Purely in terms of the logistics of experimentation—of how long it takes to cause a mutation in a gene and then see what happens—mice offer speed and convenience. Females have a three-week gestation period and are fertile again only six weeks after birth, so the generation time can be as brief as nine weeks.

Despite the obvious differences in physical size and appearance, mice are the equal of humans in the only measure that matters in genetics: the size and complexity of their genetic endowment. Like humans, mice are believed to possess approximately 40,000 genes. Like humans, they have a genome that comprises about 3 billion base pairs of DNA. Like humans, they are not only vertebrates but mammals. And as a growing number of cases attest, mice possess a biology and physiology that translate very nicely into their human equivalents and can serve to study human diseases.

Because the evolution of mice diverged from human evolution an estimated 60 million years ago, there are important differences between the species—but even the differences are significant. Mice are evolutionarily close enough to mimic human biology but far enough apart that most of the portions saved, or conserved, must be important. An additional reason why mice—instead of rats or guinea pigs or hamsters—became the mammal of choice lies in the accident of history known as "fancy mice."

"Back in the 17th century, in both China and Japan, mice were collected and bred for their beauty," Tilghman explains. Prized mutant mice were bred, much as people breed roses, for strikingly different colors and appearance. Nineteenth- century stock books and catalogs for so-called mouse fanciers list many special strains with ornamental characteristics and names like red cream, ruby-eyed yellow, white English sable, and creamy buff.

"This idea spread to Europe in the 19th century," says Tilghman, "and although the individuals who did the breeding were not geneticists and had no deep appreciation for what they were doing, they were creating inbred lines of mice that were genetically homogenous." That is, hobbyists inadvertently created strains of mice that were genetically identical, and that laid the groundwork for an incredibly powerful scientific approach to understanding biology.

When a Massachusetts woman, Abbie Lathrop, began in 1900 to breed fancy mice as pets on her farm, she also bred mice for researchers at Harvard University. This operation led indirectly to the Jackson Laboratory in Bar Harbor, Maine, which was inaugurated in 1929 with the aim of providing inbred lines of mice—fancy mice and their descendants—to biologists involved in studying cancer and other medical problems.

— Stephen S. Hall

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A sketch of "Miss Abbie C. Lathrop" published in 1913, shows her gazing at one of the fancy mice she bred on her farm as pets.

Image: From Springfield Sunday Republican, October 5, 1913 (adapted by RCW Communication Design)

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