SYSTEMS OF MATING 



23 



intercrosses Incrosses P 



lAR\Ar x ARjAr\ 

 \aR\ar x aRjar } 



BackcrossesP^' * ^/f), 

 \aRjar x aR/ArJ 



_ /Aff/ar x i4fl/ar\ 

 Intercrosses P ' ' • 



\aRjAr x aRjAr) 



In each mating type of the crosses for the r-locus, the standard inbred strain is 

 on the left, the mutant stock on the right. 



CYCLE 

 m 







GENERATION 

 n 







I 



2 

 3 

 4 



Fig. 5. The cross-intercross system. 



MUTANT 



Rr 



INBRED 

 STRAIN 



CROSS 



INTERCROSS 



CROSS 



INTERCROSS 



CROSS 



With respect to the a-locus, the incrosses, with probability p m , in the first generation 

 of C m , each produce one kind of offspring which yield incrosses in the next generation 

 of the same cycle. These, in turn, contribute exclusively to the incrosses of the first 

 generation of the next cycle, since all progeny other than rr are discarded. The crosses, 

 with probability q m , also each produce one kind of offspring, all doubly heterozygous. 

 These when intercrossed produce three kinds of rr progeny AA (or aa), Aa, and aa 



