Practice problems--Autumn 2000 week 1

Answer key

About these problems

Each week's problems run the scale from very simple to somewhat complex. The intent is to get you used to problem-solving. Questions on the homeworks and exams will be on the complex end of the scale. For samples of homework/exam problems, look in the back of the course notes package and here.

Problem-solving guideline

  • Your first order of business is to understand what exactly is being asked. Read the question carefully! Ask yourself -- do I already have all the information I need in order to answer the question? Is there something I need to figure out before I can answer the question?
    • For example -- in Week 1 Question 1, you are asked to figure out the proportion of brown-eyed children. But in order to do that, you need to know the genotypes of the parents. So what are their genotypes? What information in the question will let you figure out their genotypes?
      • For the dad -- it's pretty clear, since he shows the recessive phenotype
      • For the mom -- she shows the dominant phenotype, so she could be homozygous dominant or heterozygous. Is there any information about her parentage that would let you figure out her genotype?

This example was really simple, but the idea is the same for all problems -- identify what information you already have, and what information you need to figure out. Sometimes the question may be giving you information that you don't realize -- read the questions carefully!

  • After you have answered the question, check your answer to see if anything that was said in the question contradicts anything you have said in your answer

Some common mistakes

  • Answering something that was not asked, and not answering what was really asked (i.e., not reading the question carefully)
  • Answers that are not internally consistent -- i.e., answers that contradict themselves. For example, if you identify one parent of a cross as being homozygous dominant, and later on in your answer you say that the progeny of that cross are going to show the recessive phenotype, that's an internally inconsistent answer unless you also explain why the offspring don't express the dominant allele.
  • Failing to demonstrate your logic. When we grade your answers, we are more interested in seeing whether you understand how to answer the question, than in whether you got the numerically correct answer. You may have made a mathematical error along the way -- but if you've shown your work, and we can see that your logic was correct, you'll probably get close to full credit. If you didn't show your work, all we can see is the final incorrect answer -- you get NO credit.

Week 1

Questions 5 and 10 will be covered in quiz section. (Others may be covered on request, time permitting.)

1.

Assume (for this problem and the next) that in humans eye color is inherited as though brown eye color is dominant and blue is recessive. A blue-eyed man marries a brown-eyed woman whose mother had blue eyes. What proportion of their children would you expect to be brown-eyed?

2. A brown-eyed man marries a blue-eyed woman. They have ten children, all brown-eyed. Based on just this information, can you determine the man's genotype with certainty? If their eleventh child is blue-eyed, will that change your answer?

3. Which would be easier to eliminate from a mouse colony, a recessive allele for black fur, or a dominant allele for yellow fur? Why?

4. Tail-less Manx cats do not breed true. Interbreeding these cats yields tail-less and tailed progeny in 2:1 ratio. Explain these results.

5. Ornamental chili peppers can be either red or yellow. Crosses between ornamental chili pepper plants were made as follows:

Parents Progeny
red x red 58 red, 19 yellow
red x red 70 red
red x yellow 42 red
red x yellow 39 red, 41 yellow
yellow x yellow 57 yellow

(a) Which phenotype is dominant?
(b) What are the genotypes of the parents and progeny in each cross? Make up your own gene designation, using an upper case letter for dominant and lower case for recessive.

6. A white pygmy marmot was crossed to a brown pygmy marmot and the resulting progeny were all brown. These F1 progeny were crossed as follows (remember that "P" refers to the parental generation, i.e., the original brown marmot and white marmot):

Cross Progeny
P brown x F1 brown 9 brown
F1 brown x F1 brown 8 brown, 3 white
P white x F1 brown 5 brown, 6 white

(a) Which phenotype is dominant and which is recessive?
(b) What are the genotypes of the original parents and the F1 progeny? Make up your own gene designation, using an upper case letter for dominant and lower case for recessive.
(c) Of the three crosses listed in the table above, which is the only one where you can state the genotypes each of the progeny? What are those genotypes?
(d) If the F1 brown progeny are crossed to F2 white, what progeny phenotypes would you expect, and in what proportions?
(e) If you were given a single brown pygmy marmot (of unknown parentage), how would determine whether it was homozygous or heterozygous for coat color? Assume that you have a stock of brown and white marmots to do crosses with.

7. Nucleotide sequence analysis of a yeast chromosome shows that its adenine content (compared to the other bases) is 30%. If the chromosome is 650,000 basepairs long, how many G bases are there in the chromosome? (Assume that the chromosome has double-stranded DNA.)

8. Some viruses have RNA as their genetic material instead of DNA. One difference between RNA and DNA is that RNA has the base uracil (U) in place of thymine--so RNA forms A-U base pairs instead of A-T base pairs. The base content of two different RNA viruses is as follows:

(i) 2322 A; 1476 C; 1476 G; 2322 U

(ii) 2740 A; 1458 C; 1981 G; 3133 U
(a) Based on what you know about the rules of base-pairing, which of these viruses has an unusual base composition? (What's the unusual aspect of this base composition?)
(b) What would you conclude about the structure of that viral genome?

9.

With respect to ABO blood types, what is the one "cross" that can give all four possible phenotypes among the progeny?

10. Four babies, of blood types A, B, AB, and O, were born in a hospital one night. Two of these babies were twins (non-identical twins, produced from two different fertilized eggs). The three sets of parents had the following blood types:

A and B

O and B  <-- parents of the twins

O and AB

Assuming that the twins were born to the parents with blood types O and B, which baby belonged to which parent? What were the genotypes of the parents and babies in each case?

11. While on a hike near Mt. Rainier, you discover a patch of plants that, according to your botanist friend, all belong to the same species. However, the plants came in three varieties--some produce red flowers, some blue, and some white. You surreptitiously dig up two red-flowered plants, one blue-flowered, and one white-flowered plant and bring them back to your lab. The results of various crosses are as follows:

Cross Progeny
(a) Red-flowered plant #1, selfed 3/4 red-, 1/4 blue-flowered**
(b) Red-flowered plant #2, selfed 3/4 red-, 1/4 white-flowered
(c) Blue-flowered plant, selfed 3/4 blue-, 1/4 white-flowered
(d) Red plant #1 x Red plant #2 3/4 red-, 1/4 blue-flowered
(e) Red plant #1 x blue 1/2 red-, 1/2 blue-flowered
(f) Blue-flowered x white-flowered 1/2 blue-, 1/2 white-flowered
(g) White-flowered plant, selfed All white-flowered
(h) Red plant #2 x blue  ???
(**i.e., 3/4 of the progeny make red flowers and 1/4 make blue flowers--NOT that each flower is 3/4 red and 1/4 blue!)

Come up with a unifying hypothesis to explain these results. Your answer must show the genotypes of all the plants concerned. Based on your hypothesis, predict the results of cross (h).