Segregation in Man — Multiple Allelism 37 



the other type, a more moderate anemia, is phenotypic expression may involve complete, 



called microcytemia, or thalassemia minor. partial, or no dominance, in no case does 



Pedigree and family studies all support the this have an effect either on the genes them- 



hypothesis that t. major children are homo- selves or upon their segregation and recom- 



zygotes (//) for a pair of genes. Both their bination. 



parents have t. minor and are heterozygotes Another important point to remember is 



{Tt) for this gene. Analysis on a population that initially someone had to collect the 



level has resulted in the classification of more phenotypic data in pedigree and family stud- 



than 100,000 people in Italy as TT, Tt, or //. ies, and then apply the principles known 



Notice that in the case of thalassemia, neither about genes to explain these data genetically, 



r nor / is completely dominant (nor recessive). using the simplest suitable explanations in 



You have seen, therefore, that some much the same way as was illustrated here for 



morphological and some chemical character- albinism and for MN and ABO blood types, 



istics of man are based upon segregating Sometimes the data are insufficient and the 



genes. While the relation between the alleles investigator is left with several equally prob- 



in the heterozygote with respect to their able genetic explanations. 



SUMMARY AND CONCLUSIONS 



Data furnished in pedigree and family studies provide evidence that there are a number of 

 human traits whose occurrence is based upon the effect of a pair of genes. These traits 

 are of a morphological as well as of a chemical nature, the alleles sometimes showing com- 

 plete, partial, or no phenotypic dominance in the heterozygote. 



It was necessary to postulate that a gene can exist in any one of more than two alternative 

 states. 



REFERENCES 



Mohr, O. L., "Woolly Hair a Dominant Mutant Character in Man," J. Hered., 23 : 345-352, 

 1932. 



Neel, J. v., and Schull, W. J., Human Heredity, University of Chicago, 1954, pp. 83-86, 



89-91,240 241. 

 Stern, C, Principles of Human Genetics, San Francisco, Freeman, 1961. 



QUESTIONS FOR DISCUSSION 



5.1. What is the difference between the pedigree and family methods of investigation? 



5.2. What evidence is there that pigmentation (albinism vs. nonalbinism) is due to genes 

 that are segregating? 



5.3. Two nonalbinos marry and have an albino child. What is the chance that the next 

 child is albino? nonalbino? that of the next two children, both are albinos? non- 

 albinos? one is albino and one nonalbino? 



5.4. What proportion of three-child families, whose parents are both heterozygous for 

 albinism, have (a) no albino children? (b) all albino children? (c) at least one albino 

 child? 



5.5. Would you conclude that the gene for woolly hair is completely dominant to non- 

 woolly hair? Explain. 



