The role of chromosome heteromorphism in developmental anomalies

Abstract

Morphological variability of the human chromosomes has been known to occur since the 1960s. These variations are predominantly localized in the heterochromatic regions, especially the short arm and satellite regions of the acrocentric chromosomes and the long arm of the Y chromosome. The advent of new staining techniques has helped in the identification of sites of morphological variation in human chromosomes. The variable segments correspond to the regions which are genetically inert and late replicating and contain a high quantity of repetitive DNA. These variations can be detected partly by conventional staining but mostly with C- and Q-banding techniques. Chromosome variant patterns seem to be unique for each individual. The term ‘polymorphism’ has frequently been used to describe the variation in human karyotype. Since these variations are not discrete but continuous, the term ‘variant’ has been recommended (Paris Conference, 1971)1. The supplement to the Paris Conference (1975)2 subsequently advocated the term ‘heteromorphism’ to be used in describing situations where deviations from the norm of chromosome morphology are observed. Besides conventional staining, the most frequently encountered heteromorphisms are visualized by CBG (C-bands by barium hydroxide using Giemsa) and QFQ (Q-bands by fluorescence using quinacrine mustard) techniques.