INHERITANCE OF DILUTION. 



81 



It is evident that the two sets of crosses give consistently different 

 results. This difference demonstrates that dilution does not segregate 

 independently of albinism. 



An even more striking result follows from a portion of the above data. 

 F : dilutes, one of whose parents was of intense stock, were back-crossed 

 with albinos of intense stock. They gave 9 dilute, 20 albino young, 

 no intense, although these young were at least three-quarters of intense 

 stock. On the other hand, Fi intense, one of whose parents was an 

 albino of dilute stock, were back-crossed with albinos of dilute stock. 

 They gave 5 intense, 7 albinos, no dilutes, although these young were 

 at least three-quarters of dilute stock. It is clear that the hereditary 

 difference between a dilute and an intense can not be transmitted 

 through an albino. 



Table 35. 



It was emphasized above that all the intense animals used in cross 17 

 came from stocks which have never given dilutes. This was necessary 

 because in other crosses (18, 34, 41) intense by albino has given many 

 dilute young. No such precaution was taken with the dilutes used in 

 cross 16. Any available dilutes were used regardless of ancestry. In 

 fact, 1 1 of them, with 38 young, had one or both parents intense. In 

 none of the other crosses in which dilute has been crossed with albino 

 (19, 27, 38, 44) has any intense young appeared. Thus in crosses 

 with albinos an intense may transmit dilution, but a dilute never trans- 

 mits intensity. From these crosses it seems clear that intensity is dom- 

 inant over dilution. Other crosses on the whole bear this out. The 

 apparent exceptions will be ignored for the present but discussed later. 



We have reached the definite conclusion that dilute by albino can 

 never give intense, regardless of ancestry on either side. Since the 

 only thing which a variety of necessity can not transmit is a dominant 

 allelomorph of its essential factor, it follows that dilution and albinism 

 must have the same dominant allelomorph, which we will call intensity. 



There are only a few hypotheses which will satisfy this condition. 

 We already know two recessive allelomorphs of intensity, viz, red-eye 

 and albinism. It is conceivable that dilution may be due to the 

 cooperation of an independent factor (or factors) with one or more of 

 the known combinations C r C r , C r C a , and C a C a . If this is not the case, 



