Lecture 30: Linkage, gene order, chromosomal maps
1. For all parts of this question, assume that both parts of the double crossover involve the same two strands of the tetrad.
a. What is meant by the term double crossover?
b. How many linked genes must be analyzed to observe a double cross-over?
c. What is the effect of a double crossover on the linkage pattern of the genes that are used to observe it?
d. How do you compensate for the effects of double crossovers when calculating map distance between the two outside markers?
e. Explain why it is not possible to demonstrate double crossover when using only two markers.
2. Describe the steps that must be taken to identify the middle gene in a three point cross.
3. A female Drosophila heterozygous for linked autosomal genes, a, b, and c is crossed with a male that is homozygous recessive for all three genes. The female progeny exhibit the following phenotypes. +++, 340; abc, 350; ++c, 45, ab+, 55; +b+, 95; a+c, 105; a++, 4; +bc, 6. Total flies counted = 1000.
a. Which is the middle gene?
b. What is the corrected map distance between the two outside genes?
c. Why would a two point cross between the two outside genes not yield the same value as the corrected map distance?
d. What is the interference value for the double crossover?
e. Would it have made a difference in the original cross if the genes had been sex linked and the male was hemizygous for all three recessive alleles. Explain your answer.
f. Could you use a wild type male in question e? Explain your answer.
g. How would the observed results have differed if the male parent had been heterozygous and the female parent homozygous recessive?
4. You are working with three autosomal genes in Drosophila whose possible linkage is not known. A three point cross of a female heterozygous for all three genes with a male that is homozygous recessive for all three yields roughly equal numbers of all of the possible phenotypes.
a. What alternative interpretations of the data are possible?
b. What additional experiments would be needed to distinguish among the possibilities?
c. Would your answer to part b be the same if you were studying mutations in mice?
d. If you conclude in parts b or c that two of the genes are in fact linked, how would you determine the map distance between them.
5. How many different alleles can exist at a given genetic locus? Can three different alleles at the same locus be used to perform a three point cross? Explain your answer.
6. Three genetic loci, A/a, B/b, and C/c are all on the same autosome, with B located between A and C. The measured distance from A to B in a two point cross is 10 map units and the measured distance from B to C is 8 map units. The observed interference in a three point cross is 0.5.
a. What would be the expected frequency of double crossovers if there were no interference?
b. What will be the observed frequency of double crossovers?
c. What will be the frequency of each of the reciprocal double crossover phenotypes?
d. What will be the frequency of each of the reciprocal single crossover phenotypes (a total of four phenotypes).
e. What will be the frequency of each of the parental phenotypes?
7. Mutations a and b each cause distinctly different phenotypes. A female Drosophila that is heterozygous at both loci has a wild-type pheontype. When she is test crossed, four types of progeny are produced (wild type, phenotype a, phenotype b, and phenotypes a and b) in numbers that are close enough to equal so that a chi-square test does not reject the null hypothesis for a 1:1:1:1 ratio. There is no difference in phenotype between male and female progeny. When a male of the same initial genotype is test crossed, only two types of progeny are obtained, phenotype a and phenotype b.
a. How do you explain the apparent discrepancy between the results of these two test crosses?8. In test crosses of double heterozygotes, genes A and B yield ambiguous recombination frequencies that suggest possible linkage but do not statistically rule out possible independent assortment. A series of two point crosses (single crossovers) yields the following information:
b. What conclusions can you reach about sex linkage of the two loci?
c. What are the phenotypes of the true-breeding parents that were used to produce the original heterozygous females and males used in these two test crosses?
d. How would the results have differed for a test cross of the female if both loci had been sex-linked?
e. Explain how a double recessive male could be obtained for the test cross in part d (sex linked alleles).
Gene C is 20 map units from Gene A.Construct a map showing the relative positions and approximate map distances of all of these genes.
Gene D is 23 map units from gene A.
Gene D is 40 map units from gene C.
Gene E is 15 map units from gene B.
Gene F is 25 map units from gene B.
Gene F is 38 map units from gene E.
Gene D is 10 map units from gene F
a. What is the approximate distance in map units from gene A to gene B?
b. How do you explain the ambiguous data obtained in a two point cross of genes A and B.
c. Explain how the use of 3 point crosses might have simplified the process of developing your map.
9. Genetic loci A and B are located so far apart on an autosome in Drosophila that linkage is not seen in a two-point cross. A true-breeding strain exhibiting recessive mutation a is crossed with a true-breeding strain exhibiting recessive mutation b. The F1 generation is all wild-type.
a. What will the phenotypic ratios be in the F2 generation. (Remember that there is no crossing over in the male).
b. How would you determine the map distance between A and B?
c. How would the answer to question a differ if A and B were separated by 20 map units (based on a two point cross)? It will be sufficient to set up a Punnett square and indicate what must be multiplied and added to obtain the phenotypic ratios without doing all of the calculations. Alternatively, you may wish to use the tables in the textbook.
d. How would the results from part c differ if the experiment were done in a species where the crossover rate in the male was 1/2 half of that in the female (assume that map distance is defined by female crossing over in that species). For this part also, the Punnett square and an indication of what values must be combined to obtain the final answer will be adequate.
10. Describe in detail the procedures you would follow to determine the relative map positions of three genetic loci carried on the Z chromosome in chickens.
There will be a question of some sort involving a three point cross
on the examination. In addition to these questions, the end of
Chapoter 15 has numerous
questions involving three point crosses and other map related problems.