Download Problem Set 1A Answers

Problem Set 1A Answers
Due Wed. 8-30-06 at beginning of lecture
NOTE: Put both your name and your lab section on your answer sheet: Tues. 9:30AM, Tues. 12:30PM,
Thurs. 1:30PM, or Thurs. 4:30PM.
1. In no more than two sentences compare a locus and an allele. You may use a drawing to help.
A locus (A or a in the diagram) is a particular location, or position, on a chromosome. An allele is a
specific variant in the DNA at that position, such that the DNA from the father (A in this case) may have a
different sequence than the DNA from the mother (a) in this case.
Note that there is one locus that is located on one set of
homologous chromosomes, and there are two different
alleles at this locus in this drawing.
2. The Jolly Rancher beetle has a diploid chromosome number of 18. How many different types of
sperm can a male produce, even if there is no crossing over? Show the formula you used to get
your answer.
n
2 = the number of different sperm types, where n = the haploid number:
9
2 = 512
3. The cell drawn below comes from a mammal with a diploid number of 4. Which phase of cell
division is this cell at?
a.
b.
c.
d.
e.
f.
Prophase of mitosis
Metaphase of mitosis
Prophase of meiosis I
Metaphase of meiosis I
Metaphase of meiosis II
Anaphase of meiosis II
4. Blabbosus contortiosus, the twisted talking vine, has a diploid chromosome number of 6. The
chromosome 1 homologues are metacentric. The same is true of the chromosome 2 homologues.
The chromosome 3 homologues are telocentric. The plant is normal in every way except that it
doe not undergo recombination. Draw a cell of this plant (as described in Lecture 3) as the
chromosomes would appear at metaphase of mitosis. Let the chromosomes from one parent be
solid lines, and the chromosomes from the other parent be dashed lines. Your art can be REALLY
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simple as long as its clear. Heloise’s helpful hint: making your drawings larger rather than
smaller makes it easier.
5. Put 2 loci into the above B. contortiosus cell such that one locus, R, is on chromosome 1 and is
homozygous. Let the other locus, B, occur on chromosome 2 and be heterozygous.
Note that chromosome
1’s should be larger than
chromosome 2’s, which
should be larger than
chromosome 3’s.
6. Now draw this cell as it would look during anaphase of mitosis. Label the alleles in all these
drawings.
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7. Draw one of its cells as it might look in anaphase of meiosis I.
8. Consider one of the daughter cells that would have arisen from your cell in question 7 above.
Draw that daughter cell as it might look in anaphase of meiosis II.
NOTE that the cell drawn below is the one which would have arisen from the left half of the cell above.
9. Mutation is the ultimate source of genetic variation. However, two other processes that occur
during meiosis are responsible for increasing the amount of variation among gametes. __Crossing
over (or recombination)___ shuffles alleles on the same chromosome. That process occurs at
___prophase of meiosis I____(a stage of meiosis). __independent assortment____shuffles alleles
on different chromosomes and occurs at __anaphase of meiosis I__(a stage of meiosis).
10. What type of cross would you use to determine the genotype of an individual showing a dominant
phenotype? __ test cross___
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11. Use the symbols B and b to show how you would set up the cross of problem 10.
B_ x bb.
Note: If the unknown allele, “_,” was B, then all of the progeny would have the B phenotype. If the
unknown allele was b, then ½ of the progeny would have B phenotype and ½ would have the b
phenotype.
12. List the 5 stages of the cell cycle. Be sure to include G0.
a. Very roughly describe each stage with a single phrase.
b. Where are the three checkpoints we discussed in the cycle?
c. What might happen if a cell losses control of those checkpoints? (Only need one sentence.
There are a number of possible answers.)
G1: a growth phase of the cell after mitosis.
S: a DNA synthesis phase.
G2: a second growth phase after the S phase.
M: mitosis, when chromosomes separate and cell division occurs.
G0: a possible phase with no movement through the cell cycle (no replication or cell division).
The three checkpoints: G1/S, G2/M, and spindle assembly checkpoint.
Possible answers could include:
Uncontrolled cell growth could eventually lead to cancer.
Entry into mitosis before repair of DNA damage could lead to cell death due to: increased point
mutations, or breaks in the chromosomes, or loss of essential functions.
Entry into DNA synthesis before all the molecular machinery is in place for accurate replication could
increase the error rate in DNA replication.
13. Incomplete dominance can be seen in the F2 individuals which arise from a cross of F1’s (not F2’s
as was originally in the problem set!), because of the phenotype exhibited by the
a. Homozygous dominant F2’s
b. Heterozygous F2’s
c. Homozygous recessive F2’s
d. All of the above
e. None of the above
14. When a population is true breeding for some particular trait, what does that tell us genetically? (One
sentence) The individuals in the population are all homozygous and all contain the same allele for any
particular locus involved in that trait.
15. Two gene loci, A and B, are unlinked (and thus assort independently), and alleles A and B are
dominant over alleles a and b. Indicate the probabilities of producing the following. (Heloise is here
again: Remember that an AB phenotype merely means that there is at least one A allele and one B allele.)
a. An AB gamete from an AaBb individual?
½ x ½ = 1/4
b. An ab gamete from an AABB individual?
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c. An AB gamete from an AABb individual?
1 x ½ = 1/2
d. An AABB zygote from a cross AaBb × AaBb?
¼ x ¼ = 1/16
e. An AaBb zygote from a cross AaBb × AABB?
½ x ½ = 1/4
f. An AB phenotype from a cross AaBb × AaBb?
¾ x ¾ = 9/16
g. An AB phenotype from a cross aabb × AABB?
1x1=1
h. An AABB genotype from a cross AaBb x AaBb?
¼ x ¼ = 1/16
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