Sex chromosome abnormalities in children

Abstract

HE FREQUENCY of sex chromosome abnormalities in the general population is difficult to assess. What information is available is based on surveys of the nuclear sex ~ f newborn babies such as those of Moore in 19591 and Maclean et al. 2 in 1964 and on a small number of chromosome studies on selected populations/,4 The surveys in which chromosome studies have been done are undoubtedly more complete since nuclear sexing alone will not detect some mosaic patients, nor those patients, where, despite a chromosome abnormality, the phenotypic sex agrees with the nuclear sex. However, because of the labor involved, the number of individuals who can be screened at present for chromosome abnormalities is small and the samples must inevitably be unrepresentative of the population as a whole because of the segregation of individuals in institutions, in the armed forces and in other ways, and because of the undoubted loss of some individuals with chromosome abnormalities before maturity is reached. At present, therefore, estimates of frequency must be based mainly on the surveys of newborn babies. Using these data the frequency of sex chromosome abnormalities is roughly 1 in every 400 live births. The frequency for autosomal trisomy is also approximately 1 in 400 live births, 5,6 while Court Brown et al. 3 have estimated that one person in every 200 will have a rearrangement of the auto somes. One can say, therefore, that roughly one child in every 100 born alive will have a chromosome abnormality. Not all these children will appear to be abnormal since the effect of these chromosome abnormalities on the physical and mental development of the individual will vary from one type of abnormality to another. The child may have obvious malformations at birth, but in other instances the deleterious effects of the genetic abnormality may not become apparent until some time afterwards and in some cases not until after puberty. A few chromosome abnormalities appear to have no adverse effect on the individual carrying the abnormality but they may be associated with either an increased frequency of spontaneous abortion or with the production of congenitally malformed children. There are also a few types of chromosome abnormality which appear to have little or no effect on the individual or on his or her progeny. Sex chromosome abnormalities c~m fall into all these categories. Turner's syndrome is an obvious example of the situation where congenital malformations are present at birth, while in,the majority of cases of Klinefelter's syndrome the defects are not apparent until after puberty. Some females with an XXX sex chromosome From the Medical Research Council, Clinical Effects of Radiation Research Unit, Western