20 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



either died or were sterile males; 8 of the 13 animals tested have proved 

 to be heterozygous, 3 homozygous, and 2 are questionable, for the last 

 produced only agouti young, but in such small numbers that no con- 

 clusions can be drawn in regard to their zygotic formula,. It is apparent 

 that both the expected classes of agouti individuals were produced, 

 and that the heterozygotes (8) occur approximately twice as frequently 

 as the homozygotes (3 to 5). These homozygous animals are interest- 

 ing particularly because the agouti came from two sources, the wild 

 and the tame, and they produced agouti young of two sorts. It seems 

 paradoxical to assert that a homozygous agouti animal produces two 

 sorts of agouti, yet, as we have already observed, the agouti of C. 

 rufescens is distinguishable from that of C. porcellus. 



There is a sharp distinction between a factor and its allelomorph. 

 No matter how much variation there may be in the tame agouti 

 pattern, it always segregates clearly from its absence. The same has 

 been shown for the wild agouti in tables 6 to 10. There is a certain 

 amount of variability to all unit characters. This is especially true 

 of the wild agouti pattern in a heterozygous condition in hybrid 

 animals. Where the wild agouti pattern has been so modified in the 

 hybrid animals that it can be distinctly discriminated from the tame, 

 it offers splendid material for a cross with tame agouti. Although the 

 wild has been described as somewhat darker than the tame agouti, 

 hybrids arose which were nearly black, so weak was the wild agouti 

 factor (see figs. 4 to 9). Without further preliminaries, the variabiUty 

 of the wild agouti and its action in crosses with the tame may be 

 appropriately discussed. 



THE WILD AGOUTI AND TAME AGOUTI CONTRASTED. 



In the preceding discussion all agouti individuals have been classed 

 together, irrespective of the differences which have been indicated as 

 distinguishing wild from tame animals. Such is the usual method of 

 procedure in genetic studies. For instance, in crosses of English-pat- 

 terned rabbits, bearing a dominant restricting factor, with self-colored 

 rabbits, the English pattern is held to act as a unit. The differences 

 between various animals, possessing the same unit character, are 

 explained by postulating either variabiUty in this one unit character 

 or a number of similar or dissimilar genes for this one character, or 

 other modifying unit characters, such as intensity, dilution, and the like. 



Black, in crosses, is dealt with in much the same way, and differences, 

 easily discernible or seen with difficulty in different individuals, are 

 similarly explained. A clearer example of this is shown in the crosses 

 of hooded and self-patterned rats. The hooded pattern shows a very 

 wide range of variability, yet any hooded pattern acts as a unit in 

 crosses with self. Pure genotypical races as regards color in animals 

 have not been isolated. Even agouti itself, in variety crosses, has been 



