The declaration from Jason Collins that he is gay also caused some people to note that his identical twin brother is not. That led someone else (link not handy) to review some studies on correlation among identical twins on how many of them had the same sexual orientation (in other words, is "homosexuality" genetic?)
upon review of the evidence beyond this one set of twins, if i recall correctly, it appears that statistical studies of identical and fraternal twins have found that "homosexuality" is not highly correlated with heredity, though the correlation is twice as high for men as it is for women. Some have interpreted this to mean that there might be a "predisposition" based on genetic factors that requires an environmental trigger.