THE SPLIT OB Y COMB. 15 



" single comb. " But that this is not the explanation follows for two reasons : 

 first, that even in the F, generation slightly split combs are obtained, and, 

 second, that the offspring of the cup combs are much more variable than 

 those of slightly split combs (70 to 90 per cent median). What is strikingly 

 true is that, from 50 per cent up, as the proportion of the median element 

 in the parents increases the percentage of single-combed offspring rises. 



The matter may be looked at in another light. Median comb is 

 dominant over its absence. Typically, we should expect F^ to show a 

 single comb; the Y comb that we actually get is a heterozygous condition 

 due to the failure of the median comb to dominate completely. Typically 

 we should expect Fj to reveal 75 per cent single combs, of which 1 in 

 3 is homozygous and 2 in 3 are heterozygous. Owing to the failure of 

 single comb always to dominate completely in the heterozygotes, we 

 expect to find some of the 75 per cent with the Y comb. When in the 

 parents dominance has been very incomplete in the heterozygote (as is the 

 case in the per cent to 40 per cent median-combed parents) we find 

 it so in the offspring also and all heterozygotes show a Y comb of some 

 type. But when in the parents dominance has been strong in the heterozy- 

 gote (50 per cent to 90 per cent) it is so in the offspring also and only a part 

 of the heterozygotes show the Y comb; the others show the single comb 

 and thus swell the numbers of the single-combed type. The only objection 

 to this explanation is found in the reduction in the percentages of the no- 

 median type. Thus, adding together the homozygous and heterozygous 

 median-combed offspring and comparing with the non-median-combed, we 

 find these ratios : 



Parental per cent 0-40 50 60 70 80 90 



Ratio 75.3 : 24.7 76 : 23 79 : 21 82 : 18 78 : 22 86 : 15 



There is a great deviation from 25 per cent in the "non-median" 

 offspring of the 90 per cent parents, but in this particular case the total 

 number of offspring is not large, and the deviation has a greater chance of 

 being accidental. Altogether this explanation of the varying per cents of 

 single comb on the ground of inheritance of varying potency in dominance 

 seems best to fit the facts of the case. 



From the foregoing facts and considerations we may conclude that the 

 Y comb represents imperfect dominance of median over no-median comb; 

 that there is a fluctuation in the potency of the dominance, so that the 

 proportion of the median element varies from to over 90 per cent; that 

 the more potent the dominance of median element is in any parents the 

 more complete will be the dominance in the offspring and the smaller will 

 be the percentage of imperfectly dominant, or Y-combed, offspring. Domi- 

 nance varies quantitatively and the degree of dominance is inheritable. 



The index of heredity may be readily obtained in the familiar bio- 

 metric fashion from table 9. This I have calculated and found to be 0.301 ± 

 0.002. This agrees with Pearson's theoretical coefficient of correlation 



