Genetics: What Are Little Boys & Girls Made Of? (cont.)

That is what is proposed in a provocative study published this week in the eminent British journal Nature (1997;volume 387, page 705). The paper is entitled "Evidence from Turner's syndrome of an imprinted X-linked locus affecting cognitive function."

Boys (XY) always receive their single X chromosome from their mother while girls (XX) receive an X from their mother and an X from their father. As far as X chromosomes go, what separates boys from girls is not only that girls have two X chromosomes but that only girls have an X chromosome from their father (a paternal X).

Turner's syndrome is a disorder of girls. Girls with Turner's syndrome have only one intact X chromosome instead of the two Xs that normal girls have. There is usually no second sex chromosome in Turner girls. The "X-linked locus" mentioned in the report's title refers to a position (the locus) of a gene on the X chromosome.

The Nature study suggests that this area of the X chromosome can be "imprinted" (chemically altered), so that the function of the gene is different depending on whether that X chromosome came from the father or the mother. In turn, this imprintable gene locus may have some influence on "cognitive function." Cognition (from the Latin cognitio meaning "to know") is the operation of the mind by which we know, perceive, and think.

In Turner's syndrome it is sometimes said (for example, by the authors of the Nature study) that intelligence is usually normal. In fact, the average IQ score of patients with Turner's syndrome is around 90, which is clearly below the average IQ of 100 in the general population.

What is perhaps more striking about the ability to think in patients with Turner's syndrome is the specificity of certain neuropsychological defects. Turner's girls tend to have deficits in visual-spatial orientation (so they have trouble driving), deficits in social thought (so they may miss subtle social cues), and deficits in nonverbal problem solving (so they may have problems with mathematical concepts). Moreover, social adjustment problems are quite commonplace in Turner's girls.

The study in Nature exploited the fact that in the majority of girls with Turner's syndrome, their single intact X chromosome comes from their mother while in the remaining cases it comes from the father. The authors compared 55 Turner's girls who had a maternal X with 25 Turner's girls who had a paternal X. They found that the Turner's girls with a paternal X were "significantly better adjusted with superior verbal and higher-order executive function skills which mediate social interactions."