VARIATIONS IN AGOUTI PATTERN. 95 



The agouti pattern of mice was shown by Cuenot in 1903 to be a 

 unit Mendelian character dominant over its absence as found in blacks. 

 In this and later papers (1903, 1904, 1907) he demonstrated that a 

 white-bellied type of agouti and self yellow are due to members of 

 the same series of allelomorphs. Castle, 1905, demonstrated that 

 guinea-pig agouti is a simple dominant over non-agouti. 



This agouti pattern of guinea-pigs is subject to considerable varia- 

 tion. In some cases the belly hairs are entirely yellow, a condition 

 correlated with very broad yellow ticking in the dorsal fur. At the 

 other extreme, the base of the hairs on the belly is black for about half 

 the length, and the dorsal ticking is markedly decreased. This dark 

 type has been produced by repeated crossing with intense blacks 

 (BB race). Although distinctly darker than usual, all of the agoutis 

 from such crosses are distinctly yellow-bellied. 



PREVIOUS WORK. 



Detlefsen (1914) made experiments with the wild species Cavia rufes- 

 cens of Brazil. This has the agouti pattern, but is somewhat darker 

 than C. cutleri or the tame guinea-pig. The yellow bands in the dorsal 

 fur are narrower and there is usually more black on the belly, which 

 indeed is usually slightly ticked with black. The difference in appear- 

 ance is not very great. Detlefsen found, as he expected, that C. 

 rufescens was homozygous for the agouti factor. In the hybrids 

 between C. rufescens and black guinea-pigs, the agouti behaved as a 

 simple Mendelian dominant. What was not expected was a marked 

 darkening of the agoutis which occurred among the hybrids in many 

 cases. The yellow subterminal bands became so reduced on the back 

 that many of the agoutis appeared more like blacks than guinea-pig 

 agoutis at birth. Black appeared at the ends of the hairs on the belly, 

 and the appearance changed from yellow to ticked. In the early 

 generations the variations in the agouti were exceedingly erratic in 

 their hereditary behavior. Light-bellied hybrids crossed with blacks 

 often gave ticked-bellied young, and ticked-bellied hybrids gave light- 

 bellied young. Nevertheless, as more guinea-pig blood was introduced 

 by repeated back-crosses, the trend was constantly toward the ticked- 

 bellied type. In lines in which the ticked-bellied type had become 

 constant, crosses were made with typical light-bellied agouti guinea- 

 pigs. The ticked-bellied type was found to be recessive and segregated 

 out in later crosses in regular fashion. Detlefsen found that the results 

 in these lines were adequately explained by assuming that the ticked- 

 bellied type is due to an allelomorph of both the light-bellied agouti 

 factor and the non-agouti factor, recessive to the former, dominant to 

 the latter. He used the nomenclature A, A', and a for the tame agouti, 

 wild agouti, and non-agouti factors, respectively. 



