Independent Recombination by Nonalleles 



55 



4.18. Differentiate between dominance and epistasis. 



4.19. What is the maximum number of genotypes possible in the progeny if the parents 

 are monohybrids? 



4.20. Two green corn plants are crossed and produce offspring of which approximately 

 '■'\,; are green and 7 ic are white. How can you explain these results? 



4.21. Does gene interaction occur only when identical monohybrids (or identical di- 

 hybrids) are crossed? Explain. 



.MA. 



WALNUT COMB 



SINGLE COMB 



4.22. A chicken from a pure line of "rose" combs is mated with another individual 

 from a pure line of "pea" combs (see the accompanying illustration). All the 

 Fj show "walnut" combs. Crosses of two F, "walnut" type individuals provide 

 F 2 in the ratio 9 "walnut": 3 "rose": 3 "pea":l "single." Choose and define gene 

 symbols to provide a genetic explanation for these results. 



4.23. Three walnut-combed chickens were crossed to single-combed individuals. In 

 one case the progeny were all walnut-combed. In another case one of the progeny- 

 was single-combed. In the third case the progeny were either walnut-combed or 

 pea-combed. Give the genotypes of all parents and offspring mentioned. 



4.24. Matings between walnut-combed and rose-combed chickens gave 4 single, 5 pea. 

 13 rose, and 12 walnut progeny in F v What are the most probable genotypes 

 of the parents? 



4.25. A mating of two walnut-combed chickens produced the following F x with respect 

 to combs: 1 walnut, 1 rose, 1 single. Give the genotypes of the parents. 



4.26. The hornless, or polled, condition in cattle is due to a completely dominant gene, 

 P, normally horned cattle being pp. The gene for red color (R) shows no domi- 

 nance to that for white (/?'), the hybrid (RR') being roan color. Assuming in- 

 dependent segregation, give the genotypic and phenotypic expectations from the 

 following matings: 



(a) Pp RRX pp RR' 



(b) Pp RR' X pp RR 



(c) Pp RR' X Pp RR' 



(d) hornless roan (whose mother was horned) X horned white 



4.27. When dogs from a brown pure line were mated to dogs from a white pure line 

 all the numerous F] were white. When the progeny of numerous matings be- 

 tween Fj whites were scored there were 118 white. 32 black, and 10 brown. 

 How can you explain these results genetically? 



4.28. Using your answer to the preceding question, give the phenotypic and genotypic 

 expectations from a mating between the following: 



(a) a black dog (one of whose parents was brown) and a brown dog 



(b) a black dog (one parent was brown, the other was black) and a white dog 

 (one parent was brown, the other was from a pure white strain) 



