Idiographs of human chromosomes (n = 22 + X/Y)

    The human karyotype comprises 2n = 46 chromosomes. This number was not established until 1952: older textbooks often give it as 2n=48. Chromosomes are classified into seven groups, based on size and relative placement of the centromere (centromeric index). "Small," "medium," and "large" chromosomes are described as metacentric, acrocentric, and/or telocentric, based on occurrence of the centromere near the middle, towards the end, or at the end, respectively. The division into categories are arbitrary: the seven groups are those that originally could not be distinguished from other of similar size and shape. There are 22 pairs of autosomes that are identical in both sexes. The two types of sex chromosomes, X and Y, are assigned to the C and G groups, respectively. Human females are 2N=46, XX, males are 2n=46, XY. The classic cytogenetic test for gender is the number of small G-group chromosomes, either 4 in females or 5 in males.

Figure ©2002 by Griffiths et al.; all text material ©2014 by Steven M. Carr