Sex differences in the chromosome map.

Gene by gene, biologists are plotting the map of the human chromosome, and the work is progressing rapidly. "We now expect complete maps of all of the human chromosomes in three to five years," says Raymond White of the University of Utah Medical Center in Salt Lake City. The distances on these genetic maps represent the probability that a chromosome will the corresponding pieces of the partner chromosome. Such recombination may occur several times on each chromosome during meiosis, the cell division process that produces egg and sperm. The farther apart two genes are on a chromosome, the more likely it is that recombination will occur between them.

Maternal and paternal chromosomes have different maps, White reports. He says this finding is the most surprising outcome of the mapping work so far. In some pairs of chromosomes, the maternal chromosome appears longer than the paternal one, according to the recombination analysis used for genetic mapping, and in other pairs the paternal chromosomes of each longer than the maternal. Because the chromosomes of each pair (exceptk X and Y) are physically the same length, it must be the recombination rate that differs. White reports, for example, that recombination in the short arm of chromosome 11 is about three times as frequent in males as in females, but recombination in chromosome 13 is more frequent inf emales than in males.

"The biochemical basis of these observations is obscure; the evolutionary basis is also," White says. One possible explanation is that there might be sex differences in the exzymes that carry out recombination such that they act preferentially at different sites on chromosomes. White speculates that some areas of chromosome may have more of the sites recognized by enzymes in the male, and other areas may have more of the sites recognized by enzynes in the female.