K. Sankaranarayanan (1979) The role of non-disjunction in aneuploidy in man: an overview
Non-disjunction plays a major role in generating aneuploidy in man. About 50% of spontaneous abortions are chromosomally abnormal and among these, trisomies constitute the major group (~50%), followed by monosomy X (18%), triploidy (17%), tetraploidy (6%) and others. Over 30% of all trisomies are due to trisomy for chromosome 16. There seems to be an association between maternal radiation history and spontaneous abortions in the sense that the work reported, mothers of chromosomally abnormal fetuses have received higher mean gonadal doses relative to comparable controls.
About 6% of babies who die perinatally are chromosomally abnormal and the majority are trisomics especially for group E chromosomes. The results of six major surveys of consecutive newborns show that 6 out of 1000 (0.6%) carry one or another kind of chromosome anomaly. The breakdown is as follows: 0.22%, sex-chromosomal abnormalities; 0.14%, autosomal trisomies (+D, +E and +G); 0.19%, balanced structural rearrangements and 0.06%, unbalanced structural rearrangements and others. The frequency of “clinically significant” anomalies has been estimated to be about one-half of the total of all chromosomal abnormalities detected in newborns. The spontaneous “mutation rate” for numerical anomalies of chromosomes which result in liveborn children is about 15 × 10−4 per gamete per generation (9.3 × 10−4 for sex-chromosomal aneuploidies and 5.7 × 10−4 for autosomal trisomies). Autosomal aneuploidies are associated with more severe phenotypic effects than sex-chromosomal aneuploidies.
The majority of the sex-chromosomal aneuploidies are constituted by XXY, XYY, XXX and XO genotypes (and mosaics). Of these, while the XYY genotype can arise only through second division paternal non-disjunction, the others can arise as a result of either first or second division non-disjunction in either the father or the mother. There is evidence showing that in about 80% of XO (Turner's syndrome) individuals, the single remaining X is of maternal origin suggesting loss of the paternal-X. The frequency of sex-chromosomal aneuploidies in children born to survivors of the Hiroshima and Nagasaki bombings is higher than in controls, but the difference is not significant.
Down's syndrome, which is one of the relatively better known autosomal aneuploidies in man, is due to trisomy for chromosome 21, translocation of chromosome 21 with another autosome (usually chromosome 14) or mosaicism for an extra chromosome 21. Trisomy 21 which accounts for 95% of the cases, is due to non-disjunction during gametogenesis in one of the parents, more often in the mother. With quinacrine fluorescent techniques, evidence has been obtained which shows that non-disjunction can occur either in the father or in the mother at first or second meiotic division.
The incidence of primary trisomies for chromosome 21 shows a strong correlation with increasing maternal age and this is very extensively documented (gradual increase in risk from maternal age about 20 to 30–31 and a steeper increase thereafter). The data on the association between an increase in paternal age and Down's syndrome are conflicting although some data suggest that the risk may be high for fathers above 55 years of age.
Satellite associations between acrocentric chromosomes have been observed in metaphase preparations of chromosomes of lymphocytes, but the questions of whether there are preferential associations between certain chromosomes, and if so, what their relevance is for trisomy, are not yet settled. Consequently it may be premature to extrapolate the findings in somatic cells to germ cells and to the possible origin of trisomies, including that of trisomy 21.
The results of 9 retrospective and 3 prospective studies designed to examine whether parental irradiation may increase the risk of producing Down's syndrome have been published. The available evidence does suggest that there is no correlation between paternal radiation and Down's syndrome in the progeny, but is conflicting on the question of correlation between maternal irradiation and Down's syndrome. Thus, after nearly two decades of work on this aspect, no unequivocal answers are available.
Both trisomy 18 (Edward's syndrome) and trisomy 13 (Patau's syndrome) are associated with severe phenotypic effects; the incidence rates are about one on 10 000 and one in 15 000 births, respectively. These trisomies are not compatible with survival to adulthood. Other, relatively rare trisomies include those for chromosomes 8 and 22.
[A] broad summary of the frequencies of different chromosomal anomalies and their effects, relating these to one million conceptions (under the assumption that 15% of all conceptions are spontaneously aborted and 2% of the children die perinatally and making use of the different frequencies cited in the text with respect to the kinds of anomalies).