This can be described by comparing the number of base differences between genes that are common in the genome of many organisms. The smaller the number of base differences is, two different species are more closely related. Let's take for example mitochondrial gene COII, characteristic for many organisms. The difference in COII base between rats and mice is 101 base differences, and between rats and cows is 136 base differences. Since 101 base differences are less than 136 base differences, rats and mice are more similar than rats and cows.
