Pleitropy 21 



example, the genes A and a stand, respectively, for peas with 

 round and with wrinkled seeds; B and b symbolize genes for 

 yellow and green cotyledons; and C and c represent genes for 

 gray-brown and white seed coats. In organisms in which over 

 twenty-six pairs of alleles have been discovered, the alphabet is 

 insufficient to symbolize all the genes. It was soon recognized 

 that various combinations of letters were necessary to provide 

 symbols for such large numbers of genes. Thus in Drosophila 

 melanog aster, v stands for the vermilion-eye gene, vg for the 

 gene for vestigial, and ve for the gene for veinlet. When this 

 system was introduced, the second letter was written as a sub- 

 script, as Vg, Vg, Ve, and Ve, but the present system was adopted 

 because it was easier to set in type. 



Pleitropy 



Since the genes Vg and vg determine the shape of the wing if 

 all other genes that affect the wing are wild-type genes, it might 

 be inferred that they have no effect except on the shape of the 

 wing. This is not true for actually one gene may affect many 

 parts of the body. Some parts are affected in a more striking 

 manner than others, however, and a gene is usually named from 

 the most striking effect that it produces. In Drosophila melano- 

 gaster, the gene Delta produces a delta-like expansion where 

 the longitudinal vein of the wing joins the marginal vein; but 

 it also produces smaller, rougher eyes, modifications of the 

 bristles, and still other changes of an even less striking nature. 

 It receives its name and symbol, Dl, however, from one of its 

 most striking effects. Similarly the genes at the locus for white 

 eye affect not only the color of the eye but also the shape and 

 color of the spermatheca. Another rather curious case of pleio- 

 tropy, or the multiple effects of genes, is shown by the white- and 

 red-eye genes. Flies with yellow bodies may have either red or 

 white eyes. In the white-eyed flies, the gene for eye color affects 

 the yellow pigment also, and affects it in such a way that it is 

 easily extracted with alcohol. 



Often cited as examples of pleiotropy are instances of pigment 

 formation in plants where a certain gene may produce a red 

 pigment in several organs such as flowers, stems, and leaves. 

 Considering these as separate organs, one may be inclined to 

 think of the gene as simultaneously producing red flowers, red 



