Chapter 12: Patterns of Inheritance

Sex: Everybody’s got one. For mammals, sex is determined by the chromosomal "odd couple," the X and Y chromosomes. New evidence suggests that the set of sex chromosomes you own determines much more than if you can wear a skirt in public.

In sexually reproducing organisms, one member of each pair of chromosomes comes from the mother and one comes from the father. Each human egg cell carries 22 autosomes and one X chromosome. Sperm cells carry 22 autosomes and either an X or a Y chromosome. If the egg fuses with a sperm carrying an X chromosome, the embryo will be female (XX). Sperm carrying Y chromosomes produce male (XY) embryos. Whereas females get an X chromosome from both parents, males can get only an X chromosome from their mothers.

Occasionally, errors happen when chromosomes separate during mitosis or meiosis and an egg or sperm cell ends up without a complete set. If the lost chromosome is the X chromosome, the resultant embryo becomes a girl with the genotype of XO. We can see she is missing a chromosome by examining a karyotype, a picture of the chromosomes. This girl has Turner’s syndrome.

Because the error can happen in either the sperm or the egg cell, this girl’s single X chromosome derives either from the mother (Xm) or the father (Xp). Surprisingly, if the girl has the Xm chromosome, she has considerably more trouble learning behavioral skills than if she had received an Xp chromosome, suggesting that some of the genes involved in learning self-control reside on the X chromosome. Furthermore, these genes must somehow give the message "behave yourself" if they are on the paternal X chromosome and the message "act up" if they are on the maternal one.

Boys, on average, are more likely than girls to have difficulty with language development and reading skills. Significantly more boys than girls suffer from autism, a disorder characterized by extreme, antisocial behavior. Observing typical "male" behavior in Turner’s syndrome girls with Xm chromosomes suggest that chromosomes are "marked" and behave differently depending on whether they are of paternal or maternal origin.

This marking is called genomic imprinting. Whereas both paternal and maternal genes are essential for normal development, they are not equivalent. Science may redefine what it means to be "a chip off the old block" or "daddy’s little girl."

References
Iwasa, Y. 1998. The conflict theory of genomic imprinting: How much can be explained? Curr Top Dev Bio 40: 255–93.

Keverne, E. B., Martel, F. L., and Nevison, C. M. 1996. Primate brain evolution: Genetic and functional considerations. Proc R Soc Lond B 262: 689–96.

Pagel, M. 1999. Mother and father in surprise genetic agreement. Nature 397: 19–20.

Skuse, D. N., James, R. S., Bishop, D. V. M., Coppin, B., Dalton, P., Aamodt-Leeper, G., Bacarese-Hamilton, M., Creswell, C., McGurk, R., and Jacobs, P. A. 1997. Evidence from Turner’s syndrome of an imprinted X-linked locus affecting cognitive function. Nature 387: 705–8.

Thornhill, A. R., and Burgoyne, P. S. 1993. A paternally imprinted X chromosome retards the development of the early mouse embryo. Development 118: 171–4.