Phenotypic Effects of Gene Action 89 



6.31. What conclusions can you draw from the data of B. Harvald and M. Hauge 

 (J. Amer. Med. Assoc, 186:749-753, 1963) obtained from an unbiased sample 

 of Danish twins? 



One twin Both twins cancerous 



Twin pairs cancerous At same site At different sites 



Identical 1528 143 8 13 



Nonidentical 2609 292 9 39 



6.32. In what way does the study of genes help us understand normal embryonic 

 development? 



6.33. If most somatic cells have the same genetic content, why do different cells not 

 differentiate in the same way? 



6.34. In what ways can genes regulate embryonic development? 



6.35. Do the studies of Creeper, of Brachy, or of pituitary dwarfism in mice offer any 

 support for the view that most, if not all, genes have a single, primary effect? 



6.36. What is the relationship between phenogenetics, developmental genetics, physio- 

 logical genetics, and biochemical genetics? 



6.37. Discuss the comparative importance of genes that act earlier, as compared with 

 those which act later, in development. 



6.38. Do you suppose that all genes act at all times in all cells of the body? Why? 



6.39. "This chapter tells more about development than it does about genes." Do you 

 agree? Why? 



6.40. What can be learned about gene action if the gene studied ( 1 ) has only two 

 alternatives, (2) has many alternatives? 



