60 CROSSING. 



carry the gene unseen, and they may have derived it from the 

 species whose crossing into musculus gave rise to them. 



Two organisms may have the same character, and yet their 

 hybrid off-spring may have an altogether different character, 

 dominant to the corresponding one of both parents. 



A definite character of an organism, any quality, results 

 from the cooperation of numerous factors. Some of these fac- 

 tors are quite indispensable for the final result. If one such is 

 lacking, the character is not developed. If another one is lack- 

 ing the result is the same. One rat may be albino because in 

 the chain of factors necessary for pigmentation one link is 

 missing. Another rat may be albino for quite a different reason, 

 because of the lack of some other factor. The result will be, 

 that the children of two such different albinos will be coloured. 



Xhe first cases recorded of such an unexpected production of 

 pigmented organisms from the cross of two whites with the 

 same recessive character, were found by Bateson and his pupils. 

 There is first of all the case of the two white sweet-peas in 

 Emily Henderson, both whites, differing chiefly in shape of the 

 pollen. The hybrids where purple. On analysis it was proved 

 that one was white because it lacked A, and the other was white 

 because it was lacking in a gene B. Together however, the 

 gametes constituting the hybrid zygotes contained both A and 

 B, and as all the other genes necessary for pigmentation were 

 common to both parents the hybrids were coloured. Such 

 coloured hybrids will produce as many gametes with as without 

 A, and the same holds true for B, for that reason they will, if 

 test-mated to individuals lacking both A and B, produce three 

 times as many white as coloured off-spring. But if they are 

 mated black to either parent, the proportion of coloured 

 and white will be equal. For such hybrids differ from each of 

 the parents in one gene only, namely in the gene inherited 

 from the other parent. Coloured hybrids from white parents 

 have only one gene more than each parent. If we mate them 

 back with the parent having A but lacking B, we are not con- 

 cerned with A, and only with B, as A is common to parent and 



