MENDEL'S FIRST LAW 27 



strictly only to the hen and to the lower parts of the body 

 in the cock whose upper surface is very dark blue or 

 even black. 



In this case neither black nor white can be said to be 

 dominant. The blue brought in as splashes by the 

 splashed-white might indeed be regarded as dominant over 

 the black of the other (black) parent, but if so, then the 

 uniform distribution of the blue must be determined by 

 dominance of the allelomorphic gene brought in by the 

 black parent. Each parent then would contribute at the 

 same time a dominant and a recessive effect, each the 

 product of one member of the same pair of allelomorphs. 



There are other cases in which the hybrid is inter- 

 mediate in color, and, in addition, its range of variation 

 is so large that the extremes overlap one or even both 

 of the two parental types. For example : In the vinegar 

 fly, Drosophila melanogaster, there is a race with ebony 

 wings and another race with sooty wings. AVhen such 

 flies are crossed to each other, the wings of the F^ fly are 

 intermediate in color, ranging from wings like those of 

 sooty to wings as black as ebony. When the F^ flies are in- 

 bred they give rise to a series that at one extreme has gray 

 wings and at the other black wings. Separation into three 

 classes is difficult or impossible. Here it may appear that 

 the two original characters have completely blended in 

 Fi and in F^, but that there are in reality three classes 

 of flies in F2 can be demonstrated by suitable tests. If, 

 for instance, we pick out a sufficient number of F2 males 

 to give a fair sample of the population, and mate each 

 male first to an ebony female of pure stock, and then to 

 a female of sooty stock, we shall find that one-quarter of 

 the males mated to ebony give only ebony, one-quarter 

 mated to sooty give only sooty, while the remaining two- 

 quarters give, both in the back-cross to sooty, and in that 

 to ebony, a wider ranging group, which is darker on the 

 whole when mated to ebony, and lighter when mated to 



