32 GENETIC STOCKS AND BREEDING METHODS 



This system is being used by Snell for the isolation of loci affecting histo- 

 compatibility in the mouse. No strains have advanced far enough to have been 

 mentioned in the literature. 



BROTHER-SISTER INBREEDING WITH HETEROZYGOSIS FORCED BY BACKCROSSING 



The system of brother-sister inbreeding with heterozygosis forced by backcrossing 

 is useful when it is desired to put a recessive mutation and its normal allele or a semi- 

 dominant lethal mutation and its normal allele on a common inbred background. 

 Unlike the three preceding systems, the mutant-bearing animals are not crossed with 

 animals of an existing inbred strain, but are mated as brother x sister. With respect 

 to the locus of interest, all matings are rr x Rr, in the case of a recessive viable mutation 

 r, or dd x Dd in the case of a semidominant lethal mutation D. The other homo- 

 zygote is either rejected (RR) or lethal [DD). As before, let A/a be any other locus 

 whose heterozygosity is in question, and let c be the probability of crossing over between 

 the a-locus and the r-locus or ZMocus. 



There are six types of matings with probabilities denoted as follows: 



(ArjAr x ARjAr\ 

 backcrosses lncrosses r , J = p n , 



arjar x aRjar 



Crosses PI , ' n , . I = q n , 



\arjar x ARjAr) 



(Ar/Ar x ARjar\ 

 [ar/ar x aRjAr) ~ *»' 



(Arjar x ARjAr\ _ 

 \Arjar x aRjar J ~ * n ' 



(Ar/Ar x aRjAr\ 

 [ar/ar x AR/ar) ~ ni 



_, [Ar/ar x AR/ar\ 

 Intercrosses r I . ' _, . ) = v n . 



\Ar/ar x aR/Ar) 



Backcrosses D [Ar/Ar x AR/ar 



(first kind) 



Backcrosses D [Ar/ar x AR/Ar\ 



(second kind) 



Backcrosses 

 (third kind) 



If each r is replaced by d and each R by D, the matings refer to inbreeding with a 

 semidominant lethal mutation. 



The incrosses yield only incrosses in the next generation. The crosses produce two 

 kinds of progeny, aR/Ar and Arjar, or ARjar and Arjar, but under the rule of this system 

 of mating brother x sister, which are rr x Rr, they yield only one kind of mating in the 

 next generation, intercrosses. The backcrosses of the first kind produce four kinds of 

 progeny which yield incrosses, backcrosses of the second kind, backcrosses of the third 

 kind, and intercrosses in the ratio c(l — c):{\ — c) 2 :c(\ — c) in the next generation. 

 The backcrosses of the second kind also produce four kinds of progeny which yield 

 incrosses, backcrosses of the first kind, backcrosses of the second kind, and intercrosses 



