Pleiotropism, Penetrance and Expressivity 73 



tra one), you can be reasonably certain that to attribute, with assurance, similarities or 



these differences have an environmental and differences among them to genotype or to 



not a genetic basis. However, when different environment, if both of these factors are 



individuals are compared with respect to pen- varying in uncontrolled ways (as already dis- 



etrance or expressivity, it is often impossible cussed in Chapter 1). 



SUMMARY AND CONCLUSIONS 



A gene usually produces effects upon a wide variety of morphological and biochemical 

 traits. These pleiotropic effects are the consequence of a pedigree of causes traceable, in 

 some cases, to a single effect on the part of the gene. It is hypothesized that most, if not all, 

 genes have a single primary phenotypic effect of a biochemical nature. 



Penetrance and expressivity depend upon both the genotype and the environment. The 

 most practicable traits for the study of transmission genetics are those whose penetrance is 

 100^ and whose expressivity is uniform when subjected to the normal variations of en- 

 vironment. 



REFERENCES 



Dobzhansky, Th., and Holtz, A. M., "A Re-examination of Manifold Effects of Genes in 

 Drosophila melanogaster" Genetics, 28:295-303, 1943. 



Hadorn, E., "Patterns of Development and Biochemical Pleiotropy," Cold Spring Harbor 

 Symp. on Quant. Biol., 21:363-374, 1956. 



Goldschmidt, R. B., Theoretical Genetics, Berkeley and Los Angeles, University of 

 California, 1955. 



QUESTIONS FOR DISCUSSION 



10.1. In what respects are the terms penetrance and dominance similar and in what respects 

 are they different? 



10.2. Is it the gene for dull red eye color which is pleiotropic in Drosophila, or is it the allele 

 for white eye color? Explain. 



10.3. Most of the genes studied in Drosophila affect the exoskeleton of the fly. Do you 

 suppose these genes also have effects on the internal organs? Why? 



10.4. Would you expect to find individuals that are homozygous for Polydactyly? Explain. 

 What phenotype would you expect them to have? Why? 



10.5. Why are genes whose penetrance is 100% and expressivity is uniform particularly 

 valuable in a study of gene properties? 



10.6. Two normal people marry and have a single child who is polydactylous on one hand 

 only. How can you explain this? 



10.7. A certain type of baldness is due to a gene which is dominant in men and recessive 

 in women. A nonbald man marries a bald woman and they have a bald son. Give 

 the genotypes of all individuals and discuss the penetrance of the genes involved. 



10.8. A man has one brown eye and one blue eye. Explain. 



10.9. How could you distinguish whether a given phenotype was due to a rare dominant 

 gene with complete penetrance or a rare recessive gene of low penetrance? 



