SS CHAPTER 6 



6.12. A certain type ol baldness is due to a gene thai is dominant in men and recessive 



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

 Give the genotypes ol all individuals and discuss the penetrance of the genes 

 involved. 



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



6.14. How could you distinguish whether a given phenotype is due to a rare dominant 

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



6.15. In determining whether or not twins are dizygotic, why must one study traits 



tor which one or both parents arc hctcrozygotes? 



6.16. Are mistakes ever made in classifying twins as dizygotic in origin? Why? 



6.17. Can the gene P for Polydactyly be considered as being partially dominant? As 

 having pleiotropic effects? Explain. 



6.18. When nonidentical twins are discordant for ABO blood type, why must one or 

 both parents have been heterozygous for /' or / /; ? 



6.19. Invent a particular situation that would result in greater discordance for identical 

 than for nonidentical twins. 



6.20. What would be the probability of twins being dizygotic in origin if both have 

 the genotype aa Bb CC Dd Ee Ff, each pair of alleles segregating independently, 

 if the parents are genotypically Aa Bb CC DD Ee Ff and Aa BB CC dd ee FF? 



6.21. How would you test whether, in women, there is a genetic basis for the matura- 

 tion of more than one egg at a time? 



6.22. In what way can you imagine that the paternal genotype could influence the 

 frequency of twinning? 



6.23. Is tuberculosis "inherited"? Explain. 



6.24. What can twin studies by themselves tell you about genes? About genetic re- 

 combination? 



6.25. Is it valid to apply the conclusions from twin studies to nontwin members of the 

 population? Explain. 



6.26. Does this chapter present any new information about genetic properties? Explain. 



6.27. In a genetically black strain of the house mouse, W. L. Russell found a mouse 

 with a splotchy phenotype — having white spotting on the belly and occasionally 

 on the back. Splotchy X black gives both splotchy and black types of progeny. 

 Splotchy X splotchy also produces the same types, but a number of embryos die 

 in utero at 14 days of age and are characterized by a kinky tail and spina bifida. 

 Discuss the genetic basis for and the dominance relationships involved in these 

 results. 



6.28. It has been found that mouse ovaries transplanted from embryos to adult females 

 can develop to maturity and produce offspring. Describe how you would proceed 

 to determine the genotype of the abnormal embryos described in 6.27. 



6.29. Do you agree with J. H. Sang that penetrance (P) and expressivity (E) ". . . are 

 descriptive terms which cloak our ignorance of the underlying reactions which 

 determine particular values of P and E in any situation"? Explain. 



6.30. In the Japanese quail (Coturnix coturnix) matings between normal-appearing 

 individuals of certain strains produce some micromelic embryos, having a short 

 broad head with bulging eyes, which die between 11 and 16 days of incubation. 

 How would you proceed to determine whether these abnormal embryos are 

 homozygotes for a single recessive lethal gene? 



