VARIATIONS IN AGOUTI PATTERN. 



97 



to change ticked-bellied agouti to the typical light-bellied type. The 

 crosses show conclusively that they possess no such tendency. Indeed 

 when it is recalled that, in the early hybrids and C. rufescens itself, 

 light-belly was common, it seems necessary to suppose that guinea-pigs 

 possess a residual heredity which tends to darken agouti. 



Ticked-bellied agoutis, known to be heterozygous because of parent- 

 age, were crossed inter se. The results are given in cross 2. 



Aglb. Agtb. Non-ag. 

 Agtb. X Agtb 66 19 



This result is sufficiently close to the expected 3 to 1 ratio. One- 

 third of the ticked-bellied young from this cross should be homozygous 

 (A'A') and two-thirds heterozygous (A'a). Several of them have been 

 tested by crosses with blacks (cross 3, table 43). 



TABLE 43. 



The single agouti light-belly was the one of doubtful parentage men- 

 tioned above. Sixteen heterozygotes were obtained which gave agouti 

 ticked-belly and non-agouti in approximately equal numbers ; 6 possible 

 homozygotes were obtained, rather fewer than is to be expected. The 

 male AA253 with 12 agouti ticked-belly young and 2 females, AA213 

 and AA217, with 8 agouti ticked-belly young each, were quite certainly 

 homozygous and were used to establish a homozygous ticked-bellied 

 stock. They and their progeny crossed inter se have given only ticked- 

 bellies, 26 in number (cross 4). These homozygous ticked-bellies are 

 indistinguishable from heterozygotes in appearance. 



Cross 6 gives matings of homozygous light-bellied agouti guinea-pigs 

 with non-agouti hybrids. The young, 29 in number, are all light- 

 bellied. There is no tendency toward ticked-belly introduced by the 

 hybrids. 



Cross 7 gives matings of light-bellied hybrids (agouti derived from 

 guinea-pigs) with non-agoutis. All of these light-bellies were known 

 to be heterozygous from their parentage. The result, 18 light-bellies, 

 21 non-agoutis, no ticked-bellies, is in harmony with expectation 

 (19.5 :19.5). 



Crosses 8 and 9 give data on the relation of light-belly to ticked-belly. 

 Homozygous light-bellied guinea-pig by ticked-bellied hybrid gives 

 exclusively light-bellies, 50 in number. Light-belly is thus clearly 



