SOME COMPLICATIONS OF MENDELIAN INHERITANCE 



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the coincidence of two or more non-allelic genes in the same organism. 

 Today many hundreds of such nonunit characters are known, and they 

 form such a common and important type of genetic phenomena that we 

 shall need to examine a number of representative examples. 



Walnut comb in fowls. The various breeds of the domestic chicken 

 show a variety of comb forms. Among these are rose comb, pea comb, 

 walnut comb, and single comb. Breeding experiments show that when a 

 homozygous rose-comb fowl is mated with a homozygous single-comb 

 fowl, the Fi are all rose comb and the F 2 show a ratio of 3 rose comb to 1 

 single comb, a typical monohybrid cross. The cross between pea comb 

 and single comb also forms a typical monohybrid cross, with an Fi all 



^ 



Single 

 Comb 



Pea Walnut 



Comb Comb 



Fig. 20.2. Some comb shapes in the domestic fowl. 



Rose 

 Comb 



pea comb and an F 2 with 3 pea comb to 1 single comb. When, however, 

 a homozygous rose-comb fowl is mated with a homozygous pea-comb 

 fowl, the F x are all walnut comb, and the F 2 gives 9 walnut comb, 3 rose 

 comb, 3 pea comb, and 1 single comb. Walnut comb is thus not a unit 

 character but the phenotypic expression of the genes for rose and pea 

 acting together. Here, if A is used to represent the gene for rose comb 

 and a, its recessive allele, and B is used to represent the gene for pea comb 

 and b, its recessive allele, we may diagram the cross as follows: 



X 



pea comb 

 aa BB 



Parent varieties Rose comb 



AA bb 



Fi Aa Bb 



Walnut comb 



F 2 9 (AA BB, AA Bb, Aa BB, or Aa Bb) Walnut 



3 (AA bb or Aa bb) Rose 



3 (aa BB or aa Bb) Pea 



1 (aa bb) Single 



Complementary genes — the 9:7 ratio. Among the many types of 

 sweet peas, there are two white varieties (the Emily Henderson and the 

 Blanche Burpee) that differ from one another in several structural charac- 

 ters. When these two varieties are crossed, all the Fi progeny have colored 

 (purplish red) flowers, and when the F x are allowed to self-fertilize, the 

 F 2 shows a phenotypic ratio of 9 colored to 7 white flowers. Further 

 breeding has shown that this is really a ratio of %6-M6 : ^f6 : K6> in 



