Discussion Assignment #3

The Sense of Genetics

This week we are considering some genetics problems. We're going to be mating some virtual fruit flies and gerbils.

Our first matings involve a fruit fly eye color variation called "cinnabar." [Note that there are many eye-color genes in fruit flies, located in many different positions on the chromosomes.] Normal (wild type) fruit flies have dull, brick-red eyes. Cinnabar fruit flies have vivid red-orange eyes. Assume that we have carefully bred two true-breeding groups of flies, one with normal (wild type) eyes, the other with cinnabar eyes. In other words, we've got flies homozygous for each of these variants.

We'll begin with two reciprocal crosses. Reciprocal crosses are matings in which the characteristics of the parents are reversed.

Mating #1-a Parents: Female homozygous wild type X Male homozygous cinnabar.

F1 Offspring:

Mating #1-b Parents: Female homozygous cinnabar X Male homozygous wild type.

F1 Offspring:

Now consider a different eye color gene (not allelic to the one above). This one produces a variant called "white." Again, we've developed homozygous strains of both the wild type and the white eyes. And again, we're going to do reciprocal crosses.

Mating #2-a Parents: Female homozygous wild type X Male homozygous white.

F1 Offspring:

Mating #2-b Parents: Female homozygous white X Male homozygous wild type.

F1 Offspring:

Examine the results of these two pairs of reciprocal crosses. What is the difference between these two genes (the cinnabar gene and the white gene)? In other words, why don't they give the same results for their reciprocal crosses? Explain how you figured it out.

Still looking at eye color in fruit flies. Suppose we create two F2 generations, one of them from Mating #1-a and the other from mating #2-a above. Assuming that each of these matings produces 400 F2 offspring (and that your results will be statistically perfect), fill in the two charts below with the numbers of each kind of offspring which you predict:

F2 Offspring from Mating #1-a:

F2 Offspring from Mating #2-a:

What are the phenotypic ratios for these two sets of F1 offspring? Remember that a ratio is worthless if it consists only of numbers; it must include the descriptions as well.

#1-a F2 phenotypic ratio __________________________________________________

#2-a F2 phenotypic ratio __________________________________________________

Are these results identical? Explain.

Now for the gerbils. In gerbils, the wild type fur color is "Agouti," which most of us simply call brown. There are a number of fur color variants caused by several different genes. One of these is a color called "copper," which is a bright reddish-brown.

Assume that you are beginning a research program studying this variation in color. You know nothing about its genetic cause, but you begin with the hypothesis that it is created by a variant in a single gene. With that assumption, you now need to figure out if the copper allele is dominant or recessive.

Remember that you don't know the genetic constitution of any of the gerbils you are breeding--you just went to the stores and found a bunch of brown gerbils and a bunch of copper gerbils. So you make the following observations:

When you mate a brown gerbil to a brown gerbil, in some cases the baby gerbils all turn out to be brown. But with some partners, part of the babies are brown and part of them are copper.

When you breed a copper gerbil to a copper gerbil, all of the babies always turn out to be copper.

Assuming that your original supposition is correct (that this is controlled by a single gene), which is dominant, brown or copper? How do you know?

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