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A Comment on Genetics and Homosexuality

Jim Burroway

December 14th, 2006

Commenter “Lij” corrects me in my rush to make a point:

You say that:

“Twin studies suggest that for some people their sexuality is genetic or congenital. But since there are examples where one identical twin is gay and another is straight, then this can’t be true for all gay twins.”

This is not necessarily true. Gene expression may occur in one twin and not the other.

This is true. It’s easy to make too much of the word “identical” in identical twins and jump to the erroneous conclusion that identical twins are, well, identical. Researchers avoid that assumption by using the more correct term monozygotic twins, referring to the fact that they came from a single egg or zygote that splits into two separate embryos. While they share the same genetic material when they split apart, the twins do not become exact copies of each other (for example, their finger prints are different) because of a process known as epigenetic modification.

Epigenetic modification occurs as individual genes switch on and off at different times, different intensities, with different interaction with other genes or environmental factors (“environmental” can mean hormonal, chemical, climate, emotional – or any number of other external factors), causing a slowly evolving buildup of differences between the two supposedly identical twins over the course of a lifetime.

So, to be absolutely correct, the fact that one identical twin is gay and the other one is straight does not by itself disprove any genetic theories. And on that point, I stand corrected.

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