Friday, December 23, 2005

The Book of Whitey

The Book of Whitey could also be called, How the Zebra(fish) Lost its Stripes. Researchers at Penn State have found a mutation in a gene called slc24a5 in zebrafish that results in the loss of its characteristic stripes. they also found the gene has counterparts in many other species, including chickens, dogs, cows and most notably, humans. A protein produced by the gene is responsible for accumulation of melanin pigment in skin cells. When a single letter in the gene's genetic code is changed, the protein cannot perform its ion exchange function, leading to a loss in melanin accumulation. The zebrafish's black stripes disappear. In humans, dark skin turns lighter.
Scientists said yesterday that they have discovered a tiny genetic mutation that largely explains the first appearance of white skin in humans tens of thousands of years ago, a finding that helps solve one of biology's most enduring mysteries and illuminates one of humanity's greatest sources of strife.

The work suggests that the skin-whitening mutation occurred by chance in a single individual after the first human exodus from Africa, when all people were brown-skinned. That person's offspring apparently thrived as humans moved northward into what is now Europe, helping to give rise to the lightest of the world's races. (WaPo)
Human slc24a5 has two principal alleles (different genetic forms) that differ by a single amino acid at position 111 of slc24a5's third exon. The dark-skinned allele has alanine and the lighter-skinned one has threonine (paper in Science (pp 1782, - 1789, v. 310 [16 December 2005]). This single difference accounts for about a third of the skin color variation between Europeans and Africans. Other genes are likely also involved, leading to the many shades in skin color.

What's the take home lesson here? I suppose one way to put it would be to say whites are genetic freaks that had the luck to settle in a geography highly favorable for economic development (the thesis of Jared Diamond's stimulating book, Guns, Germs and Steel). But another would be that a genetic difference associated with a major racial feature, skin color, is so small as to be trivial (in this case a single amino acid change).

Take that as both a scientific and a political statement.