340 Interaction of Genes 



16. In barley, purple is dominant over white. Two pairs of cumulative, 

 duplicate genes, Pipi and P2P2, are involved. Two light purple plants, 

 PiPi P2P2 and pipi P2P2, were crossed. Explain the F2 segregation into 

 a 9 : 6 : 1 ratio. 



17. Assume that there are three pairs of noncumulative duplicate 

 genes, Aa, Bb, and Cc. The cross A A BB CC X aa bb cc was made and 

 the Fi selfed. What ratio would be obtained in the F2? 



18. In Nilsson-Ehle's wheat, what would be the phenotypes of the 

 following crosses? 



(a) RiRi r2r2 X riri R2R2 



{b) RiRi r2r2 X RiRi r2r2 



(c) R\R\ r2r2 X R\r\ R2r2 



(d) RiTi R2r2 X riTi r2r2 



(e) RiTi R2R2 X RiTi r2r2 



19. When Nilsson-Ehle crossed an RiRi R2R2 with an riVi r2r2 and 

 raised the F2 generation, he obtained a ratio of 1 : 4 : 6 : 4 : 1. Compare 

 this ratio with the coefficients of the terms obtained by expanding the 

 binomial (a + 6)"*. Is there any possible connection here? 



20. In Primula, what phenotypes would be obtained from the follow- 

 ing crosses? 



(a) fern X oak (d) fern-oak X tongue 



(b) fern X tongue (e) fern-tongue X oak 



(c) oak X tongue (/) oak-tongue X fern 



21. In Primula, what phenotypes are obtained from the following 

 crosses? 



(a) yij 00 TT MpMp X YY ooTT mpmp 

 (6) Yy Oo Tt X Yy Oo Tt 

 (c) Yy Oo tt X tjy 00 Tt 



22. In some strains of wheat, three pairs of duplicate, cumulative, 

 incompletely dominant genes are present. The cross RiRi R2R2 R3R3 

 X riTi r2r2 tsTz gave a ratio of 1 : 6 : 15 : 20 : 15 : 6 : 1. If a stands for 

 one degree of redness and b one degree of whiteness, so that the formula 

 a^6^ would mean a redder type than a^b'^, show that this same ratio can 

 be obtained by expanding the binomial (a -\- b)^. 



23. In Nemesia, orange, 0, is dominant over white, 0, nonbuff, Bu, is 

 dominant over buff, bu, and nonpale-upper, P, over pale-upper, p. Gene 



