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There is no gay gene because there is no brown eye gene.

Hatuey

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'The gay gene'.

Whenever there is a debate on homosexuality, there always seems to be one person who will claim that since science hasn't found 'the gene' which is responsible for homosexuality then it's obvious that it's not genetic and simply a life style choice. While I have no issue in somebody saying that homosexuality is a lifestyle choice( as all forms of sexuality are displayed as lifestyle choices. You can't chose who you are attracted to but you can chose whether to act on that attraction.) I do take issue with the standard of proof that is demanded. It seems also important to note that science hasn't found a 'straight gene' either yet it does not stop people from making a genetic argument for why homosexuality is unnatural.

As sexuality slowly unravels it has become clear that human sexuality is a pretty complicated subject. Not only this but even the simplest of human genetic studies involve a myriad of genes responsible for a single action. Take for example 'skin color':

Human skin color - Wikipedia, the free encyclopedia

Several genes have been invoked to explain variations of skin tones in humans, including SLC45A2,[7] ASIP, TYR, and OCA2.[8] SLC24A5 has been shown to account for a substantial fraction of the difference in melanin units between Europeans and Africans, variations in human skin tones have been correlated with mutations in another gene; the MC1R gene.[9] Harding found no differences among Africans for the amino acid sequences in their receptor proteins while among certain European non-African individuals there were.

Examination of the variation in MC1R nucleotide sequences for people of different ancestry to determine the most probable progression of the skin tone of human ancestors over the last five million years and comparing the MC1R nucleotide sequences for chimpanzees and humans in various regions of the Earth, Rogers concluded that the common ancestors of all humans had light skin tone under dark hair. By 1.2 million years ago, all people having descendants today had exactly the receptor protein of today's Africans; their skin was dark, and the intense sun killed off the progeny with any lighter skin that resulted from mutational variation in the receptor protein.[10]

However, the progeny of those humans who migrated North away from the intense African sun had another evolutionary constraint: vitamin D availability. Human requirements for vitamin D (cholecalciferol) are in part met through photoconversion of a precursor to vitamin D3. As humans migrated north from the equator, they were exposed to less intense sunlight, in part because of the need for greater use of clothing to protect against the colder climate, under these conditions, evolutionary pressures would tend to select for lighter-skinned humans as there was less photodestruction of folate and a greater need for photogeneration of cholecalciferol.[2] Hence the leading hypothesis for the evolution of human skin color proposes that:-

1. From ~1.2 million years ago for at least ~1.35 million years, the ancestors of all people alive were as dark as today's Africans.
2. The descendants of any prehistoric people who migrated North from the equator mutated to become light over time because the evolutionary constraint keeping Africans' skin dark decreased generally the further North a people migrated.[10][11] This also occurs as a result of selection for light skin due to the need to produce vitamin D by way of the penetration of sunlight into the skin (the exception being if dietary sources of vitamin D are available, as is the case among the Inuit).
3. The genetic mutations leading to light skin, though different among East Asians and Europeans,[12] suggest the two groups experienced a similar selective pressure due to settlement in northern latitudes.[3]

A variation of the vitamin D argument is that humans lived in Europe for several thousand years without their skin lightening and that it only became white after they adopted agriculture.[3][13] It is suggested that in Europe the latitude permitted enough synthesis of vitamin D combined with hunting for health, only when agriculture was adopted was there a need for lighter skin to maximize the synthesis of vitamin D , therefore it is suggested the elimination of game meat, fish, and some plants from the diet resulted in skin turning white several thousand years after modern human settlement in Europe.[14][15]
Now eye color:

Eye color - Wikipedia, the free encyclopedia

Eye color is a polygenic phenotypic character and is determined by the amount and type of pigments in the eye's iris.[1][2] Humans and other animals have many phenotypic variations in eye color, as blue, brown, gray, green and others. These variations constitute phenotypic traits.[3]

The genetics of eye color are complicated, and color is determined by multiple genes. Some of the eye-color genes include EYCL1 (a green/blue eye-color gene located on chromosome 19), EYCL2 (a brown eye-color gene) and EYCL3 (a brown/blue eye-color gene located on chromosome 15). The once-held view that blue eye color is a simple recessive trait has been shown to be wrong. The genetics of eye color are so complex that almost any parent-child combination of eye colors can occur.[4][5]

In human eyes, these variations in color are attributed to varying ratios of eumelanin produced by melanocytes in the iris.[2] The brightly colored eyes of many bird species are largely determined by other pigments, such as pteridines, purines, and carotenoids.[6]
Now, I doubt there are many people who will claim that the variations of human skin color and eye color are far more complex than the genetics of sexuality. So why is it that when an argument on homosexuality appears some person always talks about 'the gay gene'? It shows a lack of understanding of basic genetics. I don't claim to be an expert in them but all the genetic research I've seen on sexuality claims that there are MULTIPLE genes responsible for our sexuality. So please, before you ask for 'the gay gene' think first that there is no 'straight gene' there's not even an 'I like Chinese women gene'. Genes simply do not work the way you think they do. If you would like to study the many factors around sexuality here are a few places to start:

Mayr, E. (1982). The Growth of Biological Thought: Diversity, Evolution, and Inheritance. Cambridge: Harvard University Press. p598.
Zietsch, B., Morley, K., Shekar, S., Verweij, K., Keller, M., Macgregor, S., et al. (November 2008). "Genetic factors predisposing to homosexuality may increase mating success in heterosexuals". Evolution and Human Behavior 29 (6): 424–433.
Martins Y, Preti G, Crabtree CR, Runyan T, Vainius AA, Wysocki CJ (2005). "Preference for human body odors is influenced by gender and sexual orientation". Psychol Sci 16 (9): 694–701.
 
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