Biology of Homosexuality: Neurodevelopment of Human Sexual Orientation

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-Agnivo Niyogi

Sexual attraction and its inherent molecular mechanism has been a topic of study in humans for long. With technological advances and research in neurobiology, evidences were unearthed supporting the hypothesis that sexual orientation in humans may be laid down in neural circuitry during early foetal development. Behavioural and genetic investigations also lend credibility to the proposal for a heritable component to male and female sexual orientation. Linkage studies suggest of X-linked loci (which I discussed in my earlier articles).

That level of prenatal sex hormones play an important role in the development of sexual orientation related neural substrates has also been shown in research. Significantly, the notion that non heterosexual preferences may be due to neurohormonal perturbations finds no evidential support. All these findings lend credibility to the claim that homosexuality is absolutely “normal” and is ingrained in a person during prenatal neural development.

Before delving into the deeper recesses of the mind, it is opportune to recapitulate our concept of sexual orientation and the societal dichotomy attached to the definition. Sexual orientation essentially refers to the sexual “attraction” of a person for another. In biological terms, sexual orientation comprises of “a target selection, preference mechanism sensitive to gender, motivational approach behaviours towards the target, and internal cognitive processes biased towards the target (such as sexual fantasies)” [quoted from “The neurodevelopment of human sexual orientation” by Qazi Rehman in Neurodevelopment and Behavioural Sciences, 2005]. It however does not include “self conscious labelling or past sexual activities”.

Sexual preference in males is bimodal implying most males are either homosexual or heterosexual and very few showing bisexual trait. In contrast, in females bisexual genital arousing is commonly reported. Bimodal sexual orientation in humans clearly suggests a need for demarcation of sexuality early during neural development.

Genetic studies lend quite a few evidences to aid the physiological study concerning sexuality. Family studies and twin studies have provided proof of existence of a genetic component for male and female sexual orientation. Increased rate of male homosexuality is seen in case of homosexual probands whereas a maternal line of transmission is suggestive of X linkage. In females both sex linked as well as autosomal linkage is observed (Humans have 23 pairs of chromosomes, 22 pairs of autosomes or non sex chromosomes and one pair of sex chromosomes which determine the gender). Twin studies highlight the importance of environmental factors in determination of sexuality.

Family pedigree linkage analysis led to the discovery of marker on the Xq28 chromosomal region (in layman terms this could be the site of genetic determination of sexuality) in males. But genome wide scans later revealed no such markers but instead pointed to several other putative marker sites, thus widening the scope of research.

Most of the talk in this article has been delivered earlier. But I wished to summarise them again here so that the entire thesis of work on neurodevelopment of human sexuality can be complete. In later issues, we shall discuss the models of development of sexual dimorphism and orientation. Until then, enjoy the festive season.

 

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