BOOTING. 



57 



In the first hjbrid generation all somatic cells are hybrid. The feather 

 inhibitor is present in the skin of the shank, but its strength is diluted by 

 the presence in the same cells of a protoplasm devoid of the inhibiting 

 property. Consequently, the prevailing grade of the boot falls from 6 

 (or 10) to 3. Despite the dilution, inhibition is complete in about 8 per 

 cent of the offspring (grade 0) ; in about 10 per cent of the offspring the 

 inhibiting factor is so weak that the boot develops as in the pure-blooded 

 Brahma. When, as a result of inbreeding F,'s, the feather-inhibiting factor 

 is eliminated from certain offspring, and such full-feathered birds are bred 

 together, we find a return of the mode to high numbers, such as 8 to 10 

 (but also 5) . There is no doubt of segregation. 



If a heterozygous bird be mated to a recessive the variability of the 

 offspring is much increased, owing to the occurrence in the progeny of 

 both DR and RR individuals (table 40). The offspring do not, to be sure, 

 fall into two distinct and well-defined types, as in typical Mendelian cases; 

 but one part of the range of variation agrees fairly with that of pure RR's, 

 i. e., Brahmas, and the remainder with that of hetei'ozygotes. And if we 

 make the division in the middle of the middle class, viz, 5, we shall find a 

 close approximation to that equality of extracted recessives and hetero- 

 zygotes that the segregation theory calls for (table 44). 



If, again, two heterozygous birds be mated, the variability is still 

 greater and the proportion of clean-footed offspring rises to 12 per cent. 

 These, together with some of the extremely slightly booted offspring, repre- 

 sent the extracted dominants. The whole range now falls into three regions 

 divided by the middle of grades 2 and 5. These regions correspond to the 

 DD's, the DR's, and the RR's of typical cases of segregation, and their 

 relative proportions are approximately as 25 : 50 : 25. 



Finally, if a heterozygote be mated to an extracted dominant the 

 proportion of clean-footed offspring rises to about 30 per cent and the whole 

 range of variation falls readily into two parts, the one comprising grades 

 and 1, the other grades 2 and above. The first includes the DD offspring; 



