26 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



and since these were easily distinguishable from the light tame agouti, 

 several crosses were made, into which they both entered, and many 

 more are in progress. Ten different | wild females and one yV "^^^^ 

 female were used in the following crosses : 



Cross 1 : 9 247, 9 248, and 9 311 were crossed with guinea-pig males 

 homozygous in agouti. 



Cross 2: 9108, 9131, 9166, 9172, 9198, 9 203, 9 219, and 9 536 

 were crossed with male guinea-pigs heterozygous in agouti. 



The result of the first cross was a complete dominance of the tame, 

 light, and powerful agouti over the wild, dark, and weak agouti; hence 

 all the young were light yellow agoutis with light bellies. If the wild 

 heterozygous agouti is designated by A'a, and the tame homozygous 

 agouti by AA, then the gametes formed and zygotes resulting from 

 their union in this cross were: 



A'+ a gametes of hybrid. 



A + A gametes of tame. 



2AA'+ 2Aa zygotes. 



It is evident that in half the zygotes produced are found both kinds 

 of agouti, while in the other half only tame agouti occurs. Since the 

 tame agouti is dominant, all zygotes look alike, but the heterozygous 

 animals should give only tame agoutis and non-agoutis when they are 

 bred to non-agouti animals. Their gametes should be A -Fa, and 

 combined with those of a recessive, a+a, should give zygotes 2Aa-f2aa. 

 On the other hand, the animals homozygous in agouti should produce 

 gametes A and A' ; and when such animals are mated to recessive non- 

 agoutis, with gametes a-|-a, they can give only young of the two sorts 

 expressed by the formula Aa-|-A'a; that is, all agouti, but with equal 

 numbers having the dark wild coat and light tame coat. Since only 

 three heterozygous wild agouti females were mated to the homozygous 

 tame agouti males, the number of offspring produced in cross 1 was 

 small. These wild females have already been entered in table 3. 

 Referring to that table, it will be seen that 9 248 and 9 311 produced 

 5 young, and that 9 247 and 9 248 appear as possible mothers in cases 

 of doubtful motherhood; in all cases, it may be stated, the offspring 

 were light agoutis with light bellies. When the young produced by 

 cross 1 were tested individually by breeding them to non-agouti mates, 

 they were found to be characterized as follows: 



9 485 zygotic formula AA' 



c? 486 sterile, 

 cf 487 sterile. 



d" 506 



9 580 



cr581 



? 

 ? 

 AA' 



Aa 

 Aa 



