Segregation of Alleles 41 



3.20. What are the similarities and differences regarding the segregation of genes and 

 of chromosomes? 



3.21. If the genetic material is in the chromosome, is it necessary to assume that the 

 members of a gene pair occupy exactly corresponding positions in the two homo- 

 logs? Why? 



3.22. If a mitotic chromosome normally contains two identical chromatids, can we 

 decide whether each chromosome contains one gene or an identical pair of genes? 

 Justify your answer. 



3.23. In what respect do you suppose that a sample of two-child families may be 

 biased? How would you attempt to avoid this error? 



3.24. The electron microscope shows that the sperm heads of some organisms contain 

 a mass of uniformly thin threads. Do such cases offer evidence against the 

 retention of the integrity of chromosomes or of genes? Why? 



3.25. Differentiate between the genetic recombinations that occur at the time of fertiliza- 

 tion and at the time of meiosis. 



3.26. What is the difference between the study of genetics at the cellular and at the 

 organismal levels? 



3.27. Describe the environment of a single gene. 



3.28. A lack of neuromuscular coordination, ataxia, occurs in the pedigrees of certain 

 families in Sweden. How can you explain that one form of this rare anomaly 

 occurs in certain families where the parents are apparently unrelated, and an- 

 other form occurs in other families where the parents are first cousins? 



3.29. What bearing have the following facts relative to the generality of the phenomenon 

 of dominance? 



When pure lines of smooth-seeded plants and shrunken-seeded plants are 

 crossed, the Fj seeds are all smooth. Microscopic examination reveals that the 

 margins of the starch granules in the seeds are smooth in the smooth P,, highly 

 serrated or nicked in the shrunken P l9 and slightly serrated in all the F v 



