Once upon a time, I used to get into online arguments about genetics with certain self-proclaimed "race realist" types, according to whom the scientific "reality" of racial differences in intelligence, which was supposedly being denied by "politically correct" types, would soon be established incontrovertibly. It is now coming on 9 years since I explained on here at length why I regarded all such claims as pseudo-scientific rubbish, borne of a simple-minded understanding of how genes work, and disregarding entirely wrinkles such as pleiotropy and epistasis. I was ever so sagely informed at the time by a certain commenter that these phenomena were unimportant, and the day of reckoning was upon hand for I and others of my ilk who were too timid to grasp the harsh realities that only the "hbd" [sic] advocates were bold enough to embrace wie es eigentlich gewesen war, so to speak. How, then, have matters transpired over this near-decade? Have the "hbd realists" been proven right, and the "PC police" been put in their place?
The answer to all of the above - as if there were any reason to doubt it - is "Of course not!" Not only have there been no genes identified which could plausibly be said to underlie "racial" differences in intelligence, the number of plain "IQ" genes reliably* identified so far stands at an outstanding zero. Furthermore, the factors I mentioned which make difficult the identification of genes involved in complex traits are indeed turning out to be very important after all:
Epistasis is the phenomenon whereby one polymorphism's effect on a trait depends on other polymorphisms present in the genome. The extent to which epistasis influences complex traits and contributes to their variation is a fundamental question in evolution and human genetics. Although often demonstrated in artificial gene manipulation studies in model organisms, and some examples have been reported in other species, few examples exist for epistasis among natural polymorphisms in human traits. Its absence from empirical findings may simply be due to low incidence in the genetic control of complex traits, but an alternative view is that it has previously been too technically challenging to detect owing to statistical and computational issues. Here we show, using advanced computation and a gene expression study design, that many instances of epistasis are found between common single nucleotide polymorphisms (SNPs). In a cohort of 846 individuals with 7,339 gene expression levels measured in peripheral blood, we found 501 significant pairwise interactions between common SNPs influencing the expression of 238 genes (P < 2.91 × 10(-16)). Replication of these interactions in two independent data sets showed both concordance of direction of epistatic effects (P = 5.56 × 10(-31)) and enrichment of interaction P values, with 30 being significant at a conservative threshold of P < 9.98 × 10(-5). Forty-four of the genetic interactions are located within 5 megabases of regions of known physical chromosome interactions (P = 1.8 × 10(-10)). Epistatic networks of three SNPs or more influence the expression levels of 129 genes, whereby one cis-acting SNP is modulated by several trans-acting SNPs. For example, MBNL1 is influenced by an additive effect at rs13069559, which itself is masked by trans-SNPs on 14 different chromosomes, with nearly identical genotype-phenotype maps for each cis-trans interaction. This study presents the first evidence, to our knowledge, for many instances of segregating common polymorphisms interacting to influence human traits.
A similar story can be told for the importance of pleiotropy, with estimates being made of between 4 to 8 for the average number of proteins affected by a single gene. If anything, my warnings in 2005 erred on the side of being insufficient rather than exaggerated; for instance, I had no awareness at the time of the significance that epigenetics would come to possess in understanding how traits are passed between parents and offspring.
What conclusions can be drawn from all of the above? That fools rush in where wise men fear to tread; as David Dunning and Justin Kruger established quite a while ago, it takes a certain minimum level of competence to recognize the limits of one's knowledge, and those who push "HBD" claims hardest tend to be precisely the sorts of people who lack the requisite intelligence to understand why their assertions are quite unlikely to be true - a fact rich in irony, considering the nature of the "un-PC" claims they seek to have accepted.
*By which I mean identified in replicated studies - almost all sensational claim can be made in research papers turn out to fail this important test.