184 



Plant-Breeding 



B 



Rose and pea combs behave as simple dominants to 

 single comb, segregating in the F% generation in the nor- 

 mal 3 : 1 ratio. What happens when the two dominants 

 are bred together? It was found that a third type ap- 

 peared as an FI hybrid, the so-called walnut comb. 

 When these FI hybrids were bred inter se, four types of 



combs were found 

 among the F 2 prog- 

 eny ; namely, walnut, 

 pea, rose, and single 

 in the approximate 

 ratios of 9:3:3:1 

 respectively. What 

 is the explanation 

 of this unusual phe- 

 nomenon ? 



We are evidently 

 concerned with two 

 allelomorphic pairs 

 of characters, which 

 are the presence-and- 

 absence of rose comb 

 (R) and the presence-and-absence of pea comb (P). 

 As suggested by Punnett, let us denote the rose comb 

 by RRpp (containing the presence of rose and the 

 absence of pea) and the pea comb by rrPP. When these 

 are crossed together, the zygote RrPp results. This 

 differs from either and has a walnut comb. When these 

 FI hybrids are crossed together (RrPp X RrPp), the fol- 

 lowing results may be graphically expressed in the series 

 of squares : 



FIG. 46. Fowls' combs: A, pea; B, rose 

 C, single ; D, walnut. 



