SYSTEMS OF MATING 5 



systems, as well as regular systems, may be analyzed by Wright's method of path analy- 

 sis. The seven systems are of three types: those based upon relationship, those based 

 upon locus control, and those based upon both relationship and locus control. 

 The systems based on relationship are: 



1 . RANDOM-MATING SYSTEM, 



2. BROTHER-SISTER INBREEDING SYSTEM. 



Those based on controlling a locus of interest are : 



3. BACKCROSS SYSTEM, 



4. CROSS-INTERCROSS SYSTEM, 



5. CROSS-BACKCROSS-INTERCROSS SYSTEM. 



Those which use a combination of locus control and relationship are : 



6. BROTHER-SISTER INBREEDING WITH HETEROZYGOSIS FORCED BY BACKCROSSING, 



7. BROTHER-SISTER INBREEDING WITH HETEROZYGOSIS FORCED BY INTERCROSSING. 



In the first two systems, no conscious attention is paid to the pheno types, except for 

 the inevitable selection in favor of vigorous, healthy animals. 



In the third system, in each generation, (G , G x ,. . .) the mating is a backcross 

 with respect to the locus of interest. While the locus of interest is thus being controlled 

 by a backcross, the locus whose heterozygosity is in question may be undergoing an 

 incross, a cross, or a backcross. In the fourth system the initial generation (G ) and 

 all subsequent even-numbered generations (G 2 , G 4 , . . . ) are crosses and all odd- 

 numbered generations (G l5 G 3 ,...) are intercrosses with respect to the locus of 

 interest. In the fifth system, the initial, third, sixth, etc., generations (G , G 3 , G 6 , . . .) 

 are crosses; the first, fourth, seventh, etc., generations (G l5 G 4 , G 7 , . . .) are back- 

 crosses; and the second, fifth, eighth, etc., generations (G 2 , G 5 , G 8 , . . .) are inter- 

 crosses with respect to the locus of interest. 



In the sixth system, each generation, (G , G 1} G 2 , . . . ) is a backcross with respect 

 to the locus of interest with the provision that the mates are related as brother and 

 sister. In the seventh system, each generation (G , G l5 G 2 , . . .) is an intercross with 

 respect to the locus of interest, with the same provision about the mates. Table 1 

 shows the generation sequence relative to the type of mating. 



Table 1 

 Generation sequence relative to type of mating 



System cross backcross intercross 



The serial number of the generation G n given in the table shows the sequence of use of a cross, 

 backcross, or intercross in controlling the locus of interest in five of the systems of mating. 



