Living World / Genetics

Earth’s Genetic Library

Ten years ago, the genome was largely a dream. Scientists had not tallied up genetic codes of most living things, plant or animal. Today humans, chimps, cows, dogs, honeybees, yeast, rice, and bacteria have been sequenced—and elephants, orangutans, hedgehogs, guinea pigs, and kangaroos will soon join the list.

The human genome is made up of a chemical string of about 2.9 billion base pairs, or letters, encoding roughly 20,000 to 25,000 genes. But many animals have more. Lab rats, for example, contain about 30,000 genes on a genetic alphabet 2.75 billion letters long. (Approximately 90 percent of a rat’s genes have counterparts in both mice and men.) And chickens are on a par with humans: 20,000 to 23,000 genes.

Small does not mean few. The roundworm boasts 97 million base pairs and 19,000 genes, while the fruit fly contains about 13,600 genes within its 165-million-letter code. Baker’s yeast has 12 million base pairs and 6,000 genes, and the bacterium Escherichia coli has around 4.6 million letters coding for 4,400 genes.