Cats, with their keen hunting techniques and thousand ways to purringly manipulate human beings, have only 38. Though similar in basic appearance, different chromosomes vary slightly in size and shape. In addition, when chromosomes are stained with fluorescent dyes they develop distinctive patterns of bright and dark bands. These subtle differences enable cell biologists to distinguish different chromosomes from one another, much as field biologists learn to distinguish members of a pod of whales by the marks and scars on their fins. The largest chromosome of an organism is generally referred to as chromosome 1, the next largest as chromosome 2, and so on.
Different chromosomes contain different genes.
That is, each chromosome contains a specific chunk of the genome. For example, in humans the gene for alpha globin, a part of the hemoglobin protein that carries oxygen in red blood cells, is found on chromosome 16. The gene for beta globin, the other part of the hemoglobin protein, is found on chromosome 11. Stained chromosomes can be photographed and arranged in order of size to produce a karyotype, a chart scientists use to study chromosomes. A karyotype isn't detailed enough to tell you about the individual genes on a chromosome, but it can tell you whether the chromosomes as a whole are in working order and it may help doctors diagnose and understand diseases.