GENETICS AND HOMOSEXUALITY Introduction Sexual orientation is one of the most pronounced sex differences mammalian species

GENETICS AND HOMOSEXUALITY

Introduction
Sexual orientation is one of the most pronounced sex differences mammalian species. With few exceptions, the overwhelming majority of people are heterosexual: most males desire females as sexual partners and vice versa. Knowledge about whether someone has a sexual preference for males or females is one of the most reliable behavioral predictors of that individual’s biological sex, perhaps second only to gender identity (the sense of being male or female). Although heterosexuality is the norm, a small but significant proportion of individuals (2–6%) report having more homosexual attractions (Diamond, 1993). The distribution of men and women between the two extremes of sexual orientation (completely heterosexual vs. completely homosexual) shows some interesting differences. Men are bimodally distributed, with most men being mainly attracted to just one sex (Hamer, Hu, Magnuson, Hu, ; Pattatucci, 1993; Vrangalova ; Savin-Williams, 2012). On the contrary, fewer women report that they are exclusively attracted to the same sex, but more of them report attraction to both sexes compared to men (Hu et al., 1995; Vrangalova ; SavinWilliams, 2012). The search for the biological basis of sexual orientation is not a recent interest.
Richard von Krafft-Ebing, a notable Viennese sexologist, was among those who believed that the homosexual behavior was a result of defective development (Krafft-Ebing, 1965). By the late nineteenth and early twentieth centuries, the discourse had changed somewhat. The bodies of homosexuals were still seen as distinct, but they were now characterized as a third sex (Hirschfeld, 1958). In this framework, homosexuals were seen as inverts, that is, homosexual men were thought to have some innately feminine tendencies, while homosexual women were more inclined to express masculine traits. Although homosexuals are no longer considered a distinct sex, the inversion paradigm continues to influence the way research on homosexuality is presented, particularly in terms of neurological correlates (Berglund, Lindstrom, ; Savic, 2006; LeVay, 1991; Rice, Friberg, ; Gavrilets, 2012; Savic, Berglund, ; Lindström, 2005).

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In this essay, we will consider the role of epigenetics in human sexual orientation. First, we will discuss the role of genetics in influencing this trait and review significant findings from 1994 to 2014. Second. We will highlight findings suggesting a link between epigenetics and sexual orientation, with a particular focus on female sexual orientation and prenatal hormone exposure. Thirdly, we will consider data from animal models about potential epigenetic mechanisms that could underlie long-term or organizational effects of prenatal hormones.

The genetics of sexual orientation
Epigenetics and sexual orientation in humans
Effects of hormones on molecular mechanisms
Conclusion