Heredity: Research Makes It Official: Women Are Genetic Mosaics

The cocktail-party bore who laces his chatter with the tiresome cliche about "crazy, mixed-up women" has more medical science on his side than he knows—and more than medical scientists themselves have recognized until recently. Even normal women, it appears, are mixtures of two different types of cells, or what the researchers call "genetic mosaics." If both cell types are normal, so is the woman. But if one is defective, though a woman may seem to enjoy good health herself, she may pass on hereditary disorders to her children. And oddly, the victims will nearly always be her sons, not her daughters.

From what they have slowly and painstakingly pieced together about mosaicism in female cells, medical geneticists are getting a better understanding of several inherited disorders including some forms of anemia and hemophilia, and color blindness. The research has a direct bearing on cases of genetically confused sex, in which both males and females may have an extra female sex chromosome. And this in turn may have surprising importance in mental deficiency, in which sex-chromosome abnormalities are now being detected more often. The cell studies are also tied in with some forms of cancer, especially a form of chronic leukemia.

Exquisite Chemistry. Geneticists who looked through microscopes at chromosomes taken from cell nuclei had noticed long ago that in some of their slides there was an unusually dark spot. Not until 1949 did Canada's Dr. Murray Llewellyn Barr realize that the spot appeared only in female cells. This discovery alone was invaluable for determining the true or nuclear sex in many cases involving various degrees of hermaphroditism.* But what was the spot?

In a cancer-research project at the City of Hope Medical Center in Duarte, Calif., a Japanese-born geneticist nailed the answer down. Dr. Susumu Ohno showed that the spot was an X (female) chromosome. But he also determined that it was only one of the X chromosomes.

The normal human's 46 chromosomes are classified as 22 pairs of autosomes (nonsex chromosomes), one of each pair being derived from the father and one from the mother, and two gonosomes (sex chromosomes). In a woman, the gonosomes are both Xs, one each from father and mother; in a man, they are an X from mother and a Y from father. Among the autosomes, the two members of each pair behave alike. But the Ohno team showed that a woman's two Xs behave quite differently (except in her egg cells). One acts like an autosome, and so does a man's single X. But a woman's second X curls up on itself and becomes condensed, so that it appears as a heavy, dark spot under the microscope.