ABSTRACT
Early geneticists foresaw that mice could provide a useful model for the study of mammalian genetics. They generally started with “fancy” strains of mice available in pet stores. From 3 months of age, mice can generally produce litters of six or more pups in successive months; this is due to the short (18 to 21 days) gestation and possibility of post-partum matings. All of the early derived strains of mice were presumed to be from random matings, thus providing maximal genetic variability in the original populations. Selection of a few progenitors for any laboratory population progressively limited genetic variability within that breed or strain. Mouse geneticists have from the beginning deliberately inbred the stocks of mice, typically by brother-sister matings, maintained and pedigreed over successive generations. Given 20 consecutive generations of brother-sister matings, such an inbred strain is considered to be homozygous at virtually all of the estimated 30,000 genetic loci. Given such degree of inbreeding, all concurrent mice within each inbred strain are virtually as genetically identical as identical twins derived from the same zygote. Nearly 500 inbred strains of mice have been produced and listed. It is important to note, however, that inbreeding to genetic homozygosity for all genetic loci does not eliminate phenotypic variability. In fact, genetic homozygosity may make individuals more susceptible to certain environmental factors.
