Genetic Problems


Answer the following questions in a word document and e-mail it as an attachment to Dr. Bindon.  Remember to include your name since your e-mail address may not reveal this information!  Show all of your work in the document.  I am especially interested to see your logic—where you got what numbers, and why, so give me a descriptive paragraph for each problem telling me how you solved it.  That way the correctness of your work does not depend solely on the calculations and numerical answers.  Enjoy!

1.  You are a genetic counselor and you have a couple that comes to you for advice.  The mother is heterozygous for normal and sickle cell hemoglobin (HbA/HbS) and heterozygous for types A and B glucose-6-phosphate dehydrogenase (G6PDA/G6PDB) and the father is heterozygous for sickle cell hemoglobin and hemoglobin C (HbC/HbS) and hemizygous for glucose-6-phosphate dehydrogenase type A- (G6PDA-).

a.  What genotypes and phenotypes do you predict for their offspring?

b.  What frequencies of the different combinations (both genotypes and phenotypes) are expected?

c.  What Mendelian principle or principles are illustrated by this problem?

2.  In West Africa, the Fulani have an HbS allele frequency of 0.1 and the Ewe have a frequency of 0.2.

a.  Assuming only one other allele (HbA) in each population, what are the Hardy-Weinberg equilibrium frequencies of the expected genotypes for each population?

b.  If each population starts with 5,000 people, how many individuals are there by genotype for the Ewe and the Fulani?

c.  If 1,000 of the Ewe migrate to become part of the Fulani population bringing with them the exact predicted genotype frequencies, what would the frequency of the HbS allele be for each population after migration?  How would you characterize the relationship between Ewe and Fulani allele frequencies before and after migration?

3.  When you return to visit the Ewe a generation later, you conduct a survey and find that there are 2,560 individuals who are homozygous normal (HbA/HbA), 1,600 heterozygotes (HbA/HbS), and 40 with sickle cell anemia (HbS/HbS).  Ignore the migration and compare this with your answer about the Ewe from 2b above to answer the following questions:

a. What are the new and old frequencies of the HbA and HbS alleles?

b. What is the selection coefficient for each genotype?

c. What is your long term prediction for allele frequencies if the selective pressure remains the same?

4.  In the Fulani area, a mosquito abatement program wipes out malaria, removing selection against individuals with the homozygous normal (HbA/HbA) genotype.  Selection against heterozygotes (HbA/HbS) remains nearly nonexistent (selection coefficient = 0) and for individuals with sickle cell anemia (HbS/HbS) there is a substantial adaptive penalty (selection coefficient = 0.9).  Starting with the genotype frequencies you calculated in 2a above:

a. What would the genotype frequencies and allele frequencies be in the next generation?

b. What is your long term prediction for allele frequencies if selection remains the same?

5.  Considering your answers to problems 3 and 4 above, what can you say about the nature of change in selection and allele frequencies over time?