1] PCB5065 Fall 2014 - Exam 4 - Chase Name : Key
I – Non-Mendelian inheritance – 50 pt. Note there were other answers that received
partial or full credit.
1 (15 pt) This question pertains to our discussion of organelle genetics.
1a) Define heteroplasmy
3 pt – More than one organelle genotype in a cell or an individual. Note more than one
organelle genome does not work because all cells contain multiple organelle genomes
1b) Indicate whether each of the organisms or cell-types listed below is heteroplasmic or
homoplasmic and explain how the heteroplasmy or homoplasmy arises in each example.
A yeast diploid zygote recovered immediately after crossing a mating type a
strain with a mating type alpha strain, where each strain carries a different
mitochondrial genome point mutation
1 pt – heteroplasmic
2 pt – recall that both parents contribute mitochondria and mitochondrial genomes to the
initial zygote
A chlamydomonas haploid spore recovered after crossing a mating type plus
strain with a mating type minus strain, where each strain carries a
different plastid genome point mutation
1 pt – homoplasmic
2 pt – recall the inheritance pattern of chlamydomonas plastid genome is uniparental,
mating type plus due to the active degradation of the mating type minus plastid DNA
A man suffering from myoclonic epilepsy and ragged red fiber (MERRF) due to a
mitochondrial tRNALys A8344G mutation
1 pt – heteroplasmic; recall homoplasmy for deleterious human mitochondrial
genome mutations such as this example (from class) is usually lethal
2 pt – the heteroplasmy could have resulted from mutation or from a
heteroplasmic egg, if mom was also heteroplasmic for this mutation
A variegated pelargonium (geranium) plant resulting from the cross of a
green seed parent by a variegated pollen parent
1 pt – A variegated plant is heteroplasmic with respect to the chloroplast
based on the green and white sectors.
2 pt. Recall that in geranium plastids can be transmitted by the paternal parent, so
heteroplasmy could have resulted from paternal transmission, bi-parental transmission or
mutation.
or
You also got credit if you said homoplasmic based on cells within the sectors (1 pt), that
arose through somatic segregation (2 pt), although the question specifically asked about
the plant.
2] PCB5065 Fall 2014 - Exam 4 - Chase Name : Key
2) (15 pt) The Arabidopsis seth mutations were named after the brother and
murderer of the Egyptian god Osiris. Recall that Arabidopsis pollen carrying the
seth6 mutation does not function. For simplicity, assume that the penetrance of
this mutation is 100% such that no seth6 pollen functions. Note seth6 eggs
function normally and, as described above, this mutation causes gamete lethality,
not progeny lethality; this was a point of confusion for some.
2a) Fill out the expected progeny genotypes and the expected progeny genotype
ratios (e.g. 1:1, 3:1, etc.) for each of the three crosses below :
cross 1
seed parent
genotype
pollen parent
genotype
progeny genotypes and ratios
+ / seth6
+/+
+ / seth6 and + / +
1:1
4 pt
cross 2
+/ +
+ / seth6
+ / seth6 and + / +
0:1 (no seth6 pollen)
4 pt
cross 3
+ / seth6
+ / seth6
+ / seth6 and + / +
1:1 (no seth6 pollen but seth6 eggs
are fine)
4 pt
2b) Is the seth6 mutation dominant or recessive? Explain your
answer.
3 pt You really cannot say whether the seth6 mutation is dominant or
recessive with respect to the wild-type allele. Since the mutation acts in the
haploid pollen, the two alleles are not in the same cell to test. Partial credit if you
considered the whole plant phenotype, but this really isn’t correct unless the gene
has some function in diploid tissue.
3] PCB5065 Fall 2014 - Exam 4 - Chase Name : Key
3. (20 pt) In the human
pedigree shown left, shaded
individuals have vision loss in
young adulthood. Squares
represent males and circles
females. Roman numerals
indicate generations and
Arabic numerals individuals.
3-a) Based upon the pedigree shown, could this vision-loss trait be the result of a
recessive nuclear autosomal mutation that exhibits Mendelian inheritance? Explain why or
why not. 2 pt for yes; 3 pt – you need to assume all unaffected mates are carriers
or
2 pt for no if you said (3 pt) – it would be unlikely that all unaffected mates are carriers
or
2 pt for no if you said (2 pt) that the progeny ratios are not Mendelian; note these are
small families so non-Mendelian ratios are the norm
Recall we discussed this very point about this very same pedigree in class
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.
2 pt for no
3 pt for explaining that in a maternal effect gene, all progeny have the same phenotype (as
determined by the mother’s egg genotype)
3-c) Based upon the pedigree shown, could this vision-loss trait be due to a mitochondrial
gene mutation? Explain why or why not.
2 pt for yes
3 pt for explaining that all transmission is mother to offspring and variable progeny ratios
are the result of maternal heteroplasmy for mutant and wild-type mitochondrial gentoypes
sorting into the eggs
3-d) Based on the pedigree shown, could this vision-loss trait be due to an imprinted
nuclear gene? Explain why or why not.
2 pt for yes
3 pt for explaining that the pedigree is consistent with a maternally expressed gene, but
note the moms must be heterozygous for mutant and wild-type alleles, so some offspring
are mutant and some wild-type
partial credit if you answered no because you failed to consider that the maternal parent
could be heterozygous for mutant and wild-type alleles at an imprinted locus