Questions and Problems 169 



11. Assume you have found a new gene in Drosophila melanog aster. 

 Describe in detail how you would demonstrate on which of the chromo- 

 somes that gene was located. Would this problem be easier in Droso- 

 phila than in most organisms? 



12. In Drosophila melanogaster, sepia eyes (se) are recessive to red 

 (.Se), and dichaete hairs (£)) are dominant to normal {d). They are on 

 chromosome III. If the cross Se D / Se D X se d / se d is made and the 

 Fi is testcrossed with the double recessive, 15 per cent of the offspring 

 are recombinations. Hairy body (h) is recessive to hairless. The cross 

 Se H / se hX se h / seh shows 0.5 per cent of recombinations. Gene h 

 is also on the third chromosome. Which genes do you think are farther 

 apart on the chromosome, Se and D, or >Se and H? Why? 



13. If a fly whose genotype is se D / Se d (see question 12) is crossed 

 with another fly of the same genotype, w^hat are the phenotypes of the 

 offspring? What are the phenotypes of the offspring if two flies whose 

 genetic constitution is Se h / se H are mated? 



14. In the domestic rabbit, yellow fat (y) is recessive to white (Y), 

 and brown hair and skin pigmentation [b] is recessive to black {B). 

 W. E. Castle showed that these genes are linked with about 27 per cent 

 crossing over. If a YY bb animal is crossed with a yy BB and the Fi 

 is testcrossed to a double recessive, what is the ratio of the offspring? 



15. If two Fi animals from question 14 are mated, what is the ratio 

 of the phenotypes in the F2? Although the original cross is in the re- 

 pulsion phase, why is the F2 ratio not 2:1:1:0? 



16. The gene for colored (C) in rabbits is dominant over colorless (c). 

 Genes C and Y (see question 14) are linked with 14 per cent crossing 

 over. Give the phenotypic ratios from the following crosses: 



c y / cy XCY/cy 

 CY I cy XCY/cy 

 Cy / cY Xc y / cy 

 Cy I cy Xc Y / cy 



17. In maize, shrunken endosperm {sh) is recessive to full (Sh) and 

 waxy endosperm (wx) is recessive to nonwaxy (Wx). A ShWx/ shwx 

 plant was testcrossed with a sh wx / sh wx. The offspring were 142 full, 

 nonwaxy : 63 full, waxy : 57 shrunken, nonwaxy : 138 shrunken, waxy. 

 Are these genes on the same or on different chromosomes? If on the 

 same, what is the percentage of crossing over? What w^ould be the off- 

 spring from the cross Sh wx / sh Wx X shwx/ sh wx ? 



18. In maize, colored aleurone (C) is dominant over colorless (c). 

 In the cross C sh / c ShX c sh / c sh (see question 17) the following 

 offspring were obtained : 638 colored, full : 21,379 colored, shrunken : 



