On the X Chromosome of Man

The history of our knowledge of the X chromosome of man is reviewed with appropriate reference to the contributions made through study of other forms. It is noteworthy that sex-linkage was first observed in man. X-linkage has been established for about 60 traits in man. Unlike a majority of autosomal traits in man, most of the observed X-linked traits are recessives (or incomplete recessives). The genetic map of the X chromosome is known only in the most sketchy manner but is likely to be filled in rapidly in the next few years through the use of newly discovered markers, such as the Xga blood group, and cytogenetic investigations of cases of X chromosome anomalies. It appears that the color-vision locus (or loci) is on the short arm and may be rather far from the kinetochore. Based on chiasma counts of autosomes, estimates of the genetic length of the X chromosomes are of the order of 100 map units (1 morgan). Independent segregation of color blindness and night blindness, of color blindness and hemophilia B, and of Xga and hemophilia A and B is consistent with this estimate. Cytologic observations of the male meiotic chromosomes make it unlikely that any significant portions of the X and Y chromosomes are homologous. Furthermore, the pedigree data on all traits for which partial X linkage has been suggested are at the best inconclusive. The Barr body is formed from one X chromosome (or part thereof) which retains heterochromatic properties during interphase. The Lyon hypothesis suggests that on a random basis one X chromosome in each cell of the female becomes the Barr body and genetically is relatively inactive; that the decision as to which X chromosome will behave in this manner is made early in development and once made is fixed in all descendants of a given cell. The cytologic basis of the hypothesis is presented. The genetic consequences which the hypothesis would predict have been outlined and the evidence available to date reviewed. Mutation, the sex ratio, selection, consanguinity, evolution, and genetic counselling are discussed as they bear on the X chromosomes and vice versa. The limited information about the Y chromosome is reviewed briefly.