section_path: Causes/Biology/Genetic factors
section_name: Genetic factors
paragraph_text: Genetic factors
  
  Genes may be related to the development of sexual orientation. A twin study from 2001 appears to exclude genes as a major factor, while a twin study from 2010 found that homosexuality was explained by both genes and environmental factors. However, experimental design of the available twin studies has made their interpretation difficult.
  
  In 2012, a large, comprehensive genome-wide linkage study of male sexual orientation was conducted by several independent groups of researchers. Significant linkage to homosexuality was found with genes on chromosome Xq28 and chromosome 8 in the pericentromeric region. The authors concluded that "our findings, taken in context with previous work, suggest that genetic variation in each of these regions contributes to development of the important psychological trait of male sexual orientation." It was the largest study of the genetic basis of homosexuality to date and was published online in November 2014.
  
  However, in August 2019, a genome-wide association study of 493,001 individuals concluded that hundreds or thousands of genetic variants underlie homosexual behavior in both sexes, with 5 variants in particular being significantly associated. They stated that in contrast to linkage studies that found substantial association of sexual orientation with variants on the X-chromosome, they found no excess of signal (and no individual genome-wide significant variants) on Xq28 or the rest of the X chromosome.