Genetic variation and its maintenance: Chromosome polymorphism in humans

INTRODUCTION Human chromosomes exhibit structural variants which occur at considerable frequencies. Although at the Paris Conference on Standardization in Human Cytogenetics (1975) the term “chromosomal heteromorphism” was used for the description of these variants, the term “chromosomal polymorphism” is here used, “chromosomal” to indicate that the variant is at the level of microscopically detectable chromosome structures, “polymorphism” to indicate that it complies with the conventional definition of a genetic polymorphism. Chromosomal polymorphic variants are constant within a given individual. At least, no consistent differences between the different cell types of one individual are known (though there are special problems with the nucleolar organizer regions, see below). Chromosomal polymorphisms are inherited in a simple Mendelian mode. They can be demonstrated by several techniques, but not all of them by any one technique. They are usually not present in an “all or none” fashion but show different grades, and this makes their evaluation rather difficult in some cases. The chromosomal polymorphisms were reviewed a few years ago by Verma and Dosik (1980). They are of course also relevant clinically, e.g. as a contributing cause of chromosome abnormalities, as possibly associated with certain diseases, and with tumorgenesis (see, e.g., Atkin, 1977; Kivi & Mikelsaar, 1980; Sutherland, 1983; Glover et al, 1984). In this paper some considerations and new findings on the nature of the chromosomal polymorphisms are presented as well as a brief review of the more recent results of population studies. TYPES OF POLYMORPHIC REGIONS OF CHROMOSOMES Table 1 lists different types of chromosomal polymorphisms. The different chromosome bands are placed in one group because of their cytological and molecular similarities.