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Life Sciences 1b Problem Set 3
Due in TF dropbox (Science Center 2nd floor), Monday Feb 22 at 12:00pm.
1.
(5 points) The African pygmy hedgehog, Atelerix albiventris, has a diploid chromosome number of
90, including one pair of sex chromosomes.
A.
How many chromosomes would you expect to find in a pygmy hedgehog somatic cell during
metaphase of mitosis? How many chromatids? Would the chromatids be identical? Explain your
reasoning.
B.
How many chromosomes would you expect to find in a pygmy hedgehog somatic cell during
anaphase of mitosis? How many chromatids? Explain your reasoning.
C.
How many chromosomes would you expect to find in a pygmy hedgehog germ cell during
metaphase of meiosis I? How many chromatids? Would the chromatids be identical? Explain
your reasoning.
D.
How many chromosomes would you expect to find in a pygmy hedgehog germ cell during
metaphase of meiosis II? Explain your reasoning.
2.
(6 points) Methotrexate is a drug that can inhibit cell division by preventing the synthesis of purine
bases. Cells treated with methotrexate arrest at a particular stage of the cell cycle.
A.
During which stage of the cell cycle would you expect cells treated with methotrexate to arrest?
Briefly explain.
Below are histograms (graphs) showing the numbers of cells containing different amounts of DNA
in three different samples: cells treated with two different drugs (A and B), and cells that have not
been treated with a drug. The untreated cells are progressing through the cell cycle at a normal
rate, while the cells that have been treated with drugs tend to arrest at different stages in the cell
cycle.
Treated with Drug A
# of cells
# of cells
Untreated cells
x
2x
DNA content
Treated with Drug B
# of cells
B.
x
2x
DNA content
x
2x
DNA content
On the histogram of “untreated cells” indicate which bars in the graph represent cells in each of
the following stages of the cell cycle: G1, S, and G2. If there are bars that you cannot assign to
one of these stages, do not label them.
C.
Would you expect cells treated with methotrexate to produce results more similar to those shown
for Drug A or Drug B? Briefly explain your reasoning, including how the data in the graph is
consistent with the effect of methotrexate on cell cycle progression.
D.
Which of the following drugs could be drug B? Briefly explain your reasoning for each.
•
Vinblastine, which binds to tubulin and inhibits microtubule assembly
•
PPMP, a cytokinesis inhibitor
3.
(8 points) A particular species of scorpion has two pairs of homologous chromosomes, and shown
below are the karyotypes of a normal male and a normal female scorpion as well as selected
offspring from a mating between them. The larger chromosome is chromosome I, and the smaller
chromosome is chromosome II. Remember that karyotypes are made using metaphase
chromosomes, so each chromosome shown contains two sister chromatids, though they cannot
be distinguished from one another, as they are held together tightly.
Karyotype of
female parent:
Karyotypes of selected offspring:
1
2
3
Karyotype of
male parent:
4
5
A.
For this species of scorpions, n = __________ and 2n = __________
B.
For each of the offspring (1-5), identify whether the karyotype is normal or if there is an
abnormality. If there is an abnormality, identify the abnormality (e.g., monosomy chr. II), and in
which parent and which phase of meiosis (I or II, or either) the error could have occurred.
Offspring
Normal (yes/no)?
Abnormality (if
applicable)
Parent and phase of meiosis (I or II)
in which abnormality occurred (if
applicable)
1
2
3
4
5
C.
Generally, offspring with chromosomal abnormalities such as these are not viable – they die
early in development. Individuals with some chromosomal abnormalities, however, are able to
survive. What factor determines the extent of the defect caused by a particular chromosomal
abnormality (i.e., why are humans with trisomy 21 sometimes able to survive, while individuals
with trisomy 1 are not)? Why is this factor so important?
4.
(8 points) In plants, the NBS-LRR genes mediate disease resistance. Many of these genes have
highly similar sequences and are located adjacent to one another in the genome. To study
them, you have chosen the model plant Arabidopsis thaliana, which is easy to grow and
manipulate in the lab.
Consider the following cluster of NBS-LRR genes from a specific locus in Arabidopsis thaliana.
There are six NBS-LRR genes at this site in the genome (NBS-LRRA – NBS-LRRF), and the diagram
below shows their relative positions on the homologous chromosomes of an individual who is
heterozygous for all six genes (NBS-LRRA/NBS-LRRa, NBS-LRRB/NBS-LRRb, etc.).
NBS-LRRA
NBS-LRRB
NBS-LRRC
NBS-LRRD
NBS-LRRE
NBS-LRRF
NBS-LRRa
NBS-LRRb
NBS-LRRc
NBS-LRRd
NBS-LRRe
NBS-LRRf
Below are the genotypes of three gametes produced by this individual. Some of the gametes
have missing or extra NBS-LRR genes due to unequal crossing over during meiosis. For each
gamete shown (I) determine between which genes the crossover occurred on each
chromosome and (II) write out the genotypes of the other three products of meiosis. In the
genotypes, you do not need to write out the “constant” part of the gene name (NBS-LRR), only
the final identifying letter (A or a, B or b, etc.)
A.
NBS-LRRa
B.
NBS-LRRa
NBS-LRRb
NBS-LRRc
NBS-LRRd
NBS-LRRF
C.
NBS-LRRA
NBS-LRRB
NBS-LRRc
NBS-LRRd
NBS-LRRe
D.
NBS-LRRb
NBS-LRRc
NBS-LRRd
NBS-LRRD
NBS-LRRE
NBS-LRRF
NBS-LRRf
Which of the gametes from A, B, and C would you predict would produce a next-generation
individual with reduced resistance to disease as a result of the loss of one or more NBS-LRR genes?
A: ____________________________________________
B: ____________________________________________
C: ____________________________________________
NBS-LRRC
5.
(9 points) You are interested in behavioral genetics, and have selected zebrafish as your model
system. You are studying how quickly fish respond to a flash of light, and have identified several
mutants that respond more slowly than wild-type fish.
You take three females who exhibit this delayed-response phenotype, and cross each with a
homozygous wild-type male. In the progeny, you observe the following numbers of normal and
slow-responding offspring:
# of normal offspring
# of offspring with delayed
response phenotype
Cross 1
21
32
Cross 2
42
0
Cross 3
0
37
A.
For each cross, identify whether the mutant phenotype is dominant or recessive, and the
genotype of the female from the cross. Define any symbols that you use.
B.
Do you think that the mutation in the female from cross 1 is more likely to be a loss of function
mutation or a gain of function mutation? What about cross 2? Briefly explain how you came to
each decision in the space provided.
C.
You decide to study one of these mutations further by making a transgene that contains the
mutation using the strategy discussed in class (inject a plasmid containing the transgene into 1celled embryos). For this purpose, you select a dominant mutation. Why is it necessary for the
mutation to be dominant in order for this strategy to work?
D.
If you cross two of the offspring from cross 3 with each other, what proportion of individuals in the
next generation would you expect to display the delayed-response phenotype? Show your work
or explain your reasoning.
6.
(4 points) Cystic fibrosis is inherited in an autosomal recessive manner. An unaffected male has
two children with an unaffected female. One of their children, Chen is unaffected by the disease;
the other child, Chao, has cystic fibrosis
A.
What is the genotype of Chen and Chao’s mother? Briefly explain your answer.
B.
What is the probability that Chen is a carrier for cystic fibrosis? Show your work or explain your
reasoning?
C.
Chen has a child with an unaffected female named Lin. Lin has an affected brother, but neither
of her parents are affected. What is the probability that Chen and Lin’s child will be an
unaffected girl? Show your work or explain your reasoning.