chromosomal nomenclature, a standard nomenclature system for identifying chromosomes in an individual as well as any deletions or additions of specific chromosomes or parts of chromosomes. The full complement of human chromosomes is represented as 46, XX for a normal female and 46, XY for a normal male. The pairs of autosomes are numbered from 1 to 22 according to decreasing length and are divided into seven groups: 1 through 3, group A; 4 and 5, group B; 6 through 12, group C; 13 through 15, group D; 16 through 18, group E; 19 and 20, group F; and 21 and 22, group G. Chromosomal aberrations are designated by indicating the total chromosomal number, sex complement, and group or specific chromosome in which the addition or deletion occurs. For example, 47, XY, G + indicates a male with an extra chromosome in the G group; 47, XX, 21 + indicates a female with an extra chromosome 21, a chromosomal aberration that results in Down syndrome. The short arm of a chromosome is designated by p, the long arm by q, and a translocation by t. See also Denver classification.
