Inbred strain

Inbred strains (also called inbred lines, or for animals linear animals) are individuals of a particular species which are nearly identical to each other in genotype due to long inbreeding. Inbred strains of animals are frequently used in laboratories for experiments where for reproducibility of conclusions all the test animals should be as similar as possible. However, for some experiments, genetic diversity in the test population may be desired. Thus outbred strains of most laboratory animals are also available.

Certain plants including the genetic model organism Arabidopsis thaliana naturally self pollinates, which makes it quite easy to create inbred strains in the laboratory (other plants, including important genetic models such as Maize require transfer of pollen from one flower to another). For most animals, the usual procedure is mating of brother-sister pairs for a minimum of 20 generations, which will result in lines that are roughly 99% genetically identical. Many inbred strains have been inbred for many more generations and are in effect isogenic.

Effects
Inbreeding animals will sometimes lead to unwanted genetic drift. The continuous overlaying of like genetics exposes recessive gene patterns that often lead to changes in reproduction performance, fitness, and ability to survive. A decrease in these areas is known as inbreeding depression. A hybrid between two inbred strains can be used to cancel out deleterious recessive genes resulting in an increase in the mentioned areas. This is known as heterosis.

The best-known strains of laboratory animals are:

Rats and mice
"The period before World War I led to the initiation of inbreeding in rats by Dr Helen King in about 1909 and in mice by Dr C. C. Little in 1909. The latter project led to the development of the DBA strain of mice, now widely distributed as the two major sub-strains DBA/1 and DBA/2, which were separated in 1929-1930. DBA mice were nearly lost in 1918, when the main stocks were wiped out by murine paratyphoid, and only three un-pedigreed mice remained alive. Soon after World War I, inbreeding in mice was started on a much larger scale by Dr L. C. Strong, leading in particular to the development of strains C3H and CBA, and by Dr C. C. Little, leading to the C57 family of strains (C57BL, C57BR and C57L). Many of the most popular strains of mice were developed during the next decade, and some are closely related. Evidence from the uniformity of mitochondrian DNA suggests that most of the common inbred mouse strains were probably derived from a single breeding female about 150-200 years ago."

"Many of the most widely used inbred strains of rats were also developed during this period, several of them by Curtis and Dunning at the Columbia University Institute for Cancer Research. Strains dating back to this time include F344, M520 and Z61 and later ACI, ACH, A7322 and COP. Tryon's classic work on selection for maze-bright and dull rats led to the development of the TMB and TMD inbred strains, and later to the common use of inbred rats by experimental psychologists."

Inbred strains of rats

 * Wistar as a generic name for inbred strains such as Wistar-Kyoto, developed from the Wistar outbred strains.

Inbred strains of mice
• A/J

• C3H

• C57BL/6

• CBA

• DBA/2

• BALB/c

Guinea pigs
G.M. Rommel first started conducting inbreeding experiments on guinea-pigs in 1906. Strain 2 and 13 guinea-pigs, were derived from these experiments and are still in use today. Sewall Wright took over the experiment in 1915. He was faced with the task of analyzing all of the accumulated data produced by Rommel. Wright became seriously interested in constructing a general mathematical theory of inbreeding. By 1920 Wright had developed his method of path coefficients, which he then used to develop his mathematical theory of inbreeding. Wright introduced the inbreeding coefficient F as the correlation between uniting gametes in 1922, and most of the subsequent theory of inbreeding has been developed from his work. The definition of the inbreeding coefficient now most widely used is mathematically equivalent to that of Wright.