Genetics: What Are Little Boys & Girls Made Of? (cont.)
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."
Most intriguing is how the authors of the study interpret
their results. They propose that imprinting of the paternal X
permits activation and expression of one or more genes involved
in social skills. The X chromosome from the dad is more
"socially inclined" than that from mom.
(A genetically sophisticated viewer wrote us commenting that: "Imprinting is a term unlikely to be familiar to the general population....Imprinted genes are not always expressed, imprinting can also repress a gene's expression. In addition, the X chromosome is not imprinted! True, only one X is expressed in a normal female, but this is due to X inactivation and is random. This means that in two cells...one may inactivate the paternal X, the other the maternal X. Imprinting dictates that either the maternal or paternal gene (depending on the gene involved) will ALWAYS be expressed. In this case, the individuals discussed only have one X - so expression is clearly not related to whether the chromosome is imprinted or not.)
Since all boys have an X chromosome that came from
their mothers, they can only receive a Y chromosome that makes
them male from their fathers. Therefore, it follows that boys
will tend to lack the social savvy of girls.