Chapter *5 



SEGREGATION IN MAN 

 — MULTIPLE ALLELISM 



I; 



"n this Chapter we shall discuss 

 how segregation of genes may be 

 .studied in human beings and also 

 examine some traits found in mankind which 

 may be based upon the presence of a single 

 pair of genes. Although we choose to discuss 

 genes in humans, because of the great interest 

 we naturally have in ourselves, it should be 

 remembered that the genetic principles applied 

 here would be expected to hold equally well 

 for any other sexually reproducing species. 



The study of human genetics is complicated 

 by the fact that, unlike other species of plants 

 and animals, our species is not bred experi- 

 mentally. Because of this we shall have to 

 make use of special methods of study. There 

 are several approaches to the study of human 

 genetics which may prove fruitful; these 

 include the pedigree, family, population, and 

 twin methods. The present discussion deals 

 primarily with studies utilizing the first two 

 methods. 



The pedigree method uses phenotypic 

 records of families (family trees) extending 

 over several generations. In recording pedi- 

 grees certain conventional symbols are used 

 (Figure 5-1). In such pedigree charts a 

 square or cf represents a male, a circle or 

 9 represents a female; filled-in symbols 

 represent persons affected by the anomaly 

 under discussion. The family method uti- 

 lizes the phenotypes only of parents and their 

 offspring; that is, uses data from a single 

 generation. 



Albinism, lack of melanin pigment, is a rare 

 32 



disease, which occurs approximately in one 

 birth per 20,000. Studies of families, and of 

 pedigrees hke the one in Figure 5-2, yield the 

 following facts, each of which is discussed 

 relative to the hypothesis that albinism occurs 

 in homozygotes for a recessive gene "a": 



1. Both parents of albinos may be non- 

 albino, i.e., normally pigmented. This may 

 be explained by the occurrence of a homo- 

 zygous child {aa) from the marriage of two 

 heterozygotes {Aa X Aa). 



2. The trait appears most frequently in 

 progeny which share a common ancestor. 

 This is readily demonstrable, for example, in 

 Sweden and Japan where the per cent of 

 cousin marriages is less than 5% in the general 

 population but is 20-50% among parents of 

 albino children. Since albinos are rare, so 

 also is the a gene. The chance of obtaining 

 homozygous, aa, individuals will be small 

 indeed if the parents are unrelated, since even 

 if the first parent is Aa or aa the second parent 

 will most likely be AA. On the other hand, if 

 again the first parent is Aa or aa, marriage 

 with a related individual makes it much more 

 likely that the second parent will carry an a, 

 received from the ancestor held in common 

 with the first parent. 



3. In families of two children, in which al- 

 binism appears though both parents are non- 

 albino, it is observed that the nonalbino and 

 albino children are in the relative proportion 

 of 3 : 4. On the hypothesis under considera- 

 tion, the parents must be Aa X Aa. The 

 chances from such marriages that a given 

 child is nonalbino is % and that it is albino ){. 

 Each child produced from such a marriage 

 has these same chances for nonalbinism and 

 albinism, chances which are independent of 

 the genotypes (or phenotypes) of the children 

 preceding or following it in the family. Ac- 

 cordingly, of all two-child families whose 

 parents are Aa, % will have the first child non- 

 albino, and of these % will also have the 

 second child nonalbino. Thus, %& of all two- 

 child families from heterozygous parents will 



