142 Dihybrids, Trihybrids, and Polyhybrids 



quired of them. Vegetative propagation is used exclusively 

 for commercial purposes whenever the plants are highly hetero- 

 zygous and whenever they are the kinds of plants that can be 

 reproduced vegetatively. Because they notoriously fail to breed 

 true, vegetative reproduction is used to propagate many of our 

 ornamental trees, all our important fruit crops, potatoes, some 

 nut trees, and some forest trees. 



The number of F2 phenotypes, when dominance is complete, 

 can be expressed by 2'^, where n represents the number of genes 

 that are heterozygous in the Fi and consequently the number of 

 genes by which the two homozygous parents differ. Similarly, 

 these Fi plants would produce 2"' gametes, and the number of 

 different genotypes and phenotypes from a testcross would also 

 be 2". The number of genotypes in the F2, and the number of 

 F2 phenotypes if dominance were not complete for any of the 

 genes involved would be S'*. The number of combinations of Fi 

 gametes, although not all of them different, would be 4", and, 

 similarly, the smallest possible number of F2 individuals which 

 would theoretically yield all the genotypes would be 4^*. 



Goodness of Fit 



Since trihybrid and polyhybrid ratios involve more than two 

 terms, the best method of determining goodness of fit is the one 

 used for dihybrids, except, of course, the number of degrees of 

 freedom is different. 



QUESTIONS AND PROBLEMS 



1. As shown by Mendel, in the garden pea, round seeds [W] are 

 dominant over wrinkled (w) and tall plant (D) is dominant over dwarf 

 {d). What are the genotypes and phenotypes of the Fi and F2 from 

 the cross WW DD X ww dd? 



2. What are the genotypes and phenotypes of the Fi and F2 from the 

 cross WW DD X WW ddl 



3. What are the genotypes and phenotypes of the offspring of the 

 two following crosses: WW Dd X ww dd and Ww dd X ww Dd? 



4. A man crossed a round-seeded, dwarf pea with a wrinkled-seeded, 

 tall. In the F2 he got 219 round, tall : 77 round, dwarf : 80 wrinkled, 

 tall : 24 wrinkled, dwarf. Determine x^ and from it decide whether 

 these two characters were showing independent assortment. 



5. A cross between a round tall plant and a round dwarf gave 121 

 round, tall : 124 round, dwarf : 42 wrinkled, tall : 37 wrinkled, dwarf. 

 What are the genotypes of the parents? Calculate goodness of fit by x^- 



