168 Linkage and Crossing Over 



age of recombinations tends to increase. Inversions of a part of 

 one chromosome may inhibit crossing over, and anything that 

 tends to affect normal meiosis will have an effect on crossing 

 over. 



QUESTIONS AND PROBLEMS 



1. In the Emily Henderson sweet pea, Bateson and Punnett found 

 that purple flowers (Bl) were dominant to red {bl) and elongated pollen 

 (L) was dominant to round (l) . When a homozygous purple long plant 

 was crossed with a red, round F2 ratios were 177 purple, long : 15 purple, 

 round : 15 red, long : 49 red, round. Could this be due to independent 

 assortment? If not, why not? If not, how could this F2 ratio be ac- 

 counted for? 



2. By figuring the value of x^ in the previous problem, what is the 

 probability that this is a 9 : 3 : 3 : 1 ratio? 



3. If the genes bl and I are linked with 12.5 per cent crossing over, 

 what would be the percentage of recombinations if the Fi in problem 1 

 was backcrossed to a bl I / bl I plant? 



4. If the genes in problem 1 are linked with 12:5 per cent crossing 

 over, what would be the Fi gametes and the progeny of a testcross if the 

 Pi plants were Bll/ Bll and blL/blL? Show by a diagram. Is this 

 the coupling or the repulsion phase? 



5. What would be the ratio in the F2 from selfing the Fi plant of the 

 previous question? 



6. Is the redupHcation hypothesis or the theory of linkage more in 

 agreement with cytological phenomena ? Which agrees more closely with 

 our observations on gametogenesis and sporogenesis ? Why? 



7. Genes A and B are linked with 10 per cent crossing over. It has 

 been calculated that an average of one chiasma forms between genes B 

 and C in 20 per cent of the sporocytes. Do more chiasmata form be- 

 tween B and C or between A and B? Why? 



8. In an Fi plant whose genotype is LV / Iv a chiasma forms be- 

 tween L and V in 40 per cent of the sporocytes. If this plant is test- 

 crossed to the double recessive, what ratio would be obtained? 



9. Do the ratios obtained as the result of genetic crossing over and 

 the cytological observation of chiasmata seem to have any direct connec- 

 tion? How? If only one of these phenomena were known, could the 

 other be inferred on theoretical grounds ? Would such an inference prove 

 its existence? 



10. We have pointed out on many occasions that the genes are located 

 on the chromosomes. This was not always known. Do you think that 

 the discovery of linkage and of chiasmata might have helped to prove 

 the connection between genes and chromosomes? Explain. 



