Genes that affect how we look
The Human Genome Project has greatly boosted human genetic research. As well as the full sequence, detailed maps have been a major help - instead of working with three billion bases, most of which are identical in everybody, researchers can focus on a few hundred thousand ‘markers’ at known positions.
These markers have been used by the Wellcome Trust Case Control Consortium to search for the genetic factors associated with a host of common diseases. As a byproduct, these studies also turned up genes linked to weight (FTO) and height (HMGA2). Having two copies of the ‘tall’ HMGA2 variant adds just under 1 cm to someone’s height (a second gene, discovered in 2008, GDF5-UQCC, adds another 0.44 cm). Someone with two copies of the ‘big’ FTO allele is on average 3 kg heavier than a similar person with no copies.
More detailed analysis from the Wellcome Trust Case Control Consortium, and data from other similar studies, have revealed more than 20 genes affecting human height.
Meanwhile, the International HapMap Project is documenting human genetic variation across the globe. This has begun to uncover genes underlying factors that vary with geography.
Hair we go, ear we are
The International HapMap Project discovered a gene (EDAR) associated with hair thickness in Asian populations. The thick-hair variant appeared after ancestral Asian and European populations split and seems to have spread rapidly through Asia.
Human earwax comes in two forms - wet and dry. In 2006, Japanese researchers identified a single base change in the ABCC11 gene which underpinned this difference. Dry earwax is common in East Asians, though in most other groups wet wax predominates. The dry-type version of ABCC11 is found mainly in Chinese and Koreans and in ethnic populations in other countries that originated in northwest Asia.
Skin deep
Another approach is to focus specifically on genes known to be involved in a feature, such as skin pigmentation. A study of 118 such genes found eight associated with differences in skin colour, all linked to the enzymatic pathway of melanin synthesis. Notably, in different populations, different combinations of alleles showed evidence of positive selection - more rapid genetic change than expected by chance.
The genetic evidence suggests that dark skin colour is the most ancient (as expected given the likely origins of humans in Africa) and lighter skin colour has emerged independently in Asian and European populations, as an adaptation to life at higher latitudes - possibly a need to maximise vitamin D synthesis, which requires sunlight.
