2 12 READINGS IN BIOLOGICAL SCIENCE 



It was found that one baby belonged to group O and the other to group 



A. Both of the Smiths were of group O, while one of the Browns was O 

 and the other A. This proved conclusively that the baby of group A be- 

 longed to the Browns, as two O parents can produce only children of 

 group O. 



Your blood group depends upon the presence or absence of two sub- 

 stances in your blood stream, known as antigens. The four blood groups, 

 AB, A, B, and O depend upon the presence of both, or of either alone, or 

 of neither of these antigens. The antigens are known as A and B. People 

 of group O possess neither antigen, those of groups A and B possess the 

 corresponding antigens, while those of group AB possess both antigens. 



An individual's blood group is determined by heredity. It is estabhshed 

 within the individual months before birth, and cannot be altered by any 

 known environmental circumstance. Age, disease, and climate do not 

 change one's blood group. Even transfusion from one of another group 

 does not permanently alter one's group. 



Three different genes are responsible for the four blood groups. One, 

 which we shall represent by A, results in the formation of antigen A, 

 Another, which we shall represent by B, results in the formation of antigen 



B. The third gene, which we shall represent by O, produces no antigen. 

 Two of these genes, possibly A and B, mutated from the original one, 



which would have been O. As all three genes are of the same origin only 

 two of them can be present within any individual. In other words, they 

 occur in pairs, as do the genes determining whether or not one can taste 

 P.T.C. (a certain chemical.) Their behavior is a Httle less simple, however, 

 because three instead of two different kinds of genes are involved. The 

 blood groups are determined by three alleles, A, B, and O whereas only 

 two are involved in the determination of whether or not you can taste 

 P.T.C. 



Genes A and B are each dominant to gene O. But A and B show no 

 dominance to each other. People of group AB are heterozygous for genes 

 A and B, and each antigen is present to as marked degrees as in those pos- 

 sessing only one of the antigens. People of groups A and B may be either 

 homozygous, or heterozygous for O. As O is recessive, people of blood 

 group O are always homozygous. 



Knowledge of the mode of inheritance of the blood groups is now 

 widely recognized as of great practical importance in cases of disputed 

 parentage, as well as in other types of identification, such as the baby mixup 

 we have cited. If we know the blood group of the mother and child, we 

 can tell to what group or groups the father had to belong. This is, of 

 course, negative evidence in that it tells only to what groups the father 

 had to belong, and clears a man who does not belong to those groups. If 

 the accused man happens to belong to one of the possible groups of the 

 father, we have no evidence one way or the other as to his guilt. 



