Honor System


Course Topics


Homework Assignment - Genetic Linkage

100 points each (20 pts each)

Use complete sentences in all of your answers where appropriate. If you are stumped by a problem, explain as much as you can to get partial credit.

1. A homozygous groucho fly (gro, bristles about the eye) is crossed with a homozygous rough fly (ro, eye abnormality). The F1 females are test crossed to a groucho, rough fly. The following data was obtained.

Phenotype No. Observed
Groucho 514
Rough 466
Groucho, rough 11
Wild type 9

a. Are these genes linked? If so, what is the distance between them?
b. Is the F1 in coupling or repulsion phase.

2. If the F1ís in the first problem were intermated instead of testcrossed, what would the frequency of each of the four phenotypic classes. (Hint: You will need to use your results from #1 to solve this problem.)

3. The following three recessive markers are known for lab mice: h, hotfoot; o, obese; and wa, waved. A trihybrid of unknown origin was tested crossed and produced the following distribution of offspring.

Phenotype Observed
hotfoot, obese, waved 357
hotfoot, obese 74
waved 66
obese 79
wild type 343
hotfoot, waved 61
obese, waved 11
hotfoot 9

a. Are the three genes linked?, If so, what is the distance between them?
b. What is the coupling-repulsion arrangement among the three genes in the original trihybrid?
c. Is there any crossover interference? If so, how much?

4. The following is a map information of a portion of chromosome three of Drosophila.

Map Position Gene
19 javelin bristles (jv)
43 thread arista (th)
66 delta veins (Dl)
71 ebony body (e)

a. If flies heterozygous in the coupling position for javelin and ebony eyes are mated, what phenotypic ratio would you expect in the progeny. Why?

5. In human, the ABO blood type system (codominant alleles A, B, and O) is linked to the aldolase-B gene (alleles = al, al+), a gene that functions in the liver. Individuals homozygous recessive aldolase deficiency allele (al) express fructose intolerance. A normal man with blood type AB had a fructose intolerant type B father and a normal type AB mother. He and a woman with blood type O and fructose intolerance had ten children. Five of the children were type A and normal, three were fructose intolerant and type B, and two were typeA and intolerant to fructose.

a. Draw a pedigree for this family that includes the genotypes of the grandparents, parents, and offspring. (If you wish, you can use a diamond symbol with a number inside to denote the three phenotypes of the generation III individuals.)

b. In this pedigree, which blood type allele (A, B, O) is linked to which aldolase deficiency allele (al, al+)?

c. Estimate the recombination fraction (RF) for this pedigree. Then use this estimate to calculate the LOD value for linkage between the two genes. Finally, calculate two other LOD values using one RF smaller than your estimate and one larger than your estimate.