SINGLB-COMB MINORCA AND WHITE-CKESTED POWSH. 



II 



case, a repressed potentiality toward splitting of the comb at its posterior end. 

 In general, then, the hybrid condition between single comb and supranasal 

 papillae is a Y-shaped comb* and there is an absence of dominance. 



Second generation. When the birds with Y-shaped combs were interbred 

 the loi offspring were of three types, as follows : 



Expectation in this case depends on the theoretical view we take of the 

 nature of the unit characters involved. We may assume that single comb and 

 V-comb are the allelomorphs and split comb a heterozygous type, constantly 

 reproducing itself. On this assumption we should expect the proportion 

 given in column a of the foregoing table. On the other hand, following the 

 method of interpretation suggested by Cuenot (1903) and Correns (1905) in 

 such cases, we may assume two pairs of allelomorphs, namely, (la) median 

 comb and (i^) no median, and (2«) no splitting and (2^) splitting. Taking 

 median comb and splitting as dominant, the single comb combines the two 

 characters : median comb, no splitting ; the V comb combines : no median 

 comb, splitting. In generation Fj the four characters are combined : median 

 (no median), splitting (no splitting), and the comb is Y-shaped, the char- 

 acteristics put in parenthesis being recessive. The Fj generation follows 

 the law of inheritance in dihybrids : 



Per cent. Class. 



25 % {^Median X median). 



50 % {Median X 

 dian) 



(Splitting X splitting ^%\ -.ai/a 



\ 2 [Splitting X no-splitting) . . 12^ / ^**» "" ■ 



(a) 

 (*) 



ie) 

 {d) 



^5%^o.median^no.me.^^^J^^^!^^^^^^-- ^%}rW.%.. (.) 



"'^'^^' (. S^io-splitting X no-splitting. .. 6^ (/) 



Summing up, out of every 16 individuals we should expect: 



( 'iHo-splitting X no-splitting. . 6%. 



Splitting X splitting i2}i \ ,^ 



2 {Splitting X no-splitting) . . 25 / 37/z 7« • • 



yio-splitting X no-splitting. . 12^ 



Relative 

 frequency. 



3 



I 



* For further discussion of the Y-shaped comb see page 65. 



