Outbred or Random Stocks
The terms ‘random’ and ‘outbred’ are often used interchangeably. Random refers to a chance mating system or using a randomisation table. Outbreeding is not a chance selection, but a scheduled production that avoids brother-sister mating. Several outbreeding systems are recognised but the common aim of these systems is a population represented by as large a range of alleles of each gene as possible, resulting in an inbreeding coefficient of no greater than 2% per generation.
Inbred mouse strains are bred within a closed colony by brother-sister mating to maintain certain defining characteristics. Animals are considered inbred after at least 20 consecutive generations of brother-sister mating.
A congenic strain is genetically identical to an inbred strain except for a short chromosomal segment bearing the gene of interest. The formation of a congenic strain begins with the crossing of two breeding groups. One provides the genetic background and must always be an inbred strain. The other ‘donates’ the gene of interest and may or may not be an inbred strain. The mating system that produces congenic strains depends upon whether the gene of interest is dominant or recessive and the gene’s effect, when homozygous, on reproductive performance. Generally, ten backcross generations are required to yield a congenic animal, although contemporary molecular techniques can significantly shorten this period. Once established, a congenic strain is managed identically to an inbred strain.
A hybrid is the first generation progeny (F1) of parents from two different inbred strains. Genetic uniformity and hybrid vigour are two desirable features of hybrids. They are typically not bred, except on demand, as the unique features cannot be maintained beyond the F1 generation. Hybrid nomenclature places the female strain first and the name is commonly shortened. For example the C57BL/6J (Female) x DBA/2 (Male) = B6D2F1, and C57BL (Female) x BALB/c (Male) = BCF1.
As the name implies these animals display a feature of a genetic mutation. It may be as simple and benign as a coat-colour mutation or a complex mutation where animals are sexually infertile or predisposed to a disease state or death. In most cases the mutation can be transferred to both outbred and inbred animals. Mutations can be either spontaneous (e.g. the nude mutation Foxn1nu/Foxn1nu, and hairless mutation hr/hr) or induced (transgenic and knockout strains).
Recombinant Inbred Strains
Recombinant inbred (RI) strains are derived from crossing two unrelated but highly inbred progenitor strains, which have been maintained independently under a regimen of strict inbreeding since the F2 generation, as separate parallel lines. A minimum of twenty generations of inbreeding is required thereafter. This results in numerous separate inbred strains derived from two inbred strains. These lines are useful tools to investigate whether a trait is polygenic or under the control of a single gene.