The Future of Human Cytogenetics

Abstract

Continued improvements in cytogenetic technique have consistently revealed chromosome aberrations not detectable by earlier methods. The likeliest repository of additional aberrations in human populations remains the same as before: spontaneous abortions, stillbirths, infants with multiple malformations, mentally retarded individuals, and cancers, examined using still newer methods. For example, mental retardation affects up to 3% of the population. Detectable chromosome imbalance accounts for a large percentage of severe mental retardation (IQ <55) and perhaps 5–10% of milder retardation (IQ 55–70), but the cause is unknown in many cases. (1995) have taken a novel approach, based on evidence that the subtelomeric regions are generally prone to recombination and that methods now exist for detecting unequal recombinational events by probing the DNA of flow-sorted chromosomes with polymorphic markers. They estimate that 6% of unexplained mental retardation may be due to small chromosome changes in the subtelomeric regions alone. Better methods are also needed to identify paracentric insertions, which carry a 15% risk of duplications or deletions arising during meiotic segregation in carriers. Currently, as many as 90% of these insertions may be undetectable by present methods, based on their frequency in the two chromosomal regions where they can be most readily detected.