66 CHAPTER 5 



SUMMARY AND CONCLUSIONS 



\ gene can cxisl in any one of two or more genetic states. These alternatives comprise 

 a multiple allelic (sometimes isoallelic) series. 



Some alleles ol a given gene may produce apparently different qualitative phenotypic 

 effects (and therefore show no dominance when hybrid); other alleles may produce 

 different degrees of a quantitative phenotypic effect (in which case they may or may 

 not show dominance when hybrid). 



Genes are the basis for both continuous and discontinuous traits. Continuous traits 

 are usually determined by many gene pairs, each of which has a phenotypic effect that 

 is small and often matched or exceeded by the action of the environment. 



The variability of a quantitative trait is such that the larger the number of heterozy- 

 gous polygenes determining it, the narrower is the distribution curve and the smaller 

 the chance of recovering either of the extreme phenotypes in the offspring. When 

 polygenes are heterozygous, dominance has the effect of reducing the number of pheno- 

 typic classes and of placing proportionally more offspring in extreme classes. Conse- 

 quently, dominance usually causes one to underestimate the number of genes determin- 

 ing a quantitative trait. Dominance also causes regression, so that selection must be 

 continued for a number of generations to obtain a line which approaches the desired 

 phenotype. 



REFERENCES 



Crow, J. F., Genetics Notes, 5th Ed., Minneapolis: Burgess, 1962. 



Edwards. J. H.. "The Simulation of Mendelism," Acta Genet., Basel, 10:63-70. 1960. 



Falconer, D. S.. Introduction to Quantitative Genetics, New York: Ronald Press, 1961. 



Race. R. R.. and Sanger. R.. Blood Groups in Man, 4th Ed., Philadelphia: F. A. Davis, 

 1962. 



Wiener, A. S., and Wexler. I. B.. Heredity of the Blood Groups, New York: Grune 

 & Stratton. 1958. 



QUESTIONS FOR DISCUSSION 



5.1. Discuss the occurrence of dominance with respect to blood group types. 



5.2. Why was it necessary to assume that a gene may have more than two allelic 

 forms? 



5.3. A baby has blood type AB. What can you tell about the genotypes of its parents? 

 What would you predict about the blood types of children it will later produce? 



5.4. If one parent is A blood type and the other is B, give their respective genotypes 

 if they produced a large number of children whose blood types were: 



(a) All AB (c) Half AB, half A 



(b) Half AB, half B (d) h AB. Va A. *» B. U O 



