40 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



way, for the reduction of wild blood and the increase of tame blood 

 may not have any causal relation to the subsequent change in the 

 expression of the rough character. If we represent the factor for 

 roughness by Rf, and the factor for a tame smooth coat by rf, but 

 the factor for a wild smooth coat by rf ', then the gametes and zygotes 

 of the animals in table 57 are as follows: 



Rf + rf tame gametes. 



rf'+ rf pure wild gametes. 



2Rfrf' + 2rfrf' § wild zygotes. 



The smooth character of the wild may be due to something slightly 

 different from that of the tame, hence the combination Rfrf ' is different 

 from the tame heterozygous rough coat, Rfrf. Now, since the hybrids 

 in these experiments are constantly mated back to smooth guinea-pigs, 

 the great majority of hybrids must eventually carry the guinea-pig's 

 peculiar factor or factors for smooth coat (rf); hence, when the few 

 later dilute hybrids are used, the zygotic formula is probably Rfrf. 

 This means that these later hybrids would be a combination of the 

 rough character and smooth character, both derived from the tame 

 source; and since both are derived from the tame source, the rough 

 hybrids are just like the rough guinea-pigs. In other words, the almost 

 complete inhibition of the rough coat, which the | wild hybrids show, 

 is due to the smooth wild parent; but in later generations the smooth 

 character of the wild race is not likely to be present, and the hybrids 

 have the smooth character of the tame. Nehring (1894) must have 

 had a somewhat similar experience with the rough character when he 

 mated a rough guinea-pig to C. aperea. His records would indicate a 

 failure of complete dominance; but just what the degree of roughness 

 was can not be stated, for he makes no detailed description of the 

 hybrids as regards roughness. 



Table 58 records the rest of the matings of hybrid females with 

 guinea-pig males, in which one parent is heterozygous in roughness and 

 the other parent is smooth. In either case an approximate equality of 

 rough and smooth young is expected. In the first case, in which the 

 guinea-pig male is heterozygous in roughness, 8 rough and 6 smooth 

 were born. In the second case, in which the female wild hybrid was 

 heterozygous in roughness, 19 rough and 20 smooth were born. The 

 total, 27 : 26, is as close an approximation to equality as is possible in 

 an odd number of offspring. If the results of table 57 are added to 

 these, the grand total is 31 rough and 33 smooth wild hybrids. The 

 most probable expectancy is 32 : 32. The hybrids therefore produce 

 equal numbers of gametes which carry the rough factor and which 



