24 Chromosomes and Genes 



mosaic of these unit characters. Such a position is a very ex- 

 treme one and, for most characters, is undoubtedly unsound. 



In chromosome I in Drosophila melanogaster there is another 

 pair of allelic genes which affect the shape of the wing. Flies 

 homozygous for the gene m have miniature wings (Fig. 7) much 

 like the wild-type wing but considerably smaller. Wild-type 

 flies have the gene M which is dominant over the gene for minia- 

 ture wings. We had previously stated that wild-type flies had 

 the Vg gene and now we say that they have the M gene. Is this 

 inconsistent? Actually the statements that Vg produces a wild- 

 type wing and that M produces a wild-type wing are erroneous 

 although such statements are frequently made by people who 

 understand the correct situation. The wild-type wing is not 

 produced by Vg alone or by M alone but is the result of both 

 genes acting together. In other words, the wild-type wing is 

 not a unit character because it is not produced by only one gene. 

 Similarly, the miniature wing is not the result of m alone but of 

 m working in conjunction with Vg. Gene Vg, therefore, does not 

 produce a normal wing; it merely produces a normal wing if M 

 is present. Gene vg, likewise, produces a vestigial wing in the 

 presence of M but produces a type known as vestigial-miniature 

 if m is present. Practically, the last type is indistinguishable 

 phenotypically from a vestigial. 



The early notion of unit characters certainly cannot apply in 

 this situation, but it is easy to see how it might have devel- 

 oped. If a geneticist has only wild-type and vestigial stocks of 

 flies, he has flies which all have the M gene. Since his wild-type 

 flies are VgVg MM genotypically and his vestigial flies are 

 vgvg MM, the M gene is not important, and apparently wing 

 shape is determined by Vg and vg alone. Unless he later ob- 

 tained some miniature flies, the presence of gene M would never 

 be detected and normal and vestigial wings would act simply as 

 unit characters. If, now, another geneticist had only wild-type 

 {VgVg MM) and miniature {VgVg mm) flies, the presence of Vg 

 would never be revealed, and wing shape would appear to be 

 conditioned only by the genes M and yn. If these two geneticists 

 then got together and traded their stocks, by making the appro- 

 priate crosses they would learn of the presence of both pairs of 

 genes and would realize that wild-type, vestigial, and miniature 



