1] PCB5065 Fall 2014 - Exam 4 – Chase-McCarty
Name :
I – Non-Mendelian inheritance – 50 pt.
1 (15 pt) This question pertains to our discussion of organelle genetics.
1a) Define heteroplasmy.
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
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
A man suffering from myoclonic epilepsy and ragged red fiber (MERRF)
due to a mitochondrial tRNALys A8344G mutation
A variegated pelargonium (geranium) plant resulting from the cross of
a green seed parent by a variegated pollen parent
2] PCB5065 Fall 2014 - Exam 4 – Chase-McCarty
Name :
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.
2a) Fill out the expected progeny genotypes and the expected progeny genotype
ratio (e.g. 1:1, 3:1, etc.) for each of the three crosses below :
seed parent
genotype
pollen parent
genotype
cross 1
+ / seth6
+/+
cross 2
+/ +
+ / seth6
cross 3
+ / seth6
+ / seth6
progeny genotypes and ratios
2b) Is the seth6 mutation dominant or recessive? Explain your
answer.
3] PCB5065 Fall 2014 - Exam 4 – Chase-McCarty
Name :
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.
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.
3-c) Based upon the pedigree shown, could this vision-loss trait be due to a
mitochondrial gene mutation? Explain why or why not.
3-d) Based on the pedigree shown, could this vision-loss trait be due to an
imprinted nuclear gene? Explain why or why not.
4] PCB5065 Fall 2014 - Exam 4 – Chase-McCarty
Name :
II. Developmental Genetics (50 pts)
1. (10 pts) A key test of the ABC model for floral homeotic gene action was successful prediction of
double mutant phenotypes. The ABC model is summarized in this diagram.
First, diagram the phenotypes (i.e. organ identities found in each of the four whorls) for the three
classes of single mutants:
a
b
c
Based on the model, what phenotype would you expect for the following double mutants:
ab
bc
5] PCB5065 Fall 2014 - Exam 4 – Chase-McCarty
Name :
2. (40 pts total) You identify mutants in four genes (A, B, C and D) that control development of
specialized venom barbs on limbs of the giant carpathian zombie spider (Z. apocalypses).
fluorescent antibody localizations:
A (blue), B (red), C (green) and D (gold) proteins
phenotypes:
WT limb
proximal
venom barbs
a mutant
-
distal
A
B
B (a mutant)
B (c mutant)
b mutant
C
c mutant
C (a mutant)
C (b mutant)
d mutant
D
You determine that A, B and C genes encode transcription factors with distinct DNA binding specificities.
A is expressed during early limb development in a diffuse gradient along the proximal distal axis. By
contrast B is expressed in a sharply defined region in the proximal half of the limb, whereas C is
expressed exclusively in the distal half of the limb. D encodes a small peptide containing a signal
peptide that is distributed in diffuse pattern over the proximal 2/3 of the limb. The phenotypes of the
mutants are shown above. Assume you have capability for making transgenic spiders using GFP
reporters, etc.
a) (14 pts) Sequencing of a upstream regulatory region of the B locus that confers faithful expression of
a GFP reporter gene in the proximal region reveals a cluster of five A binding sites, two B binding sites
but no C binding sites. Based on known principles of gene interaction, propose a mechanism that can
account for formation of the sharply defined domain of B expression in the proximal half of the limb
(Hint: the mechanism has two distinct components.). What are the two essential components of the
mechanism? What specific role is played by the clustering of A binding sites? How would you test their
importance in vitro and in vivo? What is the role played by the B binding sites? How would you test
their significance in vivo? Which protein (s) would you expect to exhibit sigmoidal binding kinetics for
DNA binding in vitro? Which is more likely to show hyperbolic kinetics?
A sites B sites
B gene
6] PCB5065 Fall 2014 - Exam 4 – Chase-McCarty
Name :
D mRNA in situ’s
WT
b mutant
c mutant
b) (10 pts) Given the in situ data shown above and known principles of regulatory gene action, propose a
mechanism that can account for the expression of D mRNA in the proximal domain? I.e. which gene(s)
act as positive regulators of D? Which if any act as negative regulators? Using transgenic reporter
constructs, you localize a 500 bp upstream enhancer sequence in the D locus that faithfully confers
proximal expression pattern. What specific sequence features would you look for in the enhancer
sequence? How do you account for the fact that D influences differentiation of cells in the distal
domain? Based on the observed phenotypes is D also required for differentiation in the proximal
domain?
d) (6 pts) Which gene (s) would you predict would be the most likely to directly regulate or signal
downstream genes involved in barb formation (e.g. the venom biosynthetic pathway)? Which if any are
positive regulators? Which if any are negative regulators? Explain.
7] PCB5065 Fall 2014 - Exam 4 – Chase-McCarty
Name :
e) (6 pts) You hypothesize that other genes in addition to the four play essential roles in generating the
venom barb distribution in the limb. However, saturation mutagenesis and screening for venom barb
phenotypes failed to identify any additional mutants in this pathway. Suggest two possible reasons why
other mutants have not been detected. Suggest an example of a missing function in this pathway.
f) (4 pts) You discover that in addition to affecting limb development, the a mutant also has a discernible
phenotype in the embryo (small embryo). You perform the following crosses (female on left):
embryo phenotypes
embryo genotypes
AA X
aa
100 % normal
100% Aa
aa X
Aa
100% small
50% Aa 50% aa
Aa X
aa
100% normal
50% Aa 50% aa
Where and at what stage of development would you look for expression of A that might account for
these results. Explain.