people polled believed homosexuality was "genetic." 8
There is further convincing evidence that it is not. For example, since identical twins share the same chromosomal pattern, or DNA, the genetic contributions are exactly the same within each of the pairs. Therefore, if one twin is "born" homosexual, then the other should inevitably have that characteristic too. That is not the case. When one twin is homosexual, the probability is only 50 percent that the other will have the same condition. 9 Something else must be operating.
Furthermore, if homosexuality were specifically inherited, it would tend to be eliminated from the human gene pool because those who have it tend not to reproduce. Any characteristic that is not passed along to the next generation eventually dies with the individual who carries it.
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He gives us the a figure: a 50 percent correlation between one twin displaying a characteristic and the 2nd. This is actually strong evidence FOR a trait being connected to genetics, not strong evidence against. (I'm just going to assume for the sake of argument that the figure is accurate. That it was taken from study of a large number of twins with separate upbringings. Otherwise the twins would also likely have very similar environments and any correlation could come from environmental factors.)
Much like breast cancer or any number of other conditions, genetics often simply increases a predisposition, without mandating expression of a trait.
Secondly, even if homosexuality was shown to decrease the number of offspring a subject has, that certainly doesn't mean genes that might lead to a predisposition towards homosexuality will eliminated by natural selection or any other evolutionary mechanism. It just ain't that simple folks. We all loved Mendel, and his peas, and simple traits caused by simple genes, but that just ain't how people are made. We are chock full of traits influenced by piles of genes, and those piles of genes also influence piles of other genes. Genetics may be fun, but it ain't easy :)
The most obvious example of a trait that seems bad, but that won't be weeded out of the gene pool is sickle cell anemia. Being homozygous for the trait is bad, being heterozygous is pretty neutral in most cases, but very good around malaria. Many genes work this way, homozygous bad, not having it bad, heterozygous good.
The sickle cell example shows a trait that is fully expressive, everyone who is homozygous has it, everyone who is heterozygous has malaria resistance. What makes the issue a lot more complicated are the concepts of heritability and penetrance. Basically how likely a genetic trait will be expressed. If twin A having a condition means a 50% chance of twin B having the condition, and we are dealing with twins in separate environments, then the condition has a mid level penetrance. Possessing the gene greatly predisposes one to the condition, but does not insure it.
I don't actually have any idea how strong a role nature, genetics, plays in whether someone will be identified as homosexual, or how strong a role nurture, environment, etc. plays. Apologies for the rant, but I just had to attack that article the moment I read it.
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