502 E. ELEANOR CAROTHERS 
considered as a limiting case of Mendelian inheritance where 
dominance is imperfect. In what are commonly recognized as 
cases of Mendelian inheritance the hybrid is indistinguishable 
from the dominant homozygote so far as gross characters are 
concerned, and the only means of sorting the pure from the hybrid 
offspring is by inter-breeding. But this is not necessarily the case, 
as shown by Punnett (’07, pp. 29-30). In the Leghorn breed of 
poultry white plumage is dominant to colored, but not perfectly 
so. To quote directly: 
When a white and a brown Leghorn are crossed together, all the re- 
sulting offspring are white, but almost invariably have a few colored 
feathers. The presence of these ‘ticks’ is the outward and visible sign of 
the heterozygous nature of the bird on which they occur. Such birds 
give off equal numbers of gametes bearing the white and colored char- 
- acters. This is easily tested by breeding them together. It is found 
that from such matings one-quarter of the offspring are colored recessives, 
whilst the remainder are pure white, or white with a few ticks. The 
heterozygote resembles the dominant form much more closely than it 
does the recessive. Though we may speak of dominance in such a case 
it is necessary to remember the dominance is not perfect. This, how- 
ever, makes no difference to the essential feature of Mendel’s discovery, 
which is, of course, the segregation in the gametes of the factors corre- 
sponding to the dominant and recessive characters. 
This differs only in degree from typical cases of blended 
inheritance. Yet it is clearly Mendelian, and -accords perfectly 
with Hatai’s idea of incomplete dominance. The segregation 
of pure colors in the gametes proves that there has been no inter- 
action affecting the essential character of the determinants. 
A case of complete blending in the first generation, followed 
by segregation in the second, is given by Castle (11, p. 138). 
This is in regard to length of ear in maize, and has been worked 
out by East. 
As to first crosses that breed true, it seems difficult to find such 
cases. The mulatto has been cited; but Davenport, handling the 
matter in a scientific manner, finds that there is segregation in 
the ratio of one to sixteen which can be brought into harmony 
with other Mendelian results by the assumption of four factors 
for black in the negro. 
