Genes that affect how we look
Researchers are beginning to identify the genetic factors responsible for our physical appearance
Most present-day cases of blue eyes may derive from genetic changes in the HERC2 gene that occurred 6,000–10,000 years ago, during the Stone Age. However, it is not that easy to tell a person’s eye colour from their DNA. In 2009 geneticists found they had to look at six different genomic regions and could still only predict with 90 per cent accuracy whether a person had blue or brown eyes.
Studies in mice have identified thousands of short DNA stretches that contain instructions for building a face. Mice with genetic modifications in these regions were born with slight differences in skull or face shape, suggesting that the same genetic regions in humans may play a role in birth defects affecting the face.
People with two inactive alleles of the melanocortin 1 receptor (MC1R) gene have the classic ‘Celtic’ red hair and pale skin appearance. (See our Blue eyes and red hair article for more.)
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.
At least eight genes affect melanin production, and hence skin colour. 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 that 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.
Analysis from the Wellcome Trust Case Control Consortium and data from other studies have found over 20 genes affecting human height. 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).
As with height, there are multiple genes affecting human weight. One in particular, however, is FTO. One person with two copies of the ‘big’ FTO allele is on average 3 kg heavier than a similar person with no copies.
Human earwax comes in two forms – wet and dry. In 2006 Japanese researchers identified a single base change in the ABCC11 gene that underpins 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.Lead image:
All images from Wellcome Images CC BY NC ND
- A global view of the OCA2-HERC2 region and pigmentation (2012)
- Show me your DNA and I’ll tell you your eye color
- BBC: Light shed on how genes shape face
- Understanding the genetics of hair colour
- Signatures of positive selection in genes associated with human skin pigmentation as revealed from analyses of SNPs (2007)
- FTO biology and obesity: why do a billion of us weigh 3 kg more? (2011)
- High and mighty: first common height gene identified
- Common variants in the GDF5-BFZB region are associated with variation in human height (2008)
- BBC: Secretion secrets – things you didn’t know about ear wax
- A SNP in the ABCC11 gene is the determinant of human earwax type (2006)