I. Allelic and Genic Interactions:

1)  Conduct the following cross:   Aa   x   Aa

Determine the genotypic and phenotypic ratios if there is:

 - complete dominance
 - Incomplete dominance
 - Codominance
 - Overdominance
- a recessive lethal expressed before birth, that has no phenotypic effect.
- a recessive lethal expressed before birth that is phenotypically dominant (like yellow coat color in mice)

2)  Provide a cellular explanation for complete dominance, intermediate inheritance, codominance, and overdominance. A cellular explanation means to explain, at the protein level, why the heterozygote expresses a given phenotype.... like a "dosage effect" for incomplete dominance (intermediate inheritance).

3)  Consider this cross:

                                        AaBbCc    x    AaBbCC

                     - assume independent assortment of the three genes
                     - There is incomplete dominance at the A locus  (meaning A is incompletely dominant to a).
                     - There is complete dominance at the B locus.
                     - There is overdominance at the C locus.

How many genotypes are possible in the offspring?

How many phenotypes are possible in the offspring?

4) Explain how a continuously variable trait, like skin pigmentation in humans, could be governed by genes.

5) What is an epistatic interaction? Give an example.

6) The "h" allele exerts an epistatic effect over the ABO locus. What is the genotypic and phenotypic ratio you would expect from the following cross: HhAB x hhAO?

7) Although they are often very rare, every population has many lethal alleles... indeed, each of us probably has 7-10 lethal alleles. You would expect lethal alleles to be "weeded out" of a population by selection... after all, dead organisms don't reproduce. However, they can be maintained in populations even though they are lethal. List three different ways that lethal alleles can be maintained in a population (or even in an individual).