Abnormal Phenotypes Due to Autosomal Aneuploidy or Polyploidy

Abstract

Only three autosomal trisomies, those for chromosomes 13, 18, and 21, occur with an appreciable frequency in liveborn infants, and only one, trisomy 21, is compatible with long-term survival in a large proportion of cases. Trisomy 21, trisomy 18, and trisomy 13 produce three different and characteristic syndromes despite some degree of overlap. Traits they share include mental retardation, congenital heart defects, malformations in other organ systems, seizures, and growth retardation. These features are also common with other chromosome imbalances. For example, a ventricular septal defect is found in 47% of infants with trisomy 13, in 34% of 18-trisomics, and in 20% of 21-trisomies (Lewandowski and Yunis, 1977). In general, no single trait is exclusive to a particular chromosome imbalance, but a few tend to be associated with a particular chromosome imbalance, such as the persistence of fetal hemoglobin and abnormal neutrophil projections in trisomy 13. The increasingly routine use of ultrasound sometimes identifies fetal abnormalities at a stage of pregnancy at which a rapid decision is needed if termination of pregnancy is an option. Interphase FISH with DNA probes for chromosomes 13, 18, 21, X, and Y permits rapid diagnosis of aneuploidy of any of these chromosomes, or of polyploidy (Gersen et al., 1995).