Sex chromosomes & behavior

XX and XY Chromosomes and Homosexuality

I . INTRODUCTION

Human males and females behave and react to various situations differently , which has been thought to be caused by differences in hormones secreted by the gonads of each gender . In mammals , the male 's testes secrete testosterone from the Leydig cells , which acts on the brain to cause masculine patterns of development and to differentiate the brain from that of the female . The effectiveness of these effects could give the impression that all meaningful sex differences in the brain and in brain diseases

are induced by gonadal secretions (Arnold 2004

At the gross level , the brain is highly dimorphic between the sexes in terms of size and structure . The average male brain weighs about 11 more , as this difference remains significant when controlling for males overall greater physical size . The female brain , however , is simply not a downsized version of the male brain the volumes of specific regions relative to the cerebrum are known to differ between males and females For instance , the volumes of the frontal and medial paralimbic cortices are larger in women the fronto-medial cortex , amygdale and hypothalamus , on the other hand , are larger in men . These differences in sizes can be substantial (Craig et al 2004

Taken altogether , the evidence suggests that men 's and women 's brain development follows significantly different trajectories from very early in life . During development , sex hormone may direct such differentiation , and this is discussed in detail later however genetic , non-hormonal signals may also trigger specific examples of sexual differentiation of the brain . This thesis is supported by two arguments : first , direct genetic control of sexual differentiation in behavior may be as likely to evolve as hormonal control second , neural and non-neural sexual dimorphisms have been reported that are not well explained by classical theories of steroid-dependent sexual differentiation (Arnold 2004

II . BACKGROUND

The sex chromosomes are unusual in that they are differentially represented in the two sexes and , therefore , have been subject to sex-specific evolutionary pressures . Theses pressures favor X and Y genes that are sexually antagonistic in other words , which are more adaptive in one sex than in the other . The implications of this sexual bias in X and Y genes are usually discussed with regard to the primary sex organs : the gonads . The brain is also an important sex organ , as it controls functions and behaviors that are advantageous in one sex , and contains structures that are adaptive mostly in one sex (Arnold 2004

In birds and mammals , the sex chromosomes differ in size and in the type of genes that they contain . The human Y chromosome , for example , is only about one-third of the size of the X chromosome the Y-chromosome genes only encode 27 proteins as compared to 1500 proteins that are encoded by the X-chromosome genes (Arnold 2004

When looking for genes with differential expression between the sexes the obvious starting points are those loci located on the X and Y chromosomes . Apart from its direct role...