64 



INHERITANCE OF CHARACTERISTICS IN DOMESTIC FOWL. 



even show the typical narrow nostril (fig. B, a). On the other hand, in the 

 narrow-nostriled races I have never obtained any such variation. The most 

 deviation that I have seen from grade 1 is found in my strain of Dark 



Brahma bantams that fre- 

 quently give grade 2. The 

 variability of the high nos- 

 tril, the stability of the low 

 nostril, is prima facie evi- 

 dence that the former is due 

 to the presence of a particu- 

 lar factor and the latter to 

 its absence. 



Next, the heterozygotes 

 of Fj (table 46), may be ap- 

 pealed to ; but they will give 

 no critical answer. For ex- 

 pectation, dominance being 

 imperfect, is that the hybrids 

 will be intermediate, and the 

 result will be the same which- 

 ever extreme grade is taken 

 as dominant. The empirical 

 mode in the distribution of 

 the offspring is at grade 2. 

 This implies much greater 

 imperfection of dominance 

 on the hypothesis that grade 

 10 is dominant than on the 

 hypothesis that grade 1 is 

 dominant; but this very fact 

 supports the former hypoth- 

 esis, since imperfection of 

 dominance is obviously a 

 feature of the character with 

 which we are dealing. 



The critical test is af- 

 forded by the F^ generation 

 (tables 48 and 49). By hy- 

 pothesis, 25 per cent of the 

 offspring are expected to be 

 pure ("extracted") recessives, and the same number pure dominants ; and also, 

 by hypothesis, the recessives are massed at or near one grade while the domi- ■ 

 nants are variable. Now, as a matter of fact, the upper 25 per cent range 

 over 5 to 7 grades, while the lower 25 per cent are nearly massed in grade 1 



Fig. B. — Polygons of frequency of grades of openness" of nostril in 

 off sprint; of various parents, a. Both parents pure bred dominants; 6, both 

 parents extracted dominants; e, one parent heterozygous, the other a domi- 

 nant; d, both parents heterozygous; e, dominant by recessive; f, heterozy- 

 gous by recessive; g, heterozygous by extracted recessive; k, extracted 

 recesbives; i, heterozygous by dominant; k, both parents second generation 

 hybrids. 



