Twins and sexual oriantation

Sarah Nunn and Rosie Ablewhite are two, normal 29 year-old women, but they also happen to now be a researcher's dream. They are identical twins, meaning their genes are, well, identical, and they were brought up together. This means that the circumstance for both their 'nature' the nature of their genetic make-up and their 'nurture' the environment in which they were brought up, including any extraneous factors that don't relate to their genes are pretty much exactly the same. However, Sarah is straight and Rosie is gay. Whilst thankfully, it's people this isn't a big deal for most of us, but for sexual researchers it is, since Rosie and Sarah could hold the key as to where sexual orientation derives from.

Twin studies are a classic tool for examining the role of genes. Twins brought up together share a similar environment. Monozygotic twins share all their genes, while dizygotic twins share only half their genes. Early twin studies by Franz Kallmann in and Leonard Heston in reported that if one monozygotic twin was homosexual, there was a greater chance the other twin would be homosexual. The likelihood of this was greater than for dizygotic twins.

No individual gene alone makes a person gay, lesbian or bisexual ; instead, thousands of genes likely influence sexual orientation, a massive new study of the genomes of nearly half a million people suggests. The biological factors that contribute to sexual orientation are largely unknown, but many scientists suspect that genetics plays a role, given that same-sex sexual behavior appears to run in families and is seen more often in identical twins than in fraternal twins. But a precise genetic basis for sexual orientation has been elusive, largely because scientists previously had relatively small groups of volunteers to investigate.

Dear Dr. Roach: I read your recent column regarding identical twins where one of the two was balding while his twin was not. Within my extended family, there are identical twin brothers, who also were almost impossible for family members to differentiate.