ch. v] HEREDITY 69 



and the walnut combs produced by crossing them 

 have constitution RrPp. They produce four kinds 

 of germ- cells, RP, Rp, rP, rp, giving the normal 

 ratio in F 2 of 9 birds containing R and P, 3 with 

 R and p, 3 with r and P, 1 rp. This rp, containing 

 neither rose nor pea is single, which may be regarded 

 as the normal comb with no other factor superposed 

 upon it. 



In conclusion, one further fact should be noted. 

 Although the members of an allelomorphic pair differ 

 from each other in that one contains a factor lacking 

 in the other, and this present factor is commonly 

 dominant over its absence, yet a number of cases are 

 known in which the introduction of a factor from one 

 parent only is not sufficient to cause its full develop- 

 ment in the heterozygote. The crossed offspring are 

 then different from both the parental types, and are 

 commonly intermediate between them. But when 

 such heterozygous forms are mated together or self- 

 fertilised, both the homozygous parental types are 

 produced in addition to the heterozygous form, as in 

 the offspring of a heterozygous tall pea there occur 

 homozygous tails and shorts in addition to hetero- 

 zygous tails. The classical example of this condition 

 is the blue Andalusian fowl. This breed cannot be bred 

 true ; when blues are paired together about half the 

 chickens are blues and the remainder evenly divided 

 between blacks and dirty-whites. By many genera- 



