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 occurence 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
sexs. 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 medium-sized, sub-metacentrics
(C-group
chromosomes), either 16 in
females or 15 in
males.