Genetics of aneuploidy in man reconsidered by banding studies

The effects of aneuploidy are only partially known in man, but the information available suggests that they are remarkably similar to those in Drosophila melanogaster. The fractions of the haploid genome that can be tolerated in three doses and in one dose approach 7 and 3%, respectively. Hypoploidy is more deleterious than hyperploidy. The phenotypic effects of aneuploidy result in an 'aneuploid syndrome' of developmental, mental and growth retardation and multiple malformations. Even if upon closer scrutiny a recognizable collection of dysmorphisms can be detected in some chromosomal imbalances, specific phenotypes result only as the consequence of changes in dosage-sensitive loci. The aneuploidy of man was approached by chromosomal band studies. Available data suggest that R bands may be rich in functional DNA. The proportion of R bands involved in viable imbalances was calculated, and it is concluded that nearly all chromosomal arms tolerate trisomies or monosomies of a proportion of their R band content. This evidence and the demonstration of the involvement of proportionally similar quantities of band type in viable aneuploidies contrast with the idea that imbalances with a higher R band proportion have less chances of survival. Thus band patterns seem to be adequate to provide empirical clinical phenotypes but inadequate to reflect the genetic make-up of a given chromosome.