1. How can the failure of the reciprocal crosses to produce the same result be explained? At same time Morgan was doing his work, Nettie Stevens noticed that female chromosomes of insects differed from male chromosomes

2. How can the failure of the reciprocal crosses to produce the same result be explained? –In males, one of a pair of chromosomes (the X chromosome) was much larger than the other (the Y chromosome) –Corresponding chromosomes in females were same size (pair of X chromosomes) –Morgan hypothesized that the gene controlling eye color is only found on the X chromosome (sex-linked or X-linked inheritance)

3. Mother Father Mother Father w+w+w+w+ wY Male gametes www+Yw+Y wY w+w+ Y Parental generation w+w+ w+ww+ww+Yw+Yww + wY w F 1 generation Female gametes F 1 generation Resulting phenotypes: All offspring are red-eyed Resulting phenotypes: F 1 females are red-eyed F 1 males are white-eyed FemalesMales Females Next, let the F 1 offspring mate: Figure 10.14a,b First half of reciprocal crossSecond half of reciprocal cross

4. Next, let the F 1 offspring mate: F 1 generation MotherFather w+w+ Y Male gametes Female gametes w+w+w+w+ w+Yw+Y wYw+ww+w w+w+ w F 2 generation FemalesMales Resulting phenotypes: All F 2 females are red-eyed 1/2 of F 2 males are red-eyed Figure 10.14a, lower w+ww+ww+Yw+Y