58 



( HAPTl K 5 



FIGURE 5-1. Distribution of MN blood group 

 phenotypes in different human families. 



Parents and their offspring can be tested 

 for MN blood type. The results of such 

 family studies are summarized in Figure 5-1. 

 Parents of type 6 produce offspring in the 

 proportion of 1:2:1 for M : MN : N blood 

 types. This result suggests that these blood 

 types are due to the action of a single pair 

 of genes. If we let M represent the gene 

 for blood antigen M, and M\ the allele which 

 produces the N blood antigen, mating 6 must 

 be, genetically. MM' x MM' and the off- 

 spring \ MM:2MM':\ M'M' . Note that 

 these alleles show no dominance. MM' indi- 

 viduals having both M and N blood antigens. 

 All the other family results also are con- 

 sistent with the genetic explanation pro- 

 posed. 



Different antisera can be prepared to test 

 for other blood types. One of these anti- 

 sera determines the presence or absence of 

 what is called the Rhesus or Rh factor. Red 

 blood cells from Rhesus monkeys are in- 

 jected into rabbits; if a second injection of 

 Rhesus blood is given sometime later, it will 

 be clumped. This is explained by the pres- 

 ence of an antigen carried by Rhesus red 

 blood cells against which the rabbit had 

 manufactured antibodies before its second 



exposure to Rhesus blood. The antigen in- 

 voked here is called Rh: the antibodies in- 

 duced are anti-Rh. 



When human blood is injected into a rab- 

 bit having anti-Rh antibodies in its serum, 

 it is found that 859? of all people have blood 

 which is clumped — these people have what 

 is called Rhesus-positive (or Rh-positive) 

 blood type; 15% of all people have blood 

 which is not clumped — these people have 

 Rhesus-negative (or Rh-negative) blood 

 type. Accordingly, 85 r /< of human beings 

 have the same Rh antigen as have Rhesus 

 monkeys, and 15% do not. A combination 

 of family and pedigree studies shows that 

 presence of Rh antigen in human beings is 

 controlled by a dominant gene we can repre- 

 sent by R, and its absence by a recessive 

 allele, r. 



Two other antisera, called anti-A and anti- 

 B, can be prepared.- Blood from different 

 people tested with these antisera is found 

 to be of one of four types: clumped in anti-A 

 (blood type A), clumped in anti-B (blood 

 type B), clumped in both (type AB), and 

 clumped in neither type of antiserum (O). 



Family studies of ABO blood types give 

 the phenotypic results shown in Figure 5-2. 

 Note that two kinds of results are obtained 

 from A X O and also from B X O parents. 

 In each case one kind of result (marriage 

 types 9 and 1 1 ) can be explained if one 

 assumes that the non-O parent is a hetero- 

 zygote in which the gene for O is recessive. 

 Let i be the gene for O blood type and / ' 

 the allele for A blood type, the latter being 

 dominant. Then the parents are thus: in 

 marriages type 9, /'/' X ii\ in type 10, 

 /'/' X u\ and in 13, ii X "• In order to 

 explain 11 and 12 we shall have to assume 

 the presence of a gene /'•' for B blood type, 

 which is also a dominant allele of i and from 

 which it segregates. Then mating 1 1 is 



- Based upon K. Landsteiner's work. 



