32 Genes and Characters 



gaster has been studied in considerable detail and is another 

 good example of the principle that some genes produce different 

 phenotypes if they act in different environments. Under the 

 environment in which the flies normally develop, homozygous 

 vg flies have very small and very poorly developed wings when 

 compared to those with the wild-type allele. Such flies are nor- 

 mally raised around "room temperature," or about 20° to 25° C. 

 If, however, homozygous vg flies are raised during the larval 

 stage at various temperatures from 14° to 34° C, the wings of 

 the adult will be very different in size and shape. The wings of 

 flies raised at 14° are smaller and are more poorly developed 

 than those of flies raised at 22°. At increasing larval tempera- 

 tures, the adult wings become increasingly larger and more and 

 more like the wing of a wild-type, Vg, fly. 



Penetrance and Expressivity 



The statement made in Chapter 2, that an individual that is 

 homozygous for a given recessive gene is phenotypically reces- 

 sive, is true for most of the genes that have been studied 

 thoroughly, but some genes prove to be exceptions. All flies of 

 Drosophila melanogaster that are homozygous for vestigial have 

 vestigial wings; but only about 70 per cent of the human beings 

 that are homozygous for the recessive gene for susceptibility to 

 poliomyelitis acquire the disease when exposed. Such genes as 

 vg are said to have complete penetrance whereas the gene for 

 poliomyelitis has only 70 per cent penetrance. In other words, 

 penetrance of a gene is the percentage of all the individuals pos- 

 sessing that gene and showing phenotypically the character that 

 is determined by it. If all homozygous recessives show the re- 

 cessive character, the penetrance of that gene is complete. If 

 almost but not quite all the individuals show it, the gene has a 

 high penetrance ; but if only a small percentage of the individuals 

 are phenotypically recessive, the gene has a low penetrance. 

 Dominant genes as well as recessives may differ in their pene- 

 trance. 



Genes are known with various intermediate degrees of pene- 

 trance. Because genes with high penetrance are the easiest to 

 work with, such genes have been the ones most frequently studied, 

 but in studying human genetics many genes are encountered 



