Lecture 29: Linkage: recombination, map distance

1. Do genes that are carried on the same chromosome always demonstrate linkage? Explain your answer.

2. What is a map unit? If two genes are 5 map units apart, what is their recombination frequency?

3. Can alleles of two unlinked genes affect the same phenotypic trait? Explain how you arrived at your answer.

4. Can alleles be linked? Explain your answer.

5. What is the minimum amount of information that must be provided to describe a diploid genotype? Can you infer anything about linkage from that information? Explain your answer.

6. Why is a test cross usually used in preference to production of an F2 generation to measure linkage?

7. Why were the earliest studies of linkage done with genes on the X-chromosome?

8. Describe the expected ratios of progeny in a test cross of the F1 progeny of true breeding parental strains in each of the following situations. In each case where there is linkage, assume that the two mutations are in coupling.

a. The loci that carry recesive mutations a and b are on separate chromosomes.
b. The loci that carry recessive mutations a and b are on the same chromosome and so closely linked that no recombinants are expected among the number of progeny examined.
c. The loci that carry recessive mutations a and b are on the same chromosome but separated by 10 map units.
d. The loci that carry recessive mutations a and b are on the same chromosome, but separated by 1.0 map unit.
e. The loci that carry recessive mutations a and b are on the same chromosome, but separated by over 100 map units.

9. What is a centimorgan, and why does it have that name?

10. What will be the apparent map distance between two unlinked loci if you mistakenly assume that they are linked when you set up the crosses to measure their linkage?

11. Loci a and b appear to assort independently when tested for frequency of recombination in a female Drosophila. However they are never separated when tested for possible recombination in a male Drosophila.

a. Does this prove that they are sex-linked? Explain your answer.

b. Does this prove that they are linked? Explain your answer?

c. If you answered yes in a or b, how would you measure map distance between a and b?

d. Would your answer to part a be different if the experiments were done in mice? Explain you answer.

e. How would you explain the results if the two mutations were never separated in female Drosophila and never observed in the male progeny of those females?

12. Explain the relationship between linkage groups and chromosomes.

13. Distinguish between the following pairs in a manner that makes it clear you know what each is and how they differ.

a. cis- and trans- heterozygotes
b. Coupling and repulsion
c. Test cross and F2
d. Map distance and mapping function
e. Phenotypic frequency and rate of recombination

14. Explain how double crossovers can go undetected in a two point cross. What would you do to detect such events?

15. Explain in detail how you would determine the map distance between a dominant mutation at one locus and a recessive mutation at another locus in Drosophila.

16. Describe two distinctly different types of problems that make it impractical to do direct measurements of map distance for two genetic loci that are located quite far from each other.

17. Describe an experiment that demonstrated that genetic recombination is accomplished by an actual physical exchange of parts of chromosomes.

18. Can male mice be used to measure rates of recombination? Explain any special problems that might be encountered in such studies.

19. What type of cross would you set up and how would your interpret the data to measure map distance in each of the following situations? (You have not been given the answers. You must justify whatever procedures you suggest.)

a. One of the loci that you are studying exhibits codominant phenotypic expression.
b. One of the loci exhibits incomplete dominance.
c. One of the loci exhibits recessive epistasis over expression at the other locus.
d. The two loci exhibit complementary gene action.
e. The two loci exhibit duplicate gene action.

20. Would you expect a frameshift mutation to behave any differently from a missense mutation in a two point cross? Explain the reasoning behind your answer.

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