HEREDITY 



591 



ing the three allelic genes by the letters rep- 

 resenting the antigens they produce as A, 

 B, and o (for none); group A will have the 

 genotype AA or Ao; group B, BB or Bo; 

 group AB, AB; and group O will be oo. 



Gene A and gene B are not dominant to 

 each other, but both are dominant to gene 

 o. A gamete, whether sperm or egg, must 

 carry a single gene as A, B, or o, and a 

 zygote can have only 2 of these genes in 

 duplicate or in a combination of any 2. The 

 following table (Fig. 418) shows the rela- 

 tion of the possible genotypes to the 

 blood groups. The superscripts usually used 

 to show a multiple alleles series have been 

 omitted to simplify the symbols. 



Figure 418. Table showing the inheritance of the 

 A-B-O blood groups. 



Rh factor 



Although the inheritance of the A-B-O 

 blood groups is relatively simple, the situa- 



Rh positive father 



Genotypes of parents 



tion regarding the Rh groups is more com- 

 plex. Their inheritance may be explained in 

 two ways. One school of thought holds 

 that 3 closely linked pairs of genes are in- 

 volved, while the other explanation is based 

 on an allelic series of at least 8 (probably 

 more) genes. Since this discussion is not 

 concerned with all the antigens of the Rh 

 system, but only with the one most com- 

 monly involved in clinical problems, it is 

 quite proper to think of the system in terms 

 of two genes. The gene R causes the de- 

 velopment of the Rh antigen, while its allele, 

 r, does not. Thus persons of genotype RR 

 or Rr will have the antigen and will there- 

 fore be Rh-positive, while those of genotype 

 rr will not have it, and are classified as Rh- 

 negative. In terms of dominance, one can 

 think of R as being dominant to r. However, 

 if one thinks of these genes simply as pro- 

 ducing or not producing the Rh antigen, 

 there is no need to be concerned with 

 dominance. 



In matings between Rh-positive and Rh- 

 negative individuals, two patterns have been 

 observed among the offspring. If the Rh- 

 positive parent is homozygous (RR), then 

 all the children will be Rh-positive, even 

 though they are heterozygous as shown in 

 Fig. 419. If, however, the Rh-positive parent 

 is heterozygous (Rr), then one-half of the 

 offspring are expected to be Rh-positive 



Rh negative mother 



Gametes of parents 



F, (Rh positive child) 



Figure 419. Diagram showing one way in which Rh incompatibility may be inherited. The 

 gene for the Rh-positive blood type is dominant to the gene for the Rh negative blood type. 



