Download midterm questions

Biology 4540b midterm examination example
short answer/ 3 hours
100 marks
1. There are maternal effect and zygotic lethal mutations that result in the deletion of the termini
of the embryo, the acron and telson. Mothers homozygous for loss-of-function mutations in the
three maternal-effect genes trunk (trk), torso (tor), and torso-like (tsl) lay eggs that develop
lacking their termini. Embryos laid that are homozygous for loss-of-function mutations in the
embryonic lethal tailless (tll) gene also develop lacking their termini.
a) Why would double mutant combinations between these loss-of-function mutations not give us
any information on the order of function of these genes? (5)
There exists a dominant gain-of-function mutation in the torso gene (torgf). This is a dominant
female sterile mutation which results in mothers laying eggs in which the embryos develop with
no head, thoracic, or abdominal segments, just an enlarged acron and an enlarged telson. We use
this allele to do the following two sets of experiments: A with trk and tsl, B with tll.
A)
genotype of mothers
phenotype of embryos laid
+/+ (wild-type)
trklf/trklf
tsllf/tsllf
torgf/+
normal embryos
no termini
no termini
only termini
torgf/+; trklf/trklf
torgf/+; tsllf/tsllf
only termini
only termini
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Since tailless mutants are zygotic lethal and torso gain-of-function mutants are dominant female
sterile, to do the epistatsis we have to derive tailless mutant embryos from torso gain-of-function
mothers.
B)
torso
genotype of mothers
tailless
genotype of embryos
phenotype of embryos
+/+
torgf/+
+/+
torgf/+
+/+
+/+
tlllf/tlllf
tlllf/tlllf
normal embryos
only termini
no termini
no termini
b) Assuming these genes work along a control pathway, such that the last decision is the most
important, draw the pathway encoded by these genes, and indicate below this pathway the
activity state of the functions encoded by these genes in the middle and at the termini of the
developing embryo. Briefly explain the logic for the order. (10)
c) The following pole cell transplantation experiments are performed using loss-of-function
alleles in torso, trunk and torso-like.
Genotype of pole cell donor genotype of the
recipient embryo
phenotype of
eggs laid
torlf/torlf
trklf/trklf
tsllf/tsllf
OvoD/+
OvoD/+
OvoD/+
no termini
no termini
wild-type
wild-type
wild-type
wild-type
OvoD/+; torlf/torlf
OvoD/+; trklf/trklf
OvcD/+; tsllf/tsllf
wild-type
wild-type
no termini
What does this analysis tell us about which cells synthesis these activities. Combine this
information with that gained from the study of epistatic interactions in the form of a drawn
model showing the various cells. Do not say more than the experiments presented allow you.
(10)
d) The torso gene encodes a transmembrane receptor tyrosine kinase molecule expressed
throughout the embryo. If one assumes that torso gain-of-function mutations represent a
constitutively active tyrosine kinase activity, and that torso-like encodes a secreted polypeptide,
hypothesis roles for what these two gene products normally play in determination of the termini?
(5)
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2.) The distal portion of the mouse chromosome 11 exhibits linkage conservation with the
human chromosome 17. That is the same genes on the distal portion of the mouse chromosome
11 are on the human chromosome 17.
In mice scientists have engineered the following chromosome 11. This chromosome called In11
carries a Dominant marker Agouti. Agouti confers a yellow colour to the fur of mice that would
normally be black. The chromosome In11 is homozygous lethal. On the In11 chromosome,
scientists engineered an inversion between the genes Trp53 and Wnt3. Trp 53 and Wnt 3 are 24
cM (Centa Morgans) apart on a normal chromosome. 1cM equals 1% meiotic recombination.
a) What type of chromosome have these scientists created in mice that we have seen in
Drosophila? (5)
b) In the progeny of a mouse heterozygous for In11 and a normal chromosome 11, what is the
expected frequency of recombinants between In11 and wt11 (wild-type chromosome 11) and
between the markers Trp53 and Wnt3? (5)
We use In11 in the following screen. ENU is a powerful mutagen.
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c) What are the genotypes and phenotypes (fur and viability) of the four potential genotypes in
F2? (5)
d) There are two phenotypic outcomes of the pups born in F3.
i) What is the phenotypic outcome of the pups born in F3 for mutations that result in loss-offunction alleles in genes that are not essential for embryonic development? (2.5)
ii) What is the phenotypic outcome of the pups born in F3 for mutations that result in loss-offunction alleles in genes that are essential for embryonic development? (2.5)
3. Loss-of-function mutations in the nanos gene result in a maternal effect phenotype. The first
allele, nosL7 was discovered fortuitously. To analyze nos function genetically, more mutant
alleles needed to be generated in the nos gene. This is done in the following set of crosses.
EMS
Fo
Pr
TM3 Sb
X
F1
th st pp nosL7 e
TM3 Sb
X
F2
st e
st e
st e*
or st e*
TM3 Sb
Pr
Select individual males of either
of the two genotypes, and set up
4,000 of these crosses
hand select from the
4,000 crosses females of
st e*
th st pp nosL7 e
genotype
and test for sterility
a) This is a combination of two types of screens that we have talked about in lectures.
i) What is telling you this is a screen for maternal effect mutants? (2.5)
ii) What is the other screen being used here? (2.5)
b) In which cross/generation (F0, F1, F2, F3) are we mutagenizing chromosomes, separating
mutant chromosomes, amplifying mutant chromosomes, and testing for new mutations in nos.
(5)
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4.) A number of genetic screens for mutants that affect the cuticular morphology of the insect
Tribolium castaneum have been performed. Below are cuticles of wild-type Tribolium and 3
Tribolium homozygous for loss-of-function mutations in 3 different zygotically acting genes.
The results of the mutant screens performed in Drosophila were divided into five classes of
genes: the coordinate genes, the gap genes, the pair-rule genes, the segment polarity genes, and
the homeotic genes. What Drosophila phenotypic classification(s) do these Tribolium mutant
alleles 1, 2, 3 fall into based solely on their mutant phenotype. Based solely on the phenotypes
shown above, what is the protein encoded by the wild-type allele required for in the development
of the Tribolium embryo. (20)
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5.) The maternally deposited anterior and posterior signals set up the anterior posterior axis.
a) Describe the experiment using cytoplasmic transplantation that demonstrated a major
difference between the anterior and posterior signal. (5)
b) The anterior signal is encoded by bicoid. Bicoid protein is expressed as concentration
gradient across the anterior posterior axis. First, describe the experiment that demonstrated that
the concentration gradient of Bicoid was important for its function in the regulation of zygotic
hunchback expression. Second, describe the experiment that indicated how the Bicoid
concentration gradient determined multiple developmental states. (10)
c) How does the posterior signal negatively regulate the anterior signal? (5)
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