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 mothersphenotype of embryos laid
+/+ (wild-type)normal embryos
trklf/trklfno termini
tsllf/tsllfno termini
torgf/+only termini
torgf/+; trklf/trklfonly termini
torgf/+; tsllf/tsllfonly termini
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)
torsotailless
genotype of mothersgenotype of embryosphenotype of embryos
+/++/+normal embryos
torgf/++/+only termini
+/+tlllf/tlllfno termini
torgf/+tlllf/tlllfno 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 donorgenotype of the phenotype of
recipient embryo eggs laid
torlf/torlfOvoD/+no termini
trklf/trklfOvoD/+no termini
tsllf/tsllfOvoD/+wild-type
wild-typeOvoD/+; torlf/torlfwild-type
wild-typeOvoD/+; trklf/trklfwild-type
wild-typeOvcD/+; tsllf/tsllfno 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)
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.
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-of-function 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-of-function 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
FoPrXst e
TM3 Sbst e
F1th st ppnosL7eXst e* or st e*
TM3 SbTM3 Sb Pr
Select individual males of either of the two genotypes, and set up 4,000 of these crosses
F2hand select from the
4,000 crosses females of
st e*
th st pp nosL7egenotype
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)
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)
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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