What ARE Little Boys & Girls Made Of?


JACKSONVILLE, FLORIDA-"What are little boys made of? Snips and snails, and puppy dogs' tails; That's what little boys are made of." according to the old nursery rhyme. The next verse, of course, addresses the parallel question: "What are little girls made of? Sugar and spice, and everything nice; That's what little girls are made of."

The behavioral differences between boys and girls continue to be a matter of common wisdom. For example, there was a front-page article in The New York Times on June 15 about the children of Robert F. ("Bobby") Kennedy. The reporter Deborah Sontag noted that, among Bobby's 11 children: "It was accepted that the boys had more problems than the girls, because as Mrs. Kennedy Townsend (the eldest of Bobby's offspring) said, 'boys in general get in trouble more.'"

Do boys "get in trouble more?" Even more basically, do boys and girls engage in different behaviors? If so, why? Is it all learned through our experiences (environmental)? Or, do our genes play a role, perhaps in pre-programming our behavior?

What may be truly different between boys' and girls' behaviors may not have to with boys' snips and snails and puppy dogs' tails or with girls' sugar and spice levels. Instead, it may have to do with differences between their X chromosomes.

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."

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.

One can speculate as to the evolutionary basis for this disparity. Even without active genes for social skills, would males in a hunter-gatherer culture have been at a disadvantage? Did a man need social skills to chase down and kill a wild animal? On the other hand, genes determining social skills might be useful to women working together around the campsite in a cooperative fashion, performing tasks such as cooking, making clothes, and raising children.

The report in Nature is first-authored by Dr. David H. Skuse from the Institute of Child Health in London. Dr. Skuse is one of ten authors of this study. The last -listed author is Dr. Patricia A. Jacobs. (Together with the first author, the last author is traditionally considered most important to the research). Dr. Jacobs is a senior chromosome scientist of considerable renown.


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The Pulitzer Prize winning reporter Natalie Angier noted in The New York Times on June 12 that not all researchers are buying into this association between a sex chromosome and behavior. For instance, Dr. Evan S. Balaban of the Neurosciences Institute in San Diego pointed out to Ms. Angier that, "one of the scientists on the current report had been an author on a study in 1965" which associated "violent criminal behavior" with XYY, an extra Y sex chromosome in males, an association that "proved to be statistically spurious." The scientist to whom Dr. Balaban alluded is clearly Dr. Jacobs. However, because a 1965 study failed to hold up to follow-up research is no reason at all why a 1997 study might not be right on the mark.

Time will tell whether modern genetics has found the basis for an old nursery rhyme.

For more information, please visit the TURNER'S SYNDROME site of MedicineNet.

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