Segregation in Man — Multiple AUelism 



35 



now, already present, specific antibodies to 

 bind the antigen. The antigen-antibody 

 complex then formed often causes the blood 

 to clump or agglutinate. It is simple to ar- 

 range the procedure so that this reaction may 

 be observed in a test tube or on a glass slide. 



What is done ^ is to inject red blood corpus- 

 cles from different people into different 

 rabbits, with the result that the rabbits form 

 antibodies against the antigens introduced. 

 The rabbit's blood, devoid of cells, can then 

 serve as an antiserum, containing antibodies, 

 which will clump any red blood cells added 

 to it carrying the original types of antigens. 

 It is found that two very distinct antisera 

 are formed by these rabbits, and that any per- 

 son's blood cells tested with these two anti- 

 sera can react in one of three ways: the red 

 blood cells are agglutinated or clumped either 

 in one antiserum (arbitrarily called anti-M), 

 or in the other (called anti-N), or in both of 

 these antisera. So all people can be classified 

 by their blood cell antigens as belonging to 

 either M, or N, or MN blood group, re- 

 spectively. 



When parents and their offspring are tested 

 for "MTV" blood type such family studies 

 give the results shown in Figure 5-3. 

 Parents of type 6 give offspring which are in 

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

 blood types. This suggests these blood types 

 are the phenotypic consequence of the pres- 

 ence of a pair of segregating genes. If we let 

 M = the gene for blood group antigen M, 

 M' = the allelic gene which produces the 

 N blood group antigen, mating 6 must be, 

 genetically, MM' X MM' and the offspring 

 \MM : IMM' : \M'M'. Note that these al- 

 leles show no dominance of one over the other 

 phenotypically, MM' individuals showing 

 both M and N blood group phenotype. All 

 the other family results also are consistent 

 with the genetic explanation proposed for 

 MN blood groups. 



PARENTS 



CHILDREN 



1 M X M 



2 N X N 



3 M X N 



4 MN X N 



5 MN X M 



M MN N 



ALL — — 



— — ALL 



— ALL — 



Va 



6 MN X MN Va V2 1/4 



FIGURE 5-3. Distribution of MN blood group 

 phenotypes in different human families. 



Still two other antisera, called anti-A and 

 anti-B, can be prepared.' When blood from 

 different people is tested using these antisera 

 it 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 (O). 



Family studies of "'ABO"' blood types give 

 the phenotypic results shown in Figure 5-4. 

 Note there that two kinds of results are ob- 

 tained from A X O and also from B X O 

 parents. In each case one result (marriage 

 types 9 and 1 1) may be explained by assuming 

 that the non-O parent is a heterozygote in 

 which the gene for O is recessive. Let / be 

 the gene for O blood group type and /^ the 

 gene for A blood, the latter being dominant. 

 Then the parents are: in marriage type 9 

 /'/ X //, in type 10 I^I^ X //, and in 13/7 X //. 

 In order to explain 1 1 and 12 we shall have to 

 assume the presence of a gene P for B blood 

 group, which is also a dominant allele of / 

 and from which it segregates. Then mating 

 1 1 is Pi X it and 12 is PP X //. Note that 

 we have made a new supposition with regard 

 to genes. In the former case the alternative 



^ Based upon K. Landsteiner's work. 



^ Based upon work of K. Landsteiner and P. Levine. 



