130 < HAPTER 9 



9.21. How would you prove genetically that the last division in a spore sac of Ncuro- 

 spora is a mitotic one.' 



9.22. In the absence ot crossing over, could you determine whether the alternatives 

 for two different traits were due to a single pair of genes or to two pairs of 

 linked genes? Explain. 



9.23. Draw an attached-X chromosome of Drosophila heterozygous both tor y and 

 for in. Show the kinds of gametes which could be obtained after: 



(a) No chiasma 



(b) One chiasma between the nonallelic genes 



(c) One chiasma not between the genes mentioned 



9.24. Suppose one member of a long pair of chromosomes in a plant has a large 

 knob at one of its ends, and the other has a small knob at the opposite end. 

 Suppose, moreover, that there is also a shorter pair of homologs. one member 

 terminating with a large knob, and the other at the opposite end with a small 

 knob. What combinations and configurations would you expect to readily find 

 in the gametes of this individual? 



9.25. What reasons can you present for believing that germ-line crossing over is based 

 neither upon premeiotic nor upon postmeiotic events? 



9.26. Calculate the number of crossover units between black body (b) and dumpy 

 wings (dp) in the following Drosophila crosses: 



(a) P! pure black X pure dumpy 

 P_, Fj 9 9 X black dumpy $ $ 

 F-, wild-type 272 



" black' 774 



dumpy 801 



black dumpy 239 



(b) Pj black dumpy X pure wild-type 

 P 2 Fj 9 9 X black dumpy $ $ 

 F., wild-type 360 



black 103 



dumpy 97 



black dumpy 314 



9.27. What phenotypic results would you expect in 9.26(a) and (b) if the reciprocal 

 mating had occurred in P_,? 



9.28. Test statistically the F.> results of 9.26(a) with those expected from independent 

 segregation. 



9.29. Test statistically whether the F L , results in 9.26(a) and (b) differ significantly. 



9.30. A trihybrid Aa Bb Cc is test crossed to aa bb cc. The Fj show that the trihybrid 

 produced the following gametes: 



29 ABC 21 abc 



235 ABc 215 abC 



210 Abc 239 aBC 



27 AbC 23 aBc 



(a) Which loci are linked and which are segregating independently? 



(b) Write the genotypes of both parents in view of your answer to (a). 



(c) Give the percentage of crossovers wherever applicable. 



