SYSTEMS OF MATING 27 



For c — 1/2, 6 cycles (or 12 generations) of the cross-intercross system are 

 required to exceed 95 per cent incross matings, and 8 cycles (16 generations) to exceed 

 99 per cent incross matings. For c = 1/10, 31 cycles (62 generations) are required to 

 exceed 95 per cent incross matings and 46 cycles (92 generations) to exceed 99 per 

 cent. This system has been used to put a number of mutations in mice on standard 

 inbred backgrounds. These include: C57BL/6J- T /a, C57BL/6J-/?, C57BL/6J-rw, and 

 BALB/cHu-z'y. Complete lists of these strains are mentioned by Lane. 751 Snell 1238 

 used this system to isolate single genes or small segments of chromosomes which affect 

 histocompatibility. Examples are AKR-//-2 a (syn. AKR-K), C3H-#-2 b (syn. 

 C3HSW), and C57BL/10-//-l d (syn. BIOBY). 



THE CROSS-BACKCROSS-INTERCROSS SYSTEM 



The cross-backcross-intercross system, like the cross-intercross system, is used to 

 put a recessive viable gene r on a standard inbred background. It is especially useful 

 when the mutant phenotype is not visible and easily detectable, but must be determined 

 by a laboratory test, and when the ease of making up another generation of matings 

 (the backcross) outweighs the cost of determining the phenotype. The system was 

 invented and first used by Snell for developing strains of mice with genes for resistance 

 to tumor grafts on standard inbred backgrounds. 



The matings proceed in cycles of three generations. The first mating of each 

 cycle is a cross, RR x rr, when rr is the mutant and RR is the genotype of the inbred 

 strain. The second mating is a backcross, RR x Rr, formed by mating the heterozy- 

 gous progeny of the cross with animals of the inbred strain. The third mating is an 

 intercross, Rr x Rr, of the heterozygous progeny of the backcross. The heterozygotes 

 are discovered to be so by observing which matings produce rr progeny. All other 

 matings, 3/4 of the total on the average, are wasted. The rr progeny are then crossed 

 with RR animals of the inbred strain to start another cycle (figure 7). 



Among the crosses, there are three mating types when any other locus, the a-locus, 

 is considered in addition to the locus of interest. There are two types of backcrosses 

 and three types of intercrosses. The mating types and their probabilities in C m are: 



„ (ARIAR x ArlAA 

 crosses Incrosses F _. ' , ) = p m , 



\aRjaR x arjar J 



Crosses F (AR/AR x ar/ar \ _ 



Crosses P ^ ^ x ^^J - q m , 



Backcrosses P 



(AR/AR x Ar/ar\ 

 \aR/aR x Ar/ar) 



„ (ARIAR x AR/AA 

 backcrosses Incrosses F I „.' ' > 



\aRjaR x aRjar J 



Backcrosses P 



I ARJAR x AR[ar\ 



\aRfaR x aRjAr) 



