Lecture 14: Chromosome maps, proaryotic gene mapping.
(Old Lecture 12)
1. Genetic loci a and b are linked, but separated by 10 centimorgans. Female mice that are heterozygous for recessive mutations at both loci are backcrossed to males that are homozygous recessive at both loci.
a. What will be the genotypic distribution of the gametes produced by the female mice?
b. What is the probability that a female gamete will be dominant for A and recessive for B? (Remember that crossover events produce pairs of gametes).
c. What is the probability that a female gamete will be wild-type at both loci?
d. What is the ordered probability that in 12 offspring of the cross, there will be six wild type offspring, four double mutant offspring, one that is mutant only at A, and one that is mutant only at b?
e. What is the ordered probability that the same sequence of progeny as in part d would have been obtained if the two loci were not linked?
f. What is the ratio of the ordered probability obtained by assuming linkage to the ordered probability obtained by assuming no linkage?
g. What is the lod score for this set of progeny, assuming a recombination rate of 10%?
h. What would the lod score be if you had assumed that the two loci were so thightly linked that no recombination was expected?
2. Lod scores are used extensively in studies on human genetics.
a. What is a lod score?
b. What is it used for?
c. How is it calculated?
d. What problem is likely to be encountered if zero recombination is assumed when making the calculation?
e. What lod score is viewed as strong evidence for linkage?
f. What is the significance of a negative lod score?
3. Describe the use of somatic cell hybridization for the assignment of genes to specific human chromosomes. How are the individual human chromosomes identified in such studies?
4. How many human chromosomes are there? How many linkage groups do they represent? (Think carefully about the second part before you answer!)
5. Describe four distinctly different ways in which genes can be transferred into a bacterial cell.
6. What is the shape of the linkage map of E. coli? How has this been shown?
7. How can you demonstrate that genes that are too far separated to exhibit linkage belong to the same linkage group? (You should be able to think of more than one answer).
8. Describe the use of chromosome banding in genetic research.
9. What is the difference between an F+ strain and an Hfr strain? Which is the most useful in bacterial genetic analysis and why?
10. What units are normally used to measure map distances in E. coli ? What is the basis for use of this particular type of unit?
11. What is a merozygote and what is its value in genetic research?
12. What type of bacterial mating system is used to generate stable partially diploid cells?
13. How does genetic transduction differ from transformation?
14. What was the nature of the first experimental evidence showing that DNA was capable of carrying genetic information?
15. What is an auxotrophic mutation? What term is used to describe
the corresponding non-mutant state?
16. Explain how the inability to utilize a particular substrate can be used as a genetic marker in bacteria.
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