54 



CHAPTER 7 



ROSE COMB 



PEA COMB 



WALNUT COMB 



FIGURE 7-4. Comb types in chickens. 



phenotypic expression, and not genotypic 

 recombination, is tested by phenotypes. In 

 fact it is true that segregation and independ- 

 dent segregation were first proven despite 

 the misleading phenotypic simphfications 



wrought by the occurrence of dominance, 

 and that the principle of independent segrega- 

 tion could also have been first proven from 

 crosses involving epistasis or apparently novel 

 phenotypes. 



SUMMARY AND CONCLUSIONS 



The phenotypic expression of genes depends upon their alleles, insofar as dominance is 

 involved, and upon nonalleles, insofar as epistasis (including superposition and antagonism) 

 and the production of apparently novel phenotypes are involved. The absence both of 

 dominance and epistasis will always produce phenotypic ratios which directly represent 

 genotypic ratios, whereas the occurrence of one, the other, or both reduces the number of 

 phenotypic classes. In any case, segregation and independent segregation, being genie 

 properties, are totally uninfluenced by the manner whereby genes do or do not come to 

 phenotypic expression. 



