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:
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:
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: