HEREDITY 



589 



Figure 416. Siamese cats. Pigment develops on the extremities as a result of temperatures 

 lower than the rest of the body. (Photo by Chandoha, courtesy of Puss'n Boots Cat Food.) 



perimentation has shown that whether the 

 gene responsible for the development of the 

 black extremities comes to expression de- 

 pends on the temperature in which the fur 

 develops. 



Reduced penetrance is very common in 

 man. Dominance with reduced penetrance 

 has been found in human pedigrees of short 

 metacarpals (brachymetapody) (Fig. 417), 

 webbed toes (zygodactyly), hay fever, and 

 many others. 



The discussion of penetrance thus far has 

 been limited to the dominant gene, but a 

 recessive gene which does not always pro- 

 duce a detectable effect in the homozygous 

 condition is also said to show reduced pene- 

 trance. 



Expressivity of the gene 



Expressivity refers to the variations in the 

 way in which a particular gene expresses 

 itself. Some genes have rather constant ex- 



pressivity, regularly producing a definite de- 

 gree of the trait; this is true of the genes for 

 the A-B-O blood groups. On the other hand, 

 many genes produce var}'ing degrees of a 

 trait in different individuals. For example, 

 the gene for hay fever may cause only a little 

 sneezing in one person and severe symptoms 

 in another. The causes of variable expres- 

 sivity of the gene are probably the same as 

 those for varying degrees of penetrance. 



Multiple alleles 



Genes are probably composed of complex 

 protein substances. The opportunities for 

 change in such complex structures are 

 greater than in simple ones. A single gene 

 need consist of only one molecule of a pro- 

 tein to possess the capacity to change in 

 several ways. 



We have, up to now, considered condi- 

 tions for which there are only two genes. 

 Since all individuals have two genes at each 



