Study finds epigenetics, not genetics, underlies homosexuality

An earlier study than the previous one.
Of course, more than one mechanism could be at work.

http://www.eurekalert.org/pub_releases/2012-12/nifm-sfe120612.php

Public Release: 11-Dec-2012
Study finds epigenetics, not genetics, underlies homosexuality
National Institute for Mathematical and Biological Synthesis (NIMBioS)

Epigenetics - how gene expression is regulated by temporary switches, called epi-marks - appears to be a critical and overlooked factor contributing to the long-standing puzzle of why homosexuality occurs.

According to the study, published online today in The Quarterly Review of Biology, sex-specific epi-marks, which normally do not pass between generations and are thus "erased," can lead to homosexuality when they escape erasure and are transmitted from father to daughter or mother to son.

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Epi-marks constitute an extra layer of information attached to our genes' backbones that regulates their expression. While genes hold the instructions, epi-marks direct how those instructions are carried out - when, where and how much a gene is expressed during development. Epi-marks are usually produced anew each generation, but recent evidence demonstrates that they sometimes carryover between generations and thus can contribute to similarity among relatives, resembling the effect of shared genes.

Sex-specific epi-marks produced in early fetal development protect each sex from the substantial natural variation in testosterone that occurs during later fetal development. Sex-specific epi-marks stop girl fetuses from being masculinized when they experience atypically high testosterone, and vice versa for boy fetuses. Different epi-marks protect different sex-specific traits from being masculinized or feminized - some affect the genitals, others sexual identity, and yet others affect sexual partner preference. However, when these epi-marks are transmitted across generations from fathers to daughters or mothers to sons, they may cause reversed effects, such as the feminization of some traits in sons, such as sexual preference, and similarly a partial masculinization of daughters.

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