7s IMIKKIIA.V K IN GUINEA-PIGS. 



ok gives red-eyes, hut no intense young. In cross 25, red-eyes 

 -. ,1 with jilhinos from various sources give no intense young, but 

 only red-eyes and albinos. One possible explanation of these results 

 would be the supposition that red-eye becomes dominant over its 

 normal allelomorph iii the presence of heterozygous albinism. In 

 this case intense young should appear when such heterozygous red-eyes 

 are crossed together; but, as is shown in cross 24, none such appears. 

 Here red-eyes from cross 21, mated inter se, gave 17 red-eyes, 6 albinos, 

 DO intense. Numerous results of this kind have made it clear that 

 intensity can never be recovered in any generation after a cross of red- 

 eye wit h albino. This means that neither red-eye nor albino can trans- 

 mit the normal allelomorph of the other. Now, the one thing which a 

 recessive variation, of necessity, can not transmit, is its own normal 

 allelomorph. Therefore the normal allelomorphs of red-eye and albino 

 must be identical. 



This does not yet demonstrate that albinism, red-eye, and intensity 

 form a series of three allelomorphs. There is still the possibility that 

 red-eve and albinism involve the same recessive allelomorph (C a ) of 

 norma] color (C), but differ by an independent modifying factor. 

 Symbolically we could suppose albinos to be C a C a rr, red-eyes to be 

 1 I IvR (or C a C a Er), intense guinea-pigs of ordinary stocks to be CCrr 

 (or C '( J a rr) , and intense guinea-pigs of lea stock to be CCRR (or CC a RR) . 

 We must suppose the lea stock to be homozygous for the modifier R, 

 to account for the absence of albinos. R must be a unit factor to 

 account for the simple 3 to 1 ratio in cross 24. This hypothesis fits all of 

 the facts given so far. The critical test of its truth is the possibility (as 

 it turns out, impossibility) of producing intense animals (CC a Rr) which 

 will give both red-eyes and albinos when crossed with albinos. If 

 intensity, red-eye, and albinism are triple allelomorphs, it should be 

 impossible to obtain such animals. Crosses 21 and 22 are interesting 

 as furnishing just this test. Cross 21 may be represented symbolically 

 as follows according to the two hypotheses: 



•w (BW) X intense (lea) = 9 intense + 4 red-eye. 



(1) CaCarr X CCaRR = CCaRr C a C a Rr. 



(2) CaCa X CCr = CC a CrCa. 



In oit her case the F, red-eyes crossed inter se should give 3 red-eyes 

 to 1 albino. The result obtained in cross 24 (17 red-eyes to 6 albinos) 

 is in nearly perfect agreement. But the cross of F x intense with albinos 

 gives very different results under the two hypotheses (cross 22): 



-"' X iniensi <!•',) = U\ intense + (0 red-eye) + 25 albinos. 



Rr = CCaRr + CC a rr + C a C a Rr + CaCarr. 



- CCa + CaCa- 



The complete absence of red-eyes among the 41 young, as well as the 



of albinos where an excess of intense is expected, thoroughly 



eliminate- the first hypothesis. The results agree reasonably well with 



