A marker X chromosome.

Descriptive human cytogenetics is entering a new and important phase. The major trisomies and monosomies are well described but generally appear as isolated, unpredictable, and hence unpreventable events, except when a large translocation is known. New syndromes due to smaller cytogenetic changes continue to emerge. In addition, from 3% to 5% of the population had an identifiable chromosomal variant in the Edinburgh survey (Court Brown et al., 1966), and the number and percentages of variants can be expected to increase as preparative techniques continue to improve, quantitative karyotyping becomes routine, and large numbers of persons are studied. The process of determining the clinical significance of these variants is complicated and variable. The finding of the same cytogenetic variant in both normal and abnormal phenotypes does not establish normality, since neither duplication-deficiency nor small translocations leading to an increased frequency of aneuploidy in the next generation can be ruled out without enough clinical data to treat statistically. The markerX chromosome described in this paper offers an additional mechanism by which the expression of the effects of a chromosomal variant may vary from generation to generation or individual to individual and permits several basic observations about the behavior and identification of the human X chromosome. Such heritable variants of the human karyotype may prove to be the most important group of cytogenetic abnormalities both because they are common and because they may permit prevention of clinical disease by identifying high-risk marriages and allowing subsequent amniocentesis and abortion of abnormal fetuses if requested by the family.