82 



CHAPTER 12 



The offspring appear in a 1 : 1 : 1 : 1 ratio, 

 demonstrating that the two pairs of genes are 

 segregating independently. The same result 

 and conclusions obtain from the cross of 

 ci+ci e e X ci ci e^e. Consider next crosses 

 in which sex and wing venation are studied 

 simultaneously both in ci^ci XX by ci ci XY 

 and in ci+ci XY by ci ci XX. The result found 

 in both cases is a 1 : 1 : 1 : 1 ratio of cubitus 

 male, cubitus female, normal male, normal 

 female. Here, then, the sex genes segregate 

 independently of the genes for cubitus. 

 Therefore, by our hypothesis, the ci alleles 

 are located autosomally. 



Similarly, e+e XX by e e XY or e+e XY by 

 e e XX also gives a 1 : 1 : 1 : 1 ratio, show- 

 ing that ebony is located autosomally. It may 

 be concluded also, since ebony and cubitus 

 are found to segregate independently of each 

 other, that they are located on different pairs 

 of autosomes. 



Note that the last two types of crosses 

 could be described as backcrosses of a mono- 

 hybrid which wei-e made reciprocally, that is, 

 one time the male was the hybrid parent, and 

 the other time the female was, even though 

 we cannot yet specify that male is XX or XY. 

 In either case a 1 : 1 ratio is found among the 



FIGURE 12-2. Results of 

 buckcrossing a dihybrid. 



+ + 

 ci ci e e x 



CI CI e e 



^, 7i 



sons, and a 1 : I ratio among the daughters. 

 We may now reconsider the meaning of the 

 statement made earlier (p. 9), that all crosses 

 gave the same results when made reciprocally. 

 This meant that the observed phenotypes and 

 their proportions were the same for sons as 

 for daughters, even though the crosses were 

 made reciprocally. So, for example, a cross 

 of the dihybrids ci+ci e+e X ci+ci e+e gave 

 a 9 : 3 : 3 : 1 ratio among the sons and a 

 9:3:3:1 ratio among the daughters be- 



