HEREDITY IN SOMATIC CELLS 361 



rates of growth of the daughter cells. If crossing over occurs in region I, 

 a single yellow spot would be expected. Since the genetic distance be- 

 tween y and sn is smaller than that between sn and the centromere, 

 homozygosity for y alone would be rarer than homozygosity for both 

 genes. Homozygosity for sn would be achieved by double crossing over 

 in regions I and II, and the extreme rarity of single singed spots is in ac- 

 cord with expectation. Somatic crossing over is another instance where 

 the reciprocal products of one exchange can be recovered, allowing one 

 more demonstration that crossing over occurs in the four-strand stage. 

 The existence of twin spots shows that the characters y and sn are 

 autonomous in development; neither type of cell prevents the characteris- 

 tic expression of the other, despite their adjacent location in small 

 patches. This is frequently but not always the case. In Drosophila 

 some individuals are gynandromorphs, female on one side of the body 

 and male on the other. This is due to the loss of an X chromosome from 

 one of the two cells produced by the first division of an XX zygote. The 

 cell derivative that is XX in genotype forms female tissue; that which is 

 X forms male (Figure 12.3). Apparently in Drosophila intracellular 

 factors determine sex and the eosin eye-color character as well. But 

 certain eye pigments are under hormonal control. A gynandromorph in 

 which the female tissue is heterozygous for vermilion eyes, +/v, and 

 where the male tissue has only the v gene, does not show a vermilion eye 

 on the male side. This situation was brilliantly explored by Beadle and 

 Ephrussi, who began their studies of this problem in Paris. They im- 

 planted the embryonic eye buds from vermilion larvae into the body 

 cavities of wild-type larval hosts where the implants received a substance 

 that caused the eyes to develop normal pigmentation. It was eventually 

 shown that the diffusible hormone was kynurenine, and further work has 



FIGURE 12.3. A gynandromorph of Drosophila melanogaster. The left side of the 

 head and thorax is wild type and female; the right side is eosin, forked and male 

 (except for a patch of wild type tissue in the right eye) (from Sturtevant and Beadle, 

 1939, An Introducfion to Genetics, Philadelphia, Saunders). 



