Biology 120: Genetics

Biology 1120: Genetics
Take-Home Exam I
This part of your exam is worth 60 points.
This is a take home exam. You are free to make use of any resources available to help you solve the problems.
This exam is due Tuesday, Feb. 26, no later than 1:30 p.m. Late Take-homes will be accepted through 1:30 on Thursday,
Feb. 28. All late take-homes will earn a 25% late penalty..
You must submit your exam directly to me. Do not take it to the division office. You may put it under my office door if
Please keep the following in mind as you prepare your exam.
Make certain your name is on every page you turn in.
It is your responsibility to locate and use a stapler to attach your pages together. Be sure you staple the exam
pages to your pages of answers.
You must show your work for every problem, unless the instructions specifically excuse you from this
requirement. Work must be clear and legible. I must be able to follow your reasoning from beginning to end. If
I can't figure out your work, it doesn't exist.
Double check before you decide you have completed a problem to be certain you have actually answered the
questions which I asked, and make sure those answers are clearly marked and easy to find. Label questions
clearly with the appropriate question number, and label sections clearly with the appropriates section letter.
A special note: If you invent symbols for the alleles of a gene, be sure to provide some sort of chart showing
me what symbols you chose, and what each of them represents. I can't read minds.
Please solve all of the following problems.
In Drosophila the yellow body gene is located on the X chromosome. This gene has two alleles, B (wild type gray
body) and b (yellow body). B is completely dominant to b.
You mate a homozygous wild type female Drosophila to a yellow body male.
What phenotypic ratio do you expect in their offspring?
If you back cross one of their female offspring to her father, what phenotypic ratio do you expect
them to produce?
Finally, if you performed the reciprocal of the original cross, what phenotypic ratio do you expect?
In gerbils, the allele for wild type agouti coal color (B) is dominant to the allele for black coat color (b). The
allele for white spotting (W) and the allele for solid color (w) show pseudo-dominance; the WW genotype is
lethal. These two genes are autosomal and unlinked.
An agouti spotted gerbil, whose mother was black, is mated to an agouti spotted gerbil whose father was black.
What are the genotypes of these two gerbils?
Predict the phenotypic ratio among their living offspring.
What fraction of the agouti solid offspring from this cross would you expect to be homozygous for
both agouti and solid?
What fraction of the offspring from this cross would you expect to be agouti (with or without
Read the following description carefully. There are at least two different forms of albinism in humans,
tyrosinase negative albino and tyrosinase positive albino. These are caused by recessive alleles of two
different genes. The two genes responsible for these characteristics are non-allelic to each other.
Is it possible for two albino parents to produce a normally pigmented child?
Explain your answer and diagram a cross which supports your view, showing the genotypes and
phenotypes of both parents, and showing how they can produce a normally pigmented child.
You already know about the black gene for gerbil fur (dominant B for wild type Agouti and recessive b for
black). A second fur color gene in gerbils is the Full color (wild type and dominant)/Dilute gene. Use B and b for
the black gene and D and d for the dilute gene.
If a genetically Agouti gerbil is also dd, that gerbil will be tan colored instead of brown. If a genetically black
gerbil is also dd that gerbil will be gray instead of black.
You mate a BBdd gerbil to a bbDD gerbil. What color are these parent gerbils?
What color are the hybrid offspring of this mating (the F1?
To your dismay, two of these F1 gerbils escape from you and hide in the closet. By the time you
recapture them, hanky panky has happened, and the female is pregnant. What phenotypic ratio do you
expect among their offspring (the F2)?
In chickens, the female is the heterogametic sex. Females are designated ZW, males are ZZ. There is a Zlinked gene for down color which has two alleles, a dominant allele (G) which produces silver down, and a
recessive allele (g) which produces golden down. A silver-downed female mates with a silver-downed male. They
produce twelve offspring, ten silver-downed and two golden-downed.
What gender are the golden-downed offspring?
What are the genotypes of the parents?
In a suburban hospital, SuziQ gave birth to a baby girl. SuziQ's problem is that she's not sure who the father
of her child is. There are three candidates. This hospital really counts on you to solve its problems, so you get
to figure this one out. To do it, you will be looking at blood types and color vision.
Here's what you know:
You know how inheritance works for blood type and colorblindness. (You may need to look this up.)
SuziQ has blood type A and normal color vision.
The baby girl has blood type O and is colorblind.
Man #1 has blood type O and normal vision.
Man #2 has blood type AB and is color blind.
Man #3 has blood type B and is color blind.
Tell me whether any of these men is a reasonable candidate for this baby's father. Explain for each why he is
or is not a possible father. NOTE: "Man #1 can't be the baby's father because of his blood type" is not an
explanation. If, indeed, this is true, you must explain why his blood type eliminates him as a candidate. And be
Don't forget to show the work for everything appropriate.