396 THE BIOLOGY OF BIRDS 



for blackness from one side of the house, but not from the 

 other. The offspring will all be black, for blackness is 

 dominant in relation to whiteness. Now the germ-cells of 

 these offspring will divide into two sets — one set carrying the 

 factor for blackness and the other set not. It is usual to 

 call such an organism heterozygous, which means producing 

 two kinds of germ-cells. This duality of the germ-cells is 

 sometimes demonstrable microscopically when half of the 

 germ-cells carry a nuclear rod or chromosome with a distinc- 

 tive appearance ; but the idea of segregation into two 

 contingents was arrived at as an inference from what happens 

 when the hybrid offspring are paired. They yield, to the 

 dozen let us say, 3 pure blacks (breeding true when mated 

 with others like themselves), 3 pure whites (breeding true 

 when mated with others like themselves), and 6 blacks, 

 which are, however, heterozygous, and will when mated 

 together yield the same ratios 



3BB + 6B(W) + 3WW 



(2) Fantail pigeons, as every one knows, have more than 

 double the usual dozen of tail feathers. They are also 

 without a preen-gland. Professor T. H. Morgan (1918) 

 crossed fantails showing 29, 30, and 32 tail feathers with 

 ordinary homers with 12, and the next generation (Fj) gave 

 a range of variation between 12 and 20, the highest frequency 

 being 14 feathers. " Evidently one or more of the factors 

 of the fantail act as partial dominants, producing tails that 

 have for the most part more tail feathers than has the common 

 pigeon, but less than the fantail." In the next (Eg) genera- 

 tion there w^ere many birds with a 12-feather tail, a good 

 many had 14-16, a few approached the lower range of 

 variation in the fantail (24-26). Professor Morgan also 

 points out that it is probable that the hereditary factor (or 

 gene) for more than 12 feathers, and that for no preen-gland, 

 and that for white colour are linked together, which means 

 that they are carried by the same chromosome. 



(3) As an example of the subtle characters that are 

 hereditary in definite ways, we may mention the refractive 



