144 geological time
132 4 5
6 7 8 9 10 11 12
13 14 15 16 17 18
19 20 21 22 X Y
Cytogenetic map showing the visual appearance of a chromosome when stained and examined under a microscope. Particularly impor-
tant are the visually distinct regions, called light and dark bands, that give each of the chromosomes a unique appearance. This feature
allows a person’s chromosomes to be studied in a clinical test known as a karyotype, which allows scientists to look for chromosomal
alterations.(Courtesy of Darryl Leja, NHGRI, National Institutes of Health)