DRD4 7R A Result Of Introgression?
There is a repeat polymorphism in exon III of the gene at which a variable number of 48-bp motifs can occur. The common and probable ancestral allele has four repeats (4R), whereas the allele associated with novelty seeking and ADHD has seven (7R). Ding et al. (3) show that there are essentially two allele classes at the locus: most variants are easily derived from 4R by simple recombinations or mutations, whereas 7R differs from 4R by a minimum of seven events. This means either that the coalescence of the 4R and 7R allele classes is ancient or that 7R was generated by an extremely improbable mutation and recombination event more recently. At any rate the allelic diversity within the 4R class and the lack of linkage disequilibrium with markers around it show that it has been the common variant in our species for a long time. 7R alleles, with little diversity within the class and high linkage disequilibrium with neighboring markers, must have a recent common ancestor. If the 7R class has been rare for a long time, coalescence within the class could be recent, whereas coalescence between the 7R and 4R classes could be ancient. This curious pattern, an allele that has been in the population for a very long time at a very low frequency, suggests that some kind of balancing selection has been maintaining 7R, but preventing it from becoming common until recently. An alternative is that 7R was incorporated from another hominid species during the expansion of modern humans.
3. Ding Y, Chi H-C, Grady D L, Morishima A, Kidd J, Kidd K, Flodman P, Spence M, Schuck S, Swanson J, Zhang Y-P, Moyzis R (2002) Proc Natl Acad Sci USA 99:309–314, . (First Published December 26, 2001; 10.1073/pnas.012464099).
from "In our genes"
Quoted on Wed Jan 30th, 2013