COLOR AND COAT CHARACTERS. 33 



6. EXTENSION AND RESTRICTION. 

 HOMOZYGOUS CONDITION OF EXTENSION IN CROSSES. 



Guinea-pigs of the varieties known as red, yellow, and cream agree 

 in having no black or brown pigment in their hair, but instead a yellow 

 pigment of varying intensity. Such animals I shall for convenience 

 call "red," whatever the intensity of their pigmentation. The eyes of 

 red guinea-pigs are either black or brown pigmented. Black-eyed reds 

 may transmit black coat-color in crosses, but brown-eyed reds can not do 

 so, though they are capable of transmitting brown coat-color in crosses. 



Since the black or brown pigment in a red animal is restricted to the 

 eyes and skin and does not occur in the fur, we may speak of such an 

 animal as restricted black or brown, and the gametes which transmit 

 this condition as possessing a restriction factor. Or, looking at the 

 matter from an equally justifiable point of view, a red animal is con- 

 sidered to lack the factor for extension which, in either a single or a 

 double dose, gives rise to black or brown. They may also carry, 

 unseen, that factor which acts only on black or brown, the so-called 

 agouti factor. Indeed, a number of the animals which have been 

 entered in tables dealing with the transmission of agouti were reds. 

 Similarly albino animals may be legitimately classified as regards their 

 power to transmit color characters, even though they themselves do 

 not manifest those color characters. 



No red individuals of wild Cavia rufescens are known. Just as in the 

 case of agouti, and black, in which the wild is homozygous, so, in the 

 case of extension, it was surmised the wild would prove to be homozy- 

 gous. Two guinea-pig females, known to be heterozygous in extension, 

 were mated to one of the wild males (cr33) and produced 6 young 

 (table 30), all of which had completely extended black pigmentation. 

 None of the wild, mated inter se, ever gave young with the restricted 

 (red) coat color. It is therefore safe to assume that wild individuals 

 transmit the extension factor in all gametes. 



None of the animals produced in these matings was used afterward 

 except 9 72. She proved to be homozygous in the extension factor. 

 This is not at all surprising, for there was an even chance that she 

 would be homozygous or heterozygous. All the other ^ wild females 

 were also homozygous for extension. Six of the ^ wild females were 

 mated (table 31) to guinea-pig males, carrying the extension factor in a 

 heterozygous condition (cf4, cf9246) or lacking it entirely (cf617). 

 These matings produced 29 offspring, all of which were of the extended 

 pigmentation, thus proving that, in the hybrids, extension is epistatic 

 to restriction, just the same as in guinea-pig matings. The j wild, 

 thus produced, would be of two classes, homozygous (EE) and hetero- 

 zygous (Ee). 



