82 INHERITANCE IN GUINEA-PIGS. 



dilution must be due to a new allelomorph in the albino series, let us 

 say Cd- A modifying factor which shows partial coupling would give 

 intermediate results. 



(1) Since dilution and red-eye show considerable resemblance, it 

 would be a plausible hypothesis to assume that they are due to the same 

 allelomorph in the albino series (C r ) but differ by an independent modi- 

 fying factor (D) . With this hypothesis, all stocks used (except the lea 

 and Arequipa) must needs be homozygous for the modifier in order 

 that no red-eyes should appear. Dilutes would be C r C r DD or C r C a DD 

 albinos C a C a DD in these stocks. Thus albinos of these stocks should 

 transmit the modifier and in crosses with red-eyes (C r C r dd) should 

 produce dilutes at least in F 2 . But in crosses 23 and 25, red-eyes mated 

 with such albinos have given no dilutes, nor have dilutes appeared in 

 F 2 in cross 24, among 23 young. Thus an albino can not transmit the 

 hereditary difference between a dilute and a red-eye and the hypothesis 

 is untenable. 



(2) Next to be considered is the hypothesis that there is a modifier 

 which converts into a dilute an animal which would otherwise be an 

 albino. Dilutes of ordinary stock would be C a C a DD or C a C a Dd. In 

 cross 20, dilutes of ordinary stock crossed with a pure lea male No. 

 724, a homozygous red-eye (C r C r dd), produced 5 dilute young which 

 must be of formula C r C a Dd. This shows that if the hypothesis is to 

 stand at all, it must be extended, so that the factor which converts an 

 albino into a dilute also converts a red-eye into a dilute. The fact 

 that a dilute may transmit red-eye (crosses 19 and 27) is further evi- 

 dence that this extension is necessary. In this form most of the results 

 can be explained satisfactorily. 



(3) The only other hypothesis which remains is that dilution is due 

 to a new allelomorph in the albino series making a series of four C, 

 Ca, C r , and C a . The results cited above (crosses 20, 19, and 27) make 

 it evident that dilution is dominant over red-eye. The meaning of a 

 series of four allelomorphs can be made clear by considering all of the 

 possible zygotic formulae. Every zygote must have two representatives 

 from the series, but never more than two. Intense guinea-pigs may be 

 homozygous (CC), or carry dilution (CC^), or red-eye (CC r ), or albin- 

 ism (CC a ), but can never transmit more than one of the recessive 

 conditions. Dilutes may be homozygous (CdCd) or carry red-eye 

 (CdC r ) or albinism (CdCJ, never both. Red-eyes may be homozygous 

 (C r C r ) or carry albinism (C r C a ), while albinos can only be homozygous 

 (C a C a ) and can never transmit any of the higher conditions. 



The critical test between this hypothesis of four allelomorphs and the 

 preceding one (that dilute is a modified red-eye or albino), lies in the 

 possibility or impossibility of producing animals which in crosses with 

 albinos will transmit more than one recessive condition. If an intense 

 animal can be obtained which transmits both dilution and red-eye 



