282 THE THEORY OF THE GENE 



the offspring will be blue and half white. If a blue hybrid 

 is back-crossed to a pure black bird, half the chicks will 

 be black and half blue. Both results are consistent with 

 the postulate that the genes of the blue hybrid are pure, 

 half for black and half for white. Their occurrence in the 

 same cell has not resulted in contamination or mutual 

 infection. 



In the example just given the hybrid is unlike either 

 parent and, in a sense, is intermediate between them. In 

 the next example the hybrid is indistinguishable from one 

 parent. If a black guinea pig is bred to a white one, the 

 offspring are black. If these are inbred, the offspring are 

 three blacks to one white. The extracted whites breed as 

 true as the original race of whites. The white gene has 

 not been contaminated by the black gene in their sojourn 

 together in the hybrid. 



In the next example a case is chosen in which the two 

 original forms are much alike, and the hybrids, while 

 intermediate to some degree, are so variable that, at the 

 ends of the series, they overlap the parental types. The 

 types differ in a pair of genes. 



If an ebony Drosophila is bred to a sooty one, the off- 

 spring are, as stated, intermediate, but variable. If these 

 are inbred, they produce an array of shades that give a 

 practically continuous series. There are ways, however, 

 of testing the grades. When this is done it is found that 

 the array is made up of individuals that are pure for 

 ebony, others that are hybrids, and others that are pure 

 for sooty, in the ratio of 1 : 2 : 1. Here again we have evi- 

 dence that the genes have not been mixed. The continuous 

 series of shades is merely due to overlapping variability 

 of the characters. 



All this is simple and clear because we are dealing in 

 each case with a single pair of genes that act as differen- 

 tials. These cases serve to establish the principle at stake. 



