144 Dihybrids, Trihybrids, and Polyhybrids 



15. In soybeans, broad leaf is incompletely dominant to narrow; the 

 heterozygote is intermediate. Purple flower is dominant to white. What 

 is the phenotypic F2 ratio if a broad-leaved plant which is homozygous 

 for purple flowers is crossed with a narrow-leaved, white-flowered plant ? 

 What would be the offspring of a cross between the Fi and a narrow- 

 leaved, white-flowered plant; of a cross between the Fi and an interme- 

 diate-leaved, white-flowered plant; of a broad-leaved plant that was 

 homozygous for red flow-ers? 



16. In soybeans, how would a breeder obtain true-breeding races 

 of broad-leaved, white-flowered plants; of intermediate-leaved, white- 

 flowered plants; of narrow-leaved, white-flowered plants? Assume that 

 he started with the two plants in question. 



17. The statement is made that if dominance is incomplete for each 

 of two pairs of genes that are in different chromosomes, the phenotypic 

 ratio in the F2 is the same as the genotypic. Why should this be true? 



18. In Capsella, various F2 seeds are sown to produce an F3 genera- 

 tion. What would be the genotypes of the F2 plants that produced F3 

 families that segregated as follows: (a) 3 heteris : 1 tenuis; (b) all 

 tenuis; (c) all simplex; (d) 3 heteris : 1 rhomboidea; (e) all heteris; 

 (/) 3 tenuis : 1 simplex; (g) 9 heteris : 3 tenuis : 3 rhomboidea : 1 sim- 

 plex; (h) 3 rhomboidea : 1 simplex? 



19. A BlblSsss fly (as in question 12) is testcrossed with the double 

 recessive. The offspring are selfed. What are the phenotypes from each 

 of the selfed offspring? 



20. In Drosophila melanogaster, the gene for purple eye ipr) is reces- 

 sive to the gene for red and is in chromosome II; the gene for hairless 

 {H) is in chromosome III, and the gene for bent wing {bt) is in chromo- 

 some IV. What are the genotypes and phenotypes of the Fi and F2 

 from the cross PrPr HH BtBt X prpr hh btbt ? 



21. What are the genotypes and phenotypes from a backcross of the 

 Fi in question 20 to the triple recessive? 



22. In the F2 of the cross, AA BE CC DD EE X aa bb cc dd ee, what 

 proportion will be homozygous for all the dominant genes? What pro- 

 portion will be phenotypically dominant for all the genes? 



23. What are the gametes produced by each of the following? 



Aa Bb CC dd Ee 

 Aa Bb CC Dd ee 

 aa Bb cc DD ee 

 aa bb cc DD EE 

 aa BB Cc Dd Ee 



