Genetics 307 



MULTIPLE ALLELES 



In the examples which have been cited, there were but two char- 

 acters in each allelomorphic group. Most factors are of this type; 

 however, there are some alleles which occur in series. These 

 alleles always occupy the same locus or position on homologous 

 chromosomes ; thus only two of each allelomorphic series may occur 

 in any one individual. One extreme example of this allelomorphic 

 series is that concerned with eye color in Drosophila. There are at 

 least 14 alleles, ranging in color from white through ivory, wine, buff, 

 apricot, etc., to the normal, red. The red is dominant to all others of 

 the series. 



The inheritance of blood groups in human beings is another case 

 of multiple alleles or allelomorphs. In this case, there are three allelic 

 genes named A, a", and a. A is the factor responsible for the production 

 of antigen A, a® for B, and a for neither. The genotype of the four 

 groups thus is as follows : 



Group Genotype 



AB Aa= 



B a^a" or aa° 



A AA or Aa 



O aa 



A somewhat unique situation exists here inasmuch as with the 

 genotype Aa^, each factor produces its effect completely, and both are 

 completely dominant over a. 



SEX LINKAGE 



In the discussion of sex hormones, it was pointed out that the dif- 

 ferences between the two sexes were really a matter of contrasts of 

 characters. The development of many of these characters was influ- 

 enced by the sex hormones. While in lower animals hermaphroditism 

 (testes and ovaries in the same individual) is not uncommon, among 

 the vertebrates it is a comparatively rare phenomenon. Ordinarily 

 among these latter forms there is a clear-cut distinction between the male 

 and female individuals. Secondary sexual characters are to a great 

 extent determined by the sex hormones ; fundamentally, however, sex 

 and its manifestations are controlled by the chromosomes. 



