Genomic resource centre

Gender and Genetics

Gender, Sexual Orientation, and Behavioural Genetics

Behavioural genetics research aims to determine the role genes may play in shaping human behaviour. (48) ‘Behavioural phenotypes’ are those patterns or sets of behaviours that are dependent on genotype. (49) Animals, including humans, are born with innate behaviours so that they react instinctively to some environmental stimuli in a way that enhances their prospects for survival. (50) Genes are increasingly being considered as candidates for complex behavioural traits such as sexuality and gender differences. However, as Decamp and Sugarman note, “whereas discrimination is a risk for many genetic studies, stigmatization is especially acute in the behavioural setting”. (51)

Case Example 2: Genetically determined behaviour in fruitflies
The elaborate courtship ritual of fruitflies is a unique experimental system in which to examine the basis of genetically programmed behaviour relating to gender. The sexual behaviours of Drosophila melanogaster are complex, robust, and are displayed by only one sex (males). (52) Male courtship requires products of the fruitless (fru) gene, which is spliced differently in male and female flies. Demir and Dickson have recently imposed female splicing of the gene in male flies, which resulted in a loss of male courtship behaviour and orientation. More dramatically, this single gene in the neurons of fruitflies has been recently demonstrated to control sexual behaviour and orientation so strongly that female flies engineered to manufacture male fruitless protein behave like male flies and engage in elaborate male courtship behaviour toward other females. (53) Normally, female flies do not initiate reproduction.

Behavioural traits, such as aggression or sexuality, are extremely complex and cannot be explained by one gene or genetic component. Cultural norms pertaining to gender roles and sex-related behaviours fluctuate and change with time as well as across cultures. Moreover, the existence of a gene does not guarantee the expression of that gene; indeed, expression is the product of a complex network of other genetic, developmental, biological and environmental factors. It is this complex network of interactions that constantly shape and define behaviours. (54) Any genetic component that is found to be linked to a complex behavioural trait at most indicates a probability of that behaviour manifesting, and not that the behaviour will definitely occur. Behavioural genetics research is still in its infancy, and its capacity to identify any genetic component of behaviours, such as sex -specific conduct, is limited.

Legal Implications of Gender and Behavioural Genetics
Genetics and Sexual Orientation

Legal Implications of Gender and Behavioural Genetics

Approximately 1:1000 males has an extra Y chromosome. Such individuals are characterized by greater height, severe acne, and sometimes skeletal malformations and mental deficiency. It has been contentiously suggested that the presence of an extra Y chromosome in an individual may result in greater aggression and susceptibility to criminal, anti-social behaviour. (55)

In the 1960s, in a series of high profile criminal cases in the United States of America, defence counsel argued unsuccessfully that the defendant was in possession of an additional Y chromosome and was therefore, more prone to violence. (56) The extra Y chromosome defence has since fallen into disrepute, with the New York Court of Appeal in People v. Yukl holding that there was inconclusive evidence of a causal link between the 47XYY chromosome complement and criminality. (57) The defence has fallen into disrepute in large part because the scientific research in support of the relationship between possessing an extra Y chromosome and criminal behaviour has been widely discredited. (58)

Genetics and Sexual Orientation

There has been interest among researchers in determining a biological basis for complex behavioural traits, including sexual orientation. One area of study has been how developmental processes may affect psychosexual orientation in adolescence and adulthood. For example, testosterone exerts an influence on the development of the central nervous system. It is thought that this brain imprinting may in turn affect psychosexual orientation. Another area of study has been the search for a ‘gay gene’. However, studies in this area have been inconclusive and debate continues. (60) The search for a genetic basis for sexual orientation is controversial for many reasons, one being that it is unclear whether a genetic basis will serve to enhance or diminish the stigmatization of homosexuality that occurs in some societies. (61)