See on Scoop.it - The future of medicine and health
The biology of sexual orientation has been one of the most vexing — and politically charged — questions in human genetics. For the first time, researchers have found associations between homosexuality and markers attached to DNA that can be influenced by environmental factors.
Twin studies and family trees provide strong evidence that sexual orientation is at least partly genetic. When one identical twin is gay, there is about a 20% chance that the other will be as well1. But because this rate is not 100%, it is thought that environmental factors play a role as well. One of the best characterized is the ‘older brother effect’: the chance of a man being gay increases by 33% for each older brother he has2. The reason is not clear, although one hypothesis holds that the mother’s immune system begins to react against male antigens and alter the fetus’s development.
To search for factors that could mediate a link between environment and genes, geneticist Eric Vilain at the University of California, Los Angeles (UCLA), and his colleagues looked at epigenetic markers — chemical changes to DNA that affect how genes are expressed, but not the information they contain. These ‘epi-marks’ can be inherited, but can also be altered by environmental factors such as smoking, and are not always shared by identical twins
See on nature.com
The ‘older brother effect’: the chance of a man being gay increases by 33% for each older brother he has.
from Stowe Boyd http://stoweboyd.com/post/130810699102
