This is a graphic from my book “Smart and Sexy” which details and provides the evidence for X linked inheritance of intelligence. Available free at the link. It is a succinct way of showing how intelligence can be expected to be expressed through 4 generations of a family line. A superhaplotype is explained in the book in more detail, but in short it refers to a series of genes all on the X which have recessive alleles which result in increased intelligence. Being recessive, they can be blocked partially or fully when paired with dominant alleles on an alternate X (which is only possible in women for the X). How X-inactivation relates to this is addressed in the book. In short, it doesn’t cancel this result out for various reasons. I won’t discuss that here, its in the book.
(Repeating the text in the image above, in case its difficult to read.)
Above is a diagram of four generations of inheritance of X chromosomes in which a superhaplotype is formed, passes through several generations, and then is finally destroyed. A letter indicates a gene. Capital letters are dominant alleles with average contributions to intelligence. Lower case letters are recessive intelligence boosting alleles.
(1) In the first generation, a superhaplotype is formed in the female by the random recombination of alleles on the X and is passed onto a son. All of the alleles on this superhaplotype are expressed.
(2) In the second generation, the superhaplotype son exhibits higher than average intelligence because the recessive alleles are always expressed. Since the male does not have a second X chromosome, recombination cannot break apart the superhaplotype and it is passed on to any daughters without alteration. (The small portion of the Y that does recombine is a limited exception to this).
(3) Even though the female in this generation possesses a superhaplotype, she also has a more normal X. This second X reduces the impact of the superhaplotype in boosting intelligence. During the formation of new X chromosomes, recombination with the non-superhaplotype X destroys the superhaplotype.
(4) In this generation, a brother-sister pair are shown instead of a mating pair. These children regress towards the mean population intelligence compared to their grandfather because the superhaplotype was broken in generation (3).
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