Transcript Practice exam (2012) key

PCB5065 Fall 2012 - Exam 4 - Chase
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Name Key
Question 1
___________________________
Question 2
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Question 3
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Total
Average = 44 out of 50 points
Important – please keep your answers short; confine your answers to the space provided; do
not write on the back of any pages !
PCB5065 Fall 2012 Exam 4 Chase
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Name __________________________________
1 (21 pt) In your own words, define or describe each of the following:
1-a) Hetroplasmy (as it pertains to organelle genetics)
More than one organelle genotype in a cell or individual or organelle
Not accepted: More than one organelle genome in a cell or individual or organelle - because there
are always multiple organelle genomes
1-b) Meiotic drive
Events in meiosis or subsequent gametogenesis that favor the transmission of one allele or genotype
over another
Not accepted: Events in meiosis or subsequent gametogenesis that favor the transmission of one
gene over another
1-c) Maternal effect gene
Gene expressed such that the genotype of the mother determines the phenotype of the progeny
1-d) Epigenetics
Heritable changes in gene expression that do not involve changes in the DNA sequence
1-e) Genomic imprinting
Parent of origin differences in gene expression such that only the paternal or maternal allele is
expressed
1-f) Paramutation
Changes in the expression of an allele directed by association with another allele
1-g) Gametophytic effect in plants
Technically fits the definition of meiotic drive so that definition was accepted
or
Transmission bias resulting from the effects of gene expression on the function of the haploid
gametophyte
PCB5065 Fall 2012 Exam 4 Chase
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Name __________________________________
2. (15 pt) In the human pedigree shown above, shaded individuals are affected by vision loss in
young adulthood. Squares represent males and circles females. Roman numerals indicate
generations and Arabic numerals indicate individuals.
2-a) Based upon the pedigree shown, could this vision-loss trait be the result of a recessive nuclear
autosomal mutation? Explain why or why not.
Technically yes, if you assume that unaffected mates are carriers.
If you said no because it would be unusual to have so many carriers, I accepted that, although this
would obviously depend on allele frequencies in the population, etc.
If you said no because there were not Mendelian progeny ratios you did not get full credit, because
these are very small families, so Mendelian ratios are not necessarily expected. (Remember,
flipping the coin six times, you don’t necessarily get 3 heads and 3 tails.
3-b) Based upon the pedigree shown, could this vision-loss trait be the result of a genetic mutation
in a maternal effect gene? Explain why or why not.
No. In a maternal effect, all the progeny of a single maternal parent will have the same phenotype.
That is not the case here.
3-c) Based upon the pedigree shown, could this vision-loss trait be due to a mitochondrial gene
mutation? Explain why or why not.
Yes. The trait appears to be passed from mother to offspring. Considering that mitochondrial
mutations can condition vision loss and that affected mothers can be heteroplasmic for such
mutations, variably affected progeny are to be expected.
In reality, this pedigree is from a family segregating for a mitochondrial mutation that conditions
Leber’s hereditary optic neuropathy (Wallace et al. Science 242:1427)
PCB5065 Fall 2012 Exam 4 Chase
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Name __________________________________
enhancers
[from Munshi and Duvvuri J Genet
Genom 434:93]
enhancers
3 (15 pt) The diagram illustrates the expression patterns of an imprinted gene region in
mammals. The insulin growth factor 2 (Igf2) gene is an imprinted gene. The paternal allele is
expressed and the maternal allele is silenced. In mice, loss of Igf2 function leads to a small, but
viable, mouse.
3-a) If a female mouse is heterozygous for a loss-of-function mutation at the Igf2 locus
(genotype Igf2 -/+), will this mouse have a mutant or wild-type phenotype? Explain your
answer.
This depends on whether the female mouse in question inherited the mutant or wild-type
allele from her father; the paternally inherited copy is expressed, so if dad contributed the
mutant allele, the mouse in question is mutant; if dad contributed the wild-type allele, the
mouse in question is wildtype
3-b) If the Igf2 -/+ female mouse is mated with a wild-type (Igf2 +/+) male mouse, what are
the expected progeny genotypes and phenotypes? Explain your answer.
♀ genotype
♂ genotype
expected progeny genotypes
expected progeny phenotypes
Igf2 - /+
+/+
50% + / +
50% Igf2 - / +
100% normal because only the
paternally transmitted allele is
expressed and all paternal
alleles are + in this cross
3-c) If an Igf2 +/+ female mouse is mated with an Igf2-/+ male mouse, what are the expected
progeny genotypes and phenotypes? Explain your answer.
♀ genotype
♂ genotype
expected progeny genotypes
expected progeny phenotypes
+/+
Igf2 - / +
50% + / +
50% Igf2 - / +
50% normal mouse because the
paternal + allele was transmitted
& expressed
50% small mouse because the
paternal mutant allele was
transmitted & expressed