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
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
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
better adjusted with superior verbal and higher-order executive
function skills which mediate social interactions."