London, Nov 21 (IANS): In the largest study of its kind, genetic analysis of 409 pairs of gay brothers, including sets of twins, has linked sexual orientation in men with two regions of the human genome.

Two genes, one on the X chromosome identified in 1993 by Dean Hamer from the US National Institutes of Health and another gene on chromosome 8, were responsible for sexual orientation in men, the findings showed.

"It erodes the notion that sexual orientation is a choice," lead researcher Alan Sanders from the NorthShore Research Institute, Illinois in the US was quoted as saying in a New Scientist report.

This could also refute the belief that gay people can be "treated" to make them straight.

Over five years, Sanders collected blood and saliva samples from 409 pairs of gay brothers, including non-identical twins, from 384 families.

The only trait that all 818 men shared was being gay.

All other traits, such as hair colour, height and intelligence varied by different degrees between all the subjects.

The team looked at the locations of genetic markers called single nucleotide polymorphisms (SNPs) - differences of a single letter in the genetic code - and found that only five SNPs stood out among all the samples.

They found that the ones most commonly shared among all 818 men were from two regions on the X chromosome and chromosome 8, respectively.

Sanders stresses that sexual orientation depends on multiple factors, both environmental and genetic and even individual genes are to have at most a small effect on their own.

The study appeared in the journal Psychological Medicine.