A genetic analysis of 409 pairs of gay brothers, including sets of twins, has provided the strongest evidence yet that gay people are born gay. The study clearly links sexual orientation in men with two regions of the human genome that have been implicated before, one on the X chromosome and one on chromosome 8.So this is the strongest evidence? It is extremely weak, as there were no straights in the study and the genes had only a very tiny effect.
The finding is an important contribution to mounting evidence that being gay is biologically determined rather than a lifestyle choice. ...
Whatever the results, Sanders stresses that complex traits such as sexual orientation depend on multiple factors, both environmental and genetic. Even if he has hit on individual genes, they will likely only have at most a small effect on their own
Most human behaviors are attributed to some complicated combination of nature and nurture. There is an extreme view that claims that it is 100% genetic, so far they have only shown it is about 0.01% genetic.