60 



INHERITANCE OF CHARACTERISTICS IN DOMESTIC FOWL. 



We have next to consider the grade-distribution of the offspring of the 

 narrow mated with the wide nostril. 



♦Extracted DXR. 



Table 46 gives us a picture of the nature of the dominance in this 

 case. At first sight the narrow nostril, grades 1 and 2, including 57 per cent 

 of the offspring, appears to be dominant. But, as later evidence shows, it 

 is recessive. The wide nostril is dominant, but so imperfectly that only 

 10 per cent have a nostril above one-half open. 



Let us now consider the distribution of nostril form in families whose 

 parents are hybrids of the first or later generation, crossed respectively on 

 recessives, heterozygotes, and dominants (tables 47-49). 



Table 47. — Distribution of frequency of the different grades of "openness" of nostril when one parent is 

 heterozygous and the other recessive, i. e., with closed nostril (,DBxR). 



The study of the tables 45 to 54 establishes the following conclusions: 



First, high nostril is dominant. This means that there is a factor that 



inhibits the development of the narial flap. In the absence of such a factor 



the flap goes on developing normally. This hypothesis is opposed to the 



conclusion that I reached in my report of 1906 (pp. 68, 69). I there said: 



A close agreement exists between the percentage obtained in each generation 

 and the expectation of the Mendelian theory, assuming that narrow nostril is dominant. 

 The statistics do not, however, tell the whole story. In 36 per cent of the cases in the 

 Fi generation the nostril was wider than in the " narrow " ancestor. Even in the F, gene- 

 ration nearly half of the " narrow and intermediate " were of the intermediate sort. This 

 intermediate form is evidence that dominance is imperfect and segregation is incomplete. 



