Genetic Factors in Sexual Orientation

Hereditary traits are those which are passed on through the transfer of genes from one generation to the next. We can test for these traits by examining the DNA to see if the genes that define these traits are present or not. Unlike hereditary genes and the traits they represent, the sets of genes that determine sexual orientation are present on everyone’s X chromosome. One’s sexual orientation then becomes a matter of which set(s) of genes are left on and which are turned off (methylated) as the result of the signaling that occurs based on the presence or absence of the Y chromosome.

Current methaylation studies provide us with only overall estimates, such as the percent of the genome which is mentylated during various early stages of embryonic development. Determining whether or not a specific gene set is methylated or not is well beyond our present capabilities.

The observation that sexual orientation is a nearly random occurrence and that there is almost no correlation between a person’s sexual orientation and that of his or her parents or siblings indicates that the genes that determine orientation are not inherited, but rather are universally present in each person’s genome.

The study of identical twins who share the same genome bears this out, their orientations may be different. Yet we also find that identical twins are significantly more likely to have the same orientation than random chance would allow. Identical twins occur when the zygote that results from a single fertilized egg splits in two and each develops into a separate person. If the zygote splits after the methylation has occurred which determines the orientation, then the twin’s orientation will be the same. If, instead, the zygote splits before the orientation is set, then they are as likely to be different as with any other sibling.

As to your first question, it refers to behaviors which we are all free to choose as we wish. Sexual orientation applies only to the gender of those who set off an sexual arousal response in the limbic system. Like the fight or flight reaction, this results in a faster heart rate, more fixed attention, increased blood flow to the musculature and other effects that result from an increase of norepinephrine in the system. In contrast, though, sexual arousal includes the release of phenylethylamine and oxytocin which creates an overall feeling of wellbeing and a wish to be closer to the source of this stimulation, rather than farther away.

The gender associated with the template in the limbic system which represents the primary and secondary sexual characteristics which trigger this response is determined by which set(s) of genes are left active on the X chromosome, and this response becomes manifest with the increased production of sex hormones after puberty. W (From: Family Scholar's blog Comment #29)