FACTOES OF INHERITANCE. 51 



since they are devoid of the necessary factor for the pro- 

 duction of their colour. In certain cases whiteness seems 

 to be due not to the absence of a factor, but to the 

 presence of an inhibiting factor. Such a white race can 

 behave as a dominant. A cross between a dominant 

 white and a recessive white fowl will yield some coloured 

 offspring in the second generation. So the factors may 

 influence each other's results when meeting in a zygote, 

 may interact in such a way as to produce characters 

 differing more or less completely from those produced by 

 any of them separately. 



We have seen that when two varieties are crossed 

 differing in the presence or absence of one hereditary 

 factor, as with the tall and sliort peas, a dominant and a 

 recessive form result in the second generation, in the pro- 

 portion of three to one. If, now, the varieties differed by 

 two factors, as for instance tall purple-flowered and 

 dwarf white-flowered peas, the hybrid would show both 

 the dominant characters, would be tall and purple, and 

 would give rise to four kinds of gametes in equal propor- 

 tions. One might contain both factors, one the factor 

 for tallness, one the factor for purple, and one neither 

 factor ; the gametes would appear in equal proportions, 

 since the factors segregate independently by "chance." 

 The product of random fertilisation among these gametes 

 will yield zygotes in the proportion of nine with both 

 factors, three with the factor for tallness only, three with 

 that for purple only, and one with neither, the pure 

 "extracted recessive." The number of possiole combi- 

 nations increases rapidly with the number of factors. 



The most complete analysis of the factors of inheritance 

 yet made is that of the fruit fly (Drosophila ampelophila], 

 recently worked out by Prof. Morgan and his pupils. 

 By breeding experiments about 125 different factors 

 have been distinguished, the eye-colour alone being 

 influenced by some 30. Moreover, it has been shown 

 by ingenious experiments that these factors are carried 



