HETEROCHROMOSOMES

HETEROCHROMOSOMES (from Greek heteros- different), a term that originally designated chromosomes differing in certain characteristics during cell division (synonym: allosomes); in recent times it is often used in a narrower sense: chromosomes present in somatic cells of individuals of one sex (in most cases male) in a single number (synonym: idiochromosomes, sex chromosomes), in contrast to autosomes, of which there are always pairs of each type. Thus, the original concept includes heterochromosomes in the sense of modern authors, as well as some other types of chromosomes, for example, small chromosomes in certain Rhynchota (bugs). In some cases, male cells have one unpaired chromosome and correspondingly two chromosomes of the same type in female cells. Such chromosomes are called X-chromosomes. In addition to the X-chromosome, males often have another unpaired chromosome—the Y-chromosome. More rarely (butterflies, birds)—females have one X-chromosome or X- and Y-chromosomes, while males have two X-chromosomes. Some authors give special names for this case: instead of X-chromosome—Z-chromosome, instead of Y-chromosome—W-chromosome. Sometimes there is not one X-chromosome but several (2, 5, 6 or more); in such cases the difference between the sexes in the number of chromosomes greatly increases. The sex having one X-chromosome (or one X-chromosome of each type) is called heterozygous or heterogametic, with a double number of X-chromosomes—homozygous or homogametic. The sex cells of the heterozygous sex after reduction division will be of two types: with an X-chromosome and without it (or with a Y-chromosome, if it is present); in the homozygous sex all mature sex cells are identical, with an X-chromosome. The discovery of heterochromosomes was of decisive importance in clarifying questions of sex determination (see) and sex-linked inheritance. In view of the relationship of heterochromosomes to sex determination, they are usually called sex chromosomes. Another synonym—idiochromosomes—was initially applied only to the XY type; some authors still adhere to this designation, calling heterochromosomes only X-chromosomes in the absence of Y-chromosomes.