A karyotype is the number and appearance of chromosomes in the nucleus of a eukaryotic cell. Karyotyping is a cytological technique which involves staining and then studying chromosomes under a light microscope, to ascertain size, centromere position, and, in medical diagnostics, chromosomal aberrations.

In the human karyotype, chromosomes are divided into 7 groups according to size and centromere position:

Group A
the largest
chromosomes 1, 2, and 3

Group B
chromosomes 4 and 5

Group C
medium-sized, submetacentric
chromosomes 6, 7, 8, 9, 10, 11, 12 and X

Group D
medium-sized, acrocentric
chromosomes 13, 14 and 15

Group E
small-sized, meta and submetacentric
chromosomes 16, 17 and 18

Group F
small-sized, all metacentric
chromosomes 19 and 20

Group G
small-sized, acrocentric
chromosomes 21, 22 and Y