Problem Set 2

Biological Sciences 355
Honors Genetics


Problem Set 2

The following problems are to help you sharpen and deepen your understanding of allele and intergene interactions and modified Mendelian ratios. These questions will be discussed in the recitation on Sept. 12. Problem 4 only is to be handed in but will not be graded. My intention is to spot any misunderstandings you have about these simple concepts.

1. Phenylketonuria (PKU) is a human hereditary disease caused by one Mendelian recessive allele. An individual homozygous for the PKU recessive allele cannot carry out normal metabolism of the dietary amino acid phenylalanine and, if untreated, will usually become mentally retarded.
      A couple (neither has PKU) wants to have children but seeks advice from a genetic counselor because both the man and the woman each have one sibling with PKU. There are no other cases in their families.
      Calculate the probability that their first child will have PKU. [Set up a pedigree chart. Write down genotypes where you know them, figure out which ones you don't know. If you can, work out the exact genotypes of the man and woman and calculate probabilities for their child. If you can't work out their exact genotypes, perhaps you can work out the probabilities that they will be of a certain genotype, then work out the numbers for the child.]

2. There are four babies in a maternity ward. Their blood groups are:

There are four sets of parents, with blood groups as follows: Which baby belongs to which family? Explain your reasoning.

3. Consider the general form of a dihybrid cross as AaBb x AaBb. If alleles A and B are simple dominants, we observe a 9:3:3:1 Mendelian ratio. For each of the following non-Mendelian ratios, propose a mechanism and explain your proposal using a Punnett square.

4. You do the following cross of Drosophila using the brown and scarlet eye-color mutants:

bw/bw; st+/st+ x bw+/bw+; st/st

You obtain all wild-type F1 and cross F1 siblings as follows:
bw+/bw; st+/st x bw+/bw; st+/st

You observe the following progeny:
Make a genetic hypothesis about what is happening here and show (in detail) a chi2-test to check your hypothesis. [State the "null hypothesis" precisely and calculate the probability that the null hypothesis is true.]

Click here to look up values in online chi-square table

5. [From Suzuki et al., An Introduction to Genetic Analysis] Two albinos marry and have four normal children. Explain how this happened.

6. [Klug and Cummings, Chap. 4, Prob. 17] In rats, the following genotypes of two independently assorting genes determine coat color:

A third gene (assorts independently of "A" and "B") determines whether or not any color is produced. The CC and Cc genotypes allow expression of A and B alleles as above, but cc genotype produces albinos regardless of genotype at A and B. Determine F1 phenotypic ratios for these crosses:
AAbbCC x aaBBcc
AaBBCC x AABbcc
AaBbCc x AaBbcc
AaBBCc x AaBBCc
AABbCc x AABbcc

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This information was prepared for Fall Term 2008. Report broken links or comments to: Dr. Lewis Jacobson