Linkage and Crossing Over Between Genes 129 



9.5. Name all the processes so far discussed which lead to genetic recombination. 



9.6. Do you think that one of the main principles in this chapter is that chromosomes 

 contain more than one gene? Explain. 



9.7. In light of your present knowledge how would you proceed to state a "law of 

 independent segregation"? 



9.8. What evidence do you have that crossing over does not involve the unilateral 

 movement of one gene from its position in one chromosome to a position in the 

 homologous chromosme? 



9.9. Does crossing over always result in genetic recombination? Explain. 



9.10. In what respect do you think the development of the principles of genetics in 

 this text would have been affected had the first two pairs of genes (Rr and Yy), 

 simultaneously studied in crosses, been linked? 



9.11. Assume that the gene for woolly hair (Chapter 3, p. 38) is located autosomally. 

 A nonwoolly-haired, nonhemophilic man marries a woolly-haired, nonhemophilic 

 woman. They have a woolly-haired, hemophilic son. Give the genotypes of all 

 three individuals, and the genotypes and frequencies of the gametes usually pro- 

 duced by the son. 



9.12. How would you defend the conclusion that the point of crossing over is located 

 at exactly equivalent positions on the two homologs? 



9.13. What are the relative frequencies of the phenofypes and genotypes expected from 

 a mating between two Drosophila: dihybrid vg+/+b? 



9.14. Two dihybrids, a +/+ b, for autosomally linked mutants a and b in Drosophila, 

 are crossed. If 2p equals the frequency of noncrossover eggs and 1— 2p equals the 

 frequency of crossover eggs, and if p < 0.5, give the relative frequencies of the 

 phenotypes expected among the F, of this cross. 



9.15. What result would you expect from 9.14 if the cross were between dihybrids 



a b/+ +? 



9.16. A wild-type Drosophila female, whose father had crossveinless wings and mother 

 had yellow body color, is mated to a yellow male. Give the relative frequencies 

 of genotypes and phenotypes expected in the F v 



9.17. A mating in Drosophila produces the results shown below. Give the genotypes 

 of the parents, and determine which genes are linked and which are not. 



Sons Daughters 



75 wild-type 92 wild-type 



70 yellow body color, white-eyes 75 white 



21 yellow, white, vestigial wings 28 vestigial 



27 vestigial 20 white, vestigial 



2 yellow 



1 white, vestigial 



9.18. What is the relationship in Neurospora between crossing over and first and second 

 meiotic division segregation? 



9.19. In tomatoes, the gene for tall ( + ) is dominant to short (s), and the gene for 

 smooth epidermis ( + ) is dominant to rough (/-). A cross between two plants 

 produces 208 tall smooth, 9 tall rough, 6 short smooth, 195 short rough. Give 

 the genotypes of the parents. 



9.20. What is the percentage of crossovers for two loci, in a species in which both 

 sexes undergo crossing over with equal frequency, if a mating between identical 

 dihybrids (Ab/aB) gives four equally viable classes of offspring, the smallest 

 class comprising 1% of all offspring? 



