Genetics: What Are Little Boys & Girls Made Of? (2024)

DOCTOR'S VIEW ARCHIVE

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

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

Do boys "get in trouble more?" Even morebasically, do boys and girls engage in different behaviors? Ifso, 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 puppydogs' tails or with girls' sugar and spice levels. Instead, itmay have to do with differences between their X chromosomes.

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

Boys (XY) always receive their single X chromosomefrom their mother while girls (XX) receive an X from their motherand an X from their father. As far as X chromosomes go, what separatesboys from girls is not only that girls have two X chromosomesbut that only girls have an X chromosome from their father (apaternal X).

Turner's syndrome is a disorder of girls. Girls withTurner's syndrome have only one intact X chromosome instead ofthe two Xs that normal girls have. There is usually no secondsex 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 thisarea of the X chromosome can be "imprinted" (chemicallyaltered), so that the function of the gene is different dependingon 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 cognitiomeaning "to know") is the operation of the mind by whichwe know, perceive, and think.

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

What is perhaps more striking about the ability tothink in patients with Turner's syndrome is the specificity ofcertain neuropsychological defects. Turner's girls tend to havedeficits 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 problemswith mathematical concepts). Moreover, social adjustment problemsare quite commonplace in Turner's girls.

The study in Nature exploited the factthat in the majority of girls with Turner's syndrome, their singleintact X chromosome comes from their mother while in the remainingcases it comes from the father. The authors compared 55 Turner'sgirls who had a maternal X with 25 Turner's girls who had a paternalX. They found that the Turner's girls with a paternal X were "significantlybetter adjusted with superior verbal and higher-order executivefunction skills which mediate social interactions."

Most intriguing is how the authors of the study interprettheir results. They propose that imprinting of the paternal Xpermits activation and expression of one or more genes involvedin 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 fromtheir mothers, they can only receive a Y chromosome that makesthem male from their fathers. Therefore, it follows that boyswill tend to lack the social savvy of girls.

One can speculate as to the evolutionary basis forthis disparity. Even without active genes for social skills, wouldmales 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 usefulto women working together around the campsite in a cooperativefashion, performing tasks such as cooking, making clothes, andraising children.

The report in Nature is first-authoredby Dr. David H. Skuse from the Institute of Child Health in London.Dr. Skuse is one of ten authors of this study. The last -listedauthor is Dr. Patricia A. Jacobs. (Together with the first author,the last author is traditionally considered most important tothe research). Dr. Jacobs is a senior chromosome scientist ofconsiderable renown.
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The Pulitzer Prize winning reporter Natalie Angiernoted in The New York Times on June 12 that not all researchersare buying into this association between a sex chromosome andbehavior. For instance, Dr. Evan S. Balaban of the NeurosciencesInstitute in San Diego pointed out to Ms. Angier that, "oneof the scientists on the current report had been an author ona study in 1965" which associated "violent criminalbehavior" with XYY, an extra Y sex chromosome in males, anassociation 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 researchis no reason at all why a 1997 study might not be right on themark.

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

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