200 Chromosome Maps 



the ages of four and twelve. The gene involved is a recessive, 

 incompletely sex-linked lethal. 



QUESTIONS AND PROBLEMS 



1. Why does it seem logical that the number of linkage groups should 

 equal the haploid number of chromosomes? 



2. What is the difference between a "genetic" and a "cytological" 

 map ? 



3. What are the fundamental assumptions in plotting genes on a 

 linkage map? 



4. If the order of the genes on a genetic map corresponds with the 

 order on a cytological map, why do the distances not always correspond ? 



5. Look up and discuss genetic methods used in constructing linkage 

 groups other than the method based on the frequency of crossing over. 



6. In Primula, short style (»S) is dominant over long (s), blue flower 

 (B) over nonblue (6), green stigma (G) over colored (g), and light red 

 leaf (L) over dark (/) . In the backcross of SBGL/sbglXsbgl/ 

 sb g I, the following phenotypes (male only) were obtained: 



^58SBGL ISb Gl Q5Sb g I 



7SBGI 2Sb g L 29 s B g I 



5SBg L Os BGl 3s b Gl 



25Sb GL Os Bg L 9s b g L 



S2s BGL 2118 b GL AQ7 s b g I 



270SBg I 



Determine all the linkages involved and plot the genes on their chromo- 

 some or chromosomes. 



7. Calculate the linkage values in problem 6. Using these values, cal- 

 culate the expected backcross ratio. Compare your expected ratio with 

 the observed ratio. 



8. Assume that you have conducted an experiment in Drosophila 

 melanogaster involving the following genes: cur (curved wing) ; d (dachs 

 body) ; bt (bent wing) ; se (sepia eye) ; b (black body) ; ca (claret eye) ; 

 st (scarlet eye); S (star eye); pr (purple eye); rt (rotated abdomen). 

 Assume that you obtained the following crossover values: 



cur — st 14 per cent 



d — b 18 per cent 



S — b 47 per cent 



se — rt 11 per cent 



se — cur 4 per cent 



*S — d 30 per cent 



se — st 18 per cent 



