590 



COLLEGE ZOOLOGY 



Figure 417. Short metacarpal (fourth of left hand) is due to a dommant gene which shows 

 reduced penetrance. Some individuals in the heterozygous condition do not show the trait. 

 Arrow points to short metacarpal. (From Karl A. Stiles, "The Inheritance of Brachymetapody." 

 /. of Hered., 30:87-91, 1939.) 



locus (one in each of the homologous 

 chromosomes), one may have two of the 

 same kind, or one of each of the two dif- 

 ferent kinds. Should one or both of these 

 genes undergo a mutation (change) there 

 will be 3 or more different kinds of genes for 

 this particular locus on homologous chromo- 

 somes. Such genes are called multiple alleles. 

 Any one of the genes is allelic to any other 

 gene at that locus. 



One of the best-known groups of multiple 

 alleles concerns eye color in the fruit fly. 

 The wild type is red, but the gene for this 

 color has mutated to white (Fig. 440), buff, 

 cherry, and at least 10 others. A fly must 

 have any two of these genes— two genes for 



red, or one for red and one for white, and 

 so on. 



ABO blood groups * 



In man, the inheritance of A-B-O blood 

 groups (p. 527) is determined by three such 

 allelic genes. One of these genes causes the 

 development of the A antigen, one the 

 development of the B antigen, and the third 

 results in the absence of both antigens. Since 

 each individual must have two genes at this 

 locus, one can readily ascertain that there 

 are 6 different genotypes possible. Designat- 



* The discussion of subgroups is beyond the scope 

 of this book. 



