Phenotypic Effects <>i dene Action 



87 



Richard Benedict Goldschmidt 



(1878-1958). (From Genetics, 

 vol. 45, p. I, 1960.) 



QUESTIONS FOR DISCUSSION 



6.1. Can two genetically different individuals ever have identical viabilities? Explain. 



6.2. Why can you not conclude, from the evidence presented, that the genes for MN 

 blood type in man, or for auria phenotypes in the snapdragon, are pleiotropic? 



6.3. How can genes be lethal to a genotype without producing a corpse? 



6.4. Two curly-winged, stubble-bristled Drosophila are mated. Among a large num- 

 ber of adult progeny scored the ratio obtained is: 4 curly stubble: 2 curly only: 2 

 stubble only:l neither curly nor stubble (therefore normal, wild-type). Explain 

 these results genetically. 



6.5. In Drosophila, a mating of <5 A X 9 B or of S CX $ D produces F,, Vi of 

 which turn brown and die in the egg stage. If, however, the matings are 

 t! A X 9 D or <3 C X 2 B, none of the F, eggs turn brown and die. How can 

 you explain these results genetically? 



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

 respects are they different? 



6.7. Is it the gene for dull red eye color that is pleiotropic in Drosophila, or is it the 

 allele for white eye color? Explain. 



6.8. 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? 



6.9. Would you expect to find individuals who are homozygous for Polydactyly? 

 Explain. What phenotype would you expect them to have? Why? 



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

 valuable in a study of gene properties? 



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

 hand only. How can you explain this? 



