Intergenic Linkage 115 



SUMMARY AND CONCLUSIONS 



When independent segregation fails, the nonallelic genes involved tend to be inherited in a 

 linked manner. Recombination between linked genes offers proof that a chromosome con- 

 tains more than one gene. Just as linkage is an exception to independent segregation, 

 crossing over is an exception to linkage and causes linkage to be incomplete. In any given 

 case, the degree to which linkage is incomplete, that is, the rate of crossing over, is constant 

 and independent both of the specific alleles which are present for the two different genes 

 and of the combinations these were in when they entered the individual forming the gametes. 

 Moreover, reciprocal crossover types are equally frequent. The crossover frequency in 

 different cases, as studied in different organisms, is found to vary considerably. 



REFERENCES 



Morgan, T. H., "Random Segregation Versus Coupling in Mendelian Inheritance," Science, 

 34:384, 1911. Reprinted in Great Experiments in Biology, Gabriel, M. L., and 

 S. Fogel (Eds.), Englewood Cliffs, N.J., Prentice-Hall, 1955, pp. 257-259. 



QUESTIONS FOR DISCUSSION 



15.1. Distinguish between sex linkage and the linkage of nonalleles. 



15.2. Does the linkage of two pairs of genes prove that they are located on the same chro- 

 mosome? Explain. 



15.3. Discuss the advantages and disadvantages of linkage and crossing over with respect 

 to the fitness of individuals carrying certain genotypes. 



15.4. In Drosoplula, y and spl are X-linked. A female genotypically -\ — \-/y spl produces 

 sons of which 3.0% carry either v + or + spl. 



What are the genotypes and relative frequencies of gametes produced by the 

 mother? Is the father's genotype important? Explain. 



15.5. Name all the processes so far discussed which lead to genetic recombination. 



15.6. Do you think that one of the main principles demonstrated in the present Chapter 

 is that chromosomes contain more than one gene? Explain. 



15.7. How would you proceed to state a "law of independent segregation" in the light of 

 your present knowledge? 



15.8. What evidence do you have that crossing over does not involve the unilateral move- 

 ment of one gene, from its position in one chromosome to a position in the homologous 

 chromosome? 



15.9. Does crossing over always result in genetic recombination? Explain. 



15.10. In what respect do you think the development of the principles of genetics in this 

 text would have been affected had the first two pairs of genes, simultaneously studied 

 in crosses, been linked? 



15.11. Assume the gene for woolly hair (p. 34) is located autosomally. A nonwoolly haired 

 nonhemophilic man marries a woolly haired nonhemophilic woman. They have 

 a woolly haired hemophilic son. Give the genotypes of all three individuals. Give 

 the genotypes and frequencies of the gametes usually produced by the son. 



