Females have two X chromosomes. Studies have shown that one of the X chromosomes typically is deactivated during gestation in individuals with a XY combination. Typically the maternal X is inactivated about half the time while the paternal X is inactivated about half of the time. Sometimes the inactivation of one X chromosome is favored over another. This leads to a woman’s cells having more of one type of X chromosomes. The mothers of gay males are more likely to have a skewed X-activation (so called) as compared with women who had no gay sons. This provides a strong indication that male homosexuality is a partially heritable condition and genes on the X-chromosome are likely to be involved in determining sexual orientation in males. (http://www.glreview.org/article/evolutionary-origins-of-homosexuality/)

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