Genetics of Homosexuality: Twin Studies, Part III

By Mister Curie

Chris commented on my previous post about twin studies and the genetics of homosexuality that he would like to see a comparison to "other studies of traits with unclear origins, such as handedness."  So, Chris, this post's for you.

Neuropsychologia, Volume 47, January 2009, Pages 330-337

Twins from the Australian Twin Registry and the Netherland's twins registries were asked about what hand they used to write with.  54,270 individuals from 25,732 families participated in the study.  Identical twins both wrote with the same hand about 24% of the time, while fraternal twins wrote with the same hand about 14% of the time.  The study estimated a 23.6% genetic influence on handedness with 0% shared environmental influence, and 76.4% unique environmental influence on handedness.

And apparently handedness is first demonstrated around 9-10 weeks gestation, as embryos begin to have single arm movements.  Interesting . . .

As for a trait I'm a little more familiar with research on, cholesterol levels:

New England Journal of Medicine, 1993, Volume 328, pages 1150-1156

Twins from the Swedish Twin Registry were approached to have a physical exam where cholesterol levels were measured.  302 pairs of twins participated in the study and included sets of twins that grew up together and sets of twins that were separated at birth.  Identical twins that grew up together had similar total cholesterol levels 66% of the time, while fraternal twins that grew up together had similar total cholesterol levels 41% of the time.  When the twins were separated at birth, the identical twins had similar total cholesterol levels 37% of the time and fraternal twins had similar total cholesterol levels 14% of the time.  This study estimated that cholesterol levels are influenced genetically 63%, influenced by a shared environment 18%, and by non shared environment 19%.

Handedness is interesting because it's estimated genetic influence is quite a bit less than the estimates for the genetic influence on homosexuality, like homosexuality does not appear to have a shared environmental influence (meaning the rest of the influence are unique environmental factors), and yet I don't know of anyone who believes you choose to be right or left handed.

Cholesterol levels appear to have a somewhat higher genetic influence than homosexuality.  I am familiar with the genetic research on cholesterol levels and many genes are now being identified with influencing cholesterol levels.  There are also several severe genetic defects that are known to cause huge changes in cholesterol levels, many of these are familial diseases that often result in heart disease.