Alleles 17 



present in Drosophila melanog aster, and Waddington places the 

 figure for this fruit fly and also for lilies at possibly about 

 10,000. 



Alleles 



It was pointed out in Chapter 1 that the chromosomes are 

 always found in pairs in typical organisms. Since each chromo- 

 some always has an identical mate or homologue, the genes must 



Fig. 6. Male (left) and female of wild type of Drosophila melanogaster. 

 Note the sex combs on the legs of the male. Xl9- Camera lucida 

 drawings. 



also always be found in pairs. For example, in the long, V-shaped 

 chromosome of Drosophila melanogaster, known as chromosome 

 II, a gene about one-third of the distance from one end affects 

 the shape and size of the fly's wing. Since every normal fly 

 of that species has two of those chromosomes, every normal fly 

 must have two of those genes that affect the wing. In other 

 words, on each homologous chromosome there is a gene at a 

 particular place or locus that always affects the wing. How- 

 ever, the genes at that locus on the two homologues do not 

 always affect the wing in the same manner in all flies of that 

 species. In most flies, the two genes will be alike, and each 

 will act to produce a normal, or wild-type, wing in the adult 

 (Fig. 6). Adult flies possessing those two genes will have normal 

 wings. There are other flies, however, in which the two genes 



