A study that was published in 2008 blows away the belief that monozygotic twins, also called identical twins, have the same DNA. Geneticist Carl Bruder of the University of Alabama at Birmingham and his colleagues studied the genomes of 19 pairs of adult identical twins and found sites of genetic divergence in each pair. Such divergences occur when there are a different number of copies of the same gene, a genetic state called “copy number variants.” For example, one twin in Bruder’s study had a genetic marker for leukemia – specific genes on particular chromosomes were missing. While this twin did indeed suffer from leukemia, the other twin did not.
This complicates the numerous twin studies that take place in the scientific world, including ones that explore the root of sexual orientation. While it doesn’t necessarily nullify conclusions reached during said studies, it does contradict the belief that any difference found in identical twins could only be attributed to factors that were epigenetic (having to do with the way genes are expressed during development) or otherwise environmental (the “nuture” factor).
Organizations like NARTH who claim to have science on their side will eagerly point to studies that show up to a 50% twin concordance for homosexuality, claiming that anything less than 100% “proves” that a “gay gene” doesn’t exist. And even Exodus International, a Christian organization that focuses on spiritual healing of homosexuality is an affiliate of NARTH and has this blurb on their site:
Current scientific research simply does not support the “gay gene” theory.
Researchers from all points of view have not found a 100% correlation among identical twin studies in their study groups. If homosexuality is solely a genetically based trait, there should be no variance among identical twins that share the same genetic history.
A search for the term “gay gene” on Exodus’ site yields 20 results, all in articles ranging from outright denial of genetic influence to defensive posturing that genes might determine some aspects of our lives, but not our morality.
The term “gay gene” is of course an archaic and scientifically inaccurate one, not used by anybody trying to propose a serious argument for the biological origin of non-heterosexual orientations. There is no gay gene just as there is no “left-handed gene,” though the latter trait is accepted unquestionably as biologically originated. Additionally, no serious scientific claim has been made that sexual orientation is “solely a genetic trait.” close attention has been paid to factors that influence gene expression, exposure to hormones in the womb, and physiological traits found to be common among those of a particular sexual orientation.
But now thanks to this research, scientists in all fields have a better understanding of why identical twins are so rarely completely the same – and a better understanding of why, despite genetic factors influencing to a point, one twin may be gay while the other is not.