Download Chemistry Problem Solving Drill

USMLE Step 1 - Problem Drill 09 : Genetics
Question No. 1 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems
on paper as needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
1. A 2 year old child exhibits gradually worsening muscle weakness and enlarged
calf muscles. You order genetic testing because you suspect a diagnosis of _____.
Question #01
(A) Duchenne’s muscular dystrophy
(B) Down Syndrome
(C) Steinert’s disease
(D) Hypocalcemia
(E) Edward’s Syndrome
A. Correct!
Duchenne’s muscular dystrophy results from a frame-shift mutation leading to the
deletion of the dystrophin gene and accelerated muscle breakdown.
B. Incorrect!
Down syndrome is not associated with muscular dystrophy.
C. Incorrect!
Steinert’s disease is a muscular dystrophy with adult onset.
Feedback
D. Incorrect!
Hypocalcemia does not lead to gradual weakening of muscles.
E. Incorrect!
Edward’s syndrome is a trisomy evident at birth.
Duchenne’s muscular dystrophy results from a frame-shift mutation leading to the
deletion of the dystrophin gene and accelerated muscle breakdown. A mnemonic is
Duchenne’s equals Deleted Dystrophin. The onset is before five years of age. The
weakness begins in the pelvic girdle muscles and progresses superiorly. There is
pseudohypertrophy of the calf muscles due to fibrofatty replacement of muscle and
cardiac myophathy. A characteristic of the disease is the use of Gower’ maneuver,
using the upper extremities to stand up.
Solution
(A)Duchenne’s muscular dystrophy
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Question No. 2 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems
on paper as needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
2. There are several types of DNA mutations. A ____ mutation results in a stop
codon appearing prematurely.
Question #02
(A) Missense
(B) Nonsense
(C) Random
(D) Frameshift
(E) Silent
A. Incorrect!
In missense mutations the codon codes for a different amino acid.
B. Correct!
A nonsense mutation results in a stop codon appearing prematurely.
C. Incorrect!
Random is not a type of DNA mutation.
Feedback
D. Incorrect!
A frameshift mutation results in a completely different translation from the original.
E. Incorrect!
A silent mutation is when the codn codes for the same amino acid, they often occur
in the third position of the codon.
There are several types of DNA mutations. In silent mutations the codon codes for
the same amino acid, they often occur in the third position of the codon. They can
be harmful, if the mutation occurs such that the required tRNA is not readily
available. In missense mutations the codon codes for a different amino acid. If the
new amino acid has similar properties the protein may function normally, this is
termed a “quiet” or conservative mutation. A nonsense mutation results in a stop
condon appearing prematurely, and can be remembered with the mnemonic Stop
the nonsense. These mutations can be ranked in order of the severity of damage:
nonsense is greater than missense, which is greater than silent. A frameshift
mutation results in a completely different translation from the original.
Solution
(B)Nonsense
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Question No. 3 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems
on paper as needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
3. In this type of repair a single base error caused by hydrolysis is recognized and
removed; the gap is filled and then resealed.
Question #03
(A) Mismatch repair
(B) Nucleotide excision repair
(C) Nonhomologous end joining
(D) Base excision repair
(E) Homologous recombination
A. Incorrect!
Mismatch repair corrects errors of DNA replication and recombination that result in
mispaired (but undamaged) nucleotides.
B. Incorrect!
Nucleotide excision repair recognizes bulky, helix-distorting lesions such as
pyrimidine dimers and 6,4 photoproducts.
C. Incorrect!
Double strand breaks are hazards to the cell and can be repaired by
nonhomologous end joining, microhomology-mediated end joining and homologous
recombination.
Feedback
D. Correct!
Base excision repair repairs damage to a single base caused by oxidation,
alkylation, hydrolysis, or deamination.
E. Incorrect!
Double strand breaks are hazards to the cell and can be repaired by
nonhomologous end joining, microhomology-mediated end joining and homologous
recombination.
Single strand DNA may be repaired by three different mechanisms:
Base excision repair repairs damage to a single base caused by oxidation,
alkylation, hydrolysis, or deamination. The damaged base is recognized and
removed; the gap is filled and resealed. Nucleotide excision repair recognizes bulky,
helix-distorting lesions such as pyrimidine dimers and 6,4 photoproducts. The
oligonucleotide containing the damage base is released; the gap is filled and
resealed. Mismatch repair corrects errors of DNA replication and recombination
that result in mispaired (but undamaged) nucleotides. The mismatched nucleotides
are removed; the gap is filled and resealed.
Solution
Double strand breaks are hazards to the cell and can be repaired by
nonhomologous end joining, microhomology-mediated end joining and homologous
recombination.
(D)Base excision repair
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Question No. 4 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems
on paper as needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
4. A child is born to two parents with albinism, yet the child is not affected by
albinism. What is the genetic mechanism that could explain why the child is not
affected?
Question #04
(A) Codominance
(B) Pleiptropy
(C) Dominant negative mutation
(D) Locus heterogeneity
(E) Anticipation
A. Incorrect!
Codominance occurs when neither of two alleles is dominant.
B. Incorrect!
Pleiotropy is when a single gene has more than one effect on phenotype within an
individual.
C. Incorrect!
A dominant negative mutation has a dominant phenotype.
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D. Correct!
Locus heterogeneity is when the same phenotype arises from mutations at different
loci, e.g. albinism.
E. Incorrect!
Anticipation is when the severity of disease worsens or occurs with an earlier age of
onset in succeeding generations.
Albinism is known to have variable inheritance patterns due to locus heterogeneity.
Locus heterogeneity is when the same phenotype arises from mutations at different
loci.
(D)Locus heterogeneity
Solution
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Question No. 5 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems
on paper as needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
5. Which of the following is not an assumption made in the Hardy-Weinberg
equation.
Question #05
(A) Small population size
(B) No selective pressure
(C) Random mating
(D) No mutations
(E) No emigration
A. Correct!
This is not an assumption made in the Hardy-Weinberg equation. In fact it is
assumed that there is an infinitely large population size.
B. Incorrect!
No selective pressure is an assumption of the Hardy-Weinberg equation.
C. Incorrect!
Random mating is an assumption of the Hardy-Weinberg equation.
Feedback
D. Incorrect!
No mutation is one of the assumptions of the Hardy-Weinberg equation.
E. Incorrect!
No emigration is one of the assumptions of the Hardy-Weinberg equation.
The Hardy-Weinberg principle states that both allele and genotype frequencies in a
population remain constant from generation to generation. It assumes that there is
random mating, no migration or emigration in an infinitely large population size, no
mutation, and no selective pressure for or against any traits. If so, the equation for
disease prevalence is p2 + 2pq + q2 = 1 and the equation for allele prevalence is p +
q = 1, where p and q are separate alleles. The heterozygote prevalence is 2pq and
the homozygote prevalence is p2 or q2. When the gene is sex linked the disease
prevalence for males is q and for females is q2, e.g. red-green colorblindness.
(A)Small population size
Solution
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Question No. 6 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems
on paper as needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
6. A woman diagnosed with phenylketonuria is advised she has a 25% chance of
passing this condition on to her offspring. What type of disease is phenylketonuria?
Question #06
(A) X-linked dominant
(B) Autosomal dominant
(C) Mitochondrial inheritance
(D) X-linked recessive
(E) Autosomal recessive
A. Incorrect!
In X-linked dominant inheritance transmission is through both parents: if the father
is affected, then all female offspring are diseased, and if the mother is affected, the
male or female offspring may be affected.
B. Incorrect!
In autosomal dominant inheritance many generations of both males and females are
affected and defects in structural genes are common. It is often pleiotropic, in many
cases clinical presentation occurs at puberty.
C. Incorrect!
In mitochondrial inheritance transmission is only through the mother and all
offspring may show signs of disease. Due to heteroplasmy there may be variable
expression in the population.
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D. Incorrect!
In X-linked recessive inheritance sons of heterozygous mothers have a fifty percent
chance of being affected. Typically more severe in males, although heterozygous
females can be affected. There is no male-to-male transmission.
E. Correct!
In autosomal recessive inheritance twenty-five percent of offspring are affected, it
usually only seen in one generation and enzyme deficiencies are common.
In autosomal recessive inheritance twenty-five percent of offspring are affected, it
usually only seen in one generation and enzyme deficiencies are common. Typically
more severe than dominant disorders, in many cases clinical presentation occurs in
childhood. Phenylketonuria is an autosomal recessive disorder.
(E)Autosomal recessive
Solution
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Question No. 7 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems
on paper as needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
7. With this disease, the mother may be a carrier with no symptoms due to random
inactivation of the X chromosome, yet she must be advised that any sons have a
50% chance of being affected by _____.
Question #07
(A) Cystic fibrosis
(B) Fabry’s disease
(C) Huntington’s disease
(D) Marfan’s disease
(E) Duchenne’s muscular dystrophy
A. Incorrect!
Cystic fibrosis is an autosomal recessive disease.
B. Correct!
Fabry’s disease is an x-linked recessive disorder.
C. Incorrect!
Huntington’s disease is an autosomal dominant disease.
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D. Incorrect!
Marfan’s disease is an autosomal dominant disease.
E. Incorrect!
Duchenne’s muscular dystrophy results from a frame-shift mutation leading to the
deletion of the dystophin gene and accelerated muscle breakdown.
In X-linked recessive inheritance sons of heterozygous mothers have a fifty percent
chance of being affected. Female carriers of X-linked recessive disorders can be
affected due to random inactivation of X chromosomes in each cell, but this is rare.
Fabry’s disease is an x-linked disorder, fitting these criteria.
(B)Fabry’s disease
Solution
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Question No. 8 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems
on paper as needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
8. A new born child presents with chronic pulmonary infections, pancreatic
insufficiency and meconium ileus. You order genetic testing because you suspect
_____.
Question #08
(A) Cystic fibrosis
(B) Cri-Du-Chat syndrome
(C) 22q11 Syndrome
(D) Fragile X syndrome
(E) Hypercholesterolemia
A. Correct!
Cystic fibrosis has these symptoms.
B. Incorrect!
Cri-Du-Chat syndrome is a congenital deletion of the short arm of chromosome 5.
C. Incorrect!
22q11 deletion syndrome is caused by the deletion of a small piece of chromosome
22.
Feedback
D. Incorrect!
In fragile X syndrome the defect affects the methylation and expression of the FMR1
gene and it is a triplet repeat disorder.
E. Incorrect!
Hypertcholesterolemia is an autosomal dominant disease.
Cystic fibrosis arises from an autosomal-recessive defect in the CFTR gene on
chromosome 7, most commonly deletion, of phenylalanine 508. CFTR is a chloride
channel which actively secretes chloride in the lung and GI tract and reabsorbs
chloride from sweat. The presence of a defective channel results in the secretion of
abnormally thick mucus that plugs the lungs, pancreas and liver resulting in
recurrant pulmonary infections, chronic bronchitis, bronchiectasis, pancreatic
insufficiency and meconium ileus in newborns. The diagnostic test is a sweat test
which looks for increased concentration of chloride in the sweat. It can present as a
failure to thrive in infants. Cystic fibrosis is the most common lethal genetic disease
of Caucasians.
Solution
(A)Cystic fibrosis
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Question No. 9 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems
on paper as needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
9. A 5 year old male child presents with macro-orchidism, a long face with long jaw,
and has previously been diagnosed as autistic. What do you suspect is the cause?
Question #09
(A) Down syndrome
(B) Duschenne’s muscular dystrophy
(C) Becker’s muscular dystrophy
(D) Cystic fibrosis
(E) Fragile X syndrome
A. Incorrect!
Think of a disorder linked to the X chromosome.
B. Incorrect!
Think of a disorder linked to the X chromosome.
C. Incorrect!
Think of a disorder linked to the X chromosome.
Feedback
D. Incorrect!
Think of a disorder linked to the X chromosome.
E. Correct!
Macro-orchidism, enlarged testes, long face with long jaw, large everted ears and
autism are associated with the syndrome.
In fragile X syndrome the defect affects the methylation and expression of the FMR1
gene and it is a triplet repeat disorder. It is associated with chromosomal breakage
and is the second more common genetic cause of mental retardation after Down
syndrome. Macro-orchidism, enlarged testes, long face with long jaw, large everted
ears and autism are associated with the syndrome. A mnemonic is Fragile X equals
eXtra-large testes, jaw, ears.
(E)Fragile X syndrome
Solution
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Question No. 10 of 10
Instructions: (1) Read the problem statement and answer choices carefully, (2) Work the problems
on paper as needed, (3) Pick the answer, and (4) Review the core concept tutorial as needed.
10. Which of the following results from a deletion in chromosome 5?
Question #10
(A) Cri-du-chat
(B) Cystic fibrosis
(C) Huntington’s disease
(D) Down syndrome
(E) Marfan’s syndrome
A. Correct!
Cri-du-chat is a deletion in chromosome 5.
B. Incorrect!
Cystic fibrosis results from a defect of the CFTR gene on chromosome 7.
C. Incorrect!
Huntington’s disease is a triple repeat disorder of chromosome 4.
Feedback
D. Incorrect!
Down syndrome is due to trisomy, not mutation.
E. Incorrect!
Marfan’s syndrome results from mutation of the fibrillin gene.
Cri-Du-Chat syndrome is a congenital deletion of the short arm of chromosome 5. A
mnemonic is Cri du chat = cry of the cat. Findings include microcephaly, severe
mental retardation, high-pitched crying/mewing, epicanthal folds and cardiac
abnormalities.
(A)Cri-du-chat
Solution
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