60 CHAPTER 8 



SUMMARY AND CONCLUSIONS 



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

 usually based upon 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 genes deter- 

 mining it, the narrower is the distribution curve and the smaller the chance of recovering 

 either of the extreme phenotypes in the offspring. Dominance has the effect of reducing 

 the number of phenotypic classes and of placing proportionally more offspring in extreme 

 classes. Consequently, dominance usually causes the underestimation of the number of 

 genes determining a quantitative trait. Dominance also causes regression, so that only when 

 selection of parents is continued over a number of generations will offspring eventually 

 be obtained which are phenotypically like the parents selected. 



REFERENCES 



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



Kempthorne, O., An Introduction to Genetic Statistics. New York, Wiley, 1957. 



Levene, H., "Statistical Inferences in Genetics," in Principles of Genetics, 5th Ed., E. W. 

 Sinnott, L. C. Dunn, and Th. Dobzhansky, New York, McGraw-Hill, 1958, Chap. 29, 

 pp. 388-418. 



QUESTIONS FOR DISCUSSION 



8.1. Do the genes for quantitative traits show epistasis? Explain. 



8.2. Does the environment have a more important role in determining the phenotype in 

 cases of quantitative than in cases of qualitative traits? Explain. 



8.3. Under what circumstances are only seven phenotypes possible when three pairs of 

 genes determine a quantitative trait? 



8.4. Discuss the statement: No new principles of genetics have originated from the study 

 of polygenic traits. 



8.5. Suppose each gene with a capital letter causes a plant to grow an additional inch in 

 height, aa bb cc dd ee plants being 12 inches tall. Assume independent segregation and 

 parents of the following genotypes: Aa BB cc Dd EE X aa bb CC Dd Ee. 



a. How tall are the parents? 



b. How tall will the tallest Fi be? 



c. How tall will the shortest Fi be? 



d. What proportion of all Fi will be the shortest? 



e. Is dominance and/or epistasis involved in this system? Explain. 



8.6. Assume, in man, that the difference in skin color is due primarily to two pairs of genes 

 which segregate independently: BB CC is black, bb cc is white, any three genes for 

 black produce dark skin, any two medium skin, and any one produces light skin color. 

 Give the genotypes of parents who are: 



a. Both medium, but have one black and one white child. 



b. Both black but have an albino child. 



c. Both medium and can have only medium children. 



d. Medium and light and have a large number of children: 

 % medium, % light, % dark, Vs white. 



