13 



36 

 PARENTS 



CHAPTER 5 



O X O 



CHILDREN 



AB 



FIGURE 5^. Distribution 

 of ABO blood group pheno- 

 types in different human 

 families. 



In some families. 



allelic form of / is /^ while in the latter case 

 it is P. This means that we are making the 

 additional assumption that the gene can exist 

 in more than one alternative condition, so that 

 a gene can have multiple, different, alleles. 

 Of course, while any individual has only one 

 pair of genes, these may be of the same or of 

 different types. Since the heterozygote /V^ 

 shows no dominance, and appears as AB 

 blood type, all the results indicated in the 

 table are explained. (Some readers may have 

 suspected that multiple allelism was possible 

 from the fact that three different genes were 

 involved in the presence and absence of the 

 ataxias already described.) 



There are a number of other ways to type 

 blood. One of these involves the presence or 

 absence of what is called the Rhesus or Rh 

 factor. Red blood cells from Rhesus monkeys 

 may be injected into rabbits; if a second in- 

 jection of Rhesus blood is given some time 

 later, it will be clumped. This is explained 

 by the presence 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 involved here is called Rh. Instead 

 of injecting Rhesus red blood cells into a 

 rabbit which has anti-Rh antibodies in its 

 serum, suppose human blood is injected. In 

 this event it turns out that 85% of people have 

 blood which is clumped, these people having 

 what is called the Rhesus-positive (or Rh-posi- 

 tive) blood type, whereas 15% of people have 

 blood which is not clumped, and have the 

 Rhesus-negative (or Rh-negative) blood type. 

 Accordingly, 85% of humans have the same 

 Rh antigen as have Rhesus monkeys, while 

 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, call it R, and its absence to 

 a recessive allele, say r, their distribution fol- 

 lowing the principle of segregation. 



Finally, let us consider the genetic basis 

 for certain kinds of anemia. Among Italians 

 who live in Italy or who have emigrated, there 

 may be an anemia of two special kinds. One 

 type, severe and usually fatal in childhood, is 

 called Cooley's anemia or thalassemia major; 



