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MMSC491 Exam 2 – Comprehensive Study Guide Covering
Core Course Concepts, Advanced Theoretical Principles,
Clinical Applications, Case-Based Learning, Critical Thinking
Strategies, Key Terminology, Practice Questions, and Exam
Preparation for Academic Success
EXAM COVERAGE SUMMARY
This examination comprehensively covers core concepts in medical genetics and genomics including:
fundamental principles of inheritance patterns (autosomal dominant, autosomal recessive, X-linked
recessive, X-linked dominant, Y-linked, and mitochondrial), pedigree analysis and symbol interpretation,
genotype-phenotype relationships, dominance hierarchies including codominance, chromosomal
abnormalities and their clinical consequences, mutation classification and mechanisms (including point
mutations, frameshifts, nonsense, missense, silent, and dynamic mutations), genetic heterogeneity
including allelic, locus, and phenotypic heterogeneity, penetrance and expressivity concepts, mosaicism
(somatic and germ-line), genomic imprinting and epigenetic mechanisms, X-chromosome inactivation
and lyonization, noncoding RNA function including miRNAs and RNA interference, transcriptional and
post-transcriptional regulation, cis-acting and trans-acting regulatory elements, DNA repair mechanisms
and their clinical significance, pharmacogenetics and personalized medicine, gene therapy approaches
including viral and non-viral vectors, clinical trial phases and drug development, treatment strategies
including augmentation therapy and metabolic management, molecular pathogenesis of specific disorders
including cystic fibrosis, Huntington disease, fragile X syndrome, and mitochondrial disorders,
chromosomal nomenclature and karyotype interpretation, and advanced concepts in genomic medicine
including CRISPR and genome editing technologies.
Question 1: A 45-year-old male presents with progressive choreiform movements, cognitive decline, and
psychiatric symptoms. His father died at age 58 from complications of a neurodegenerative disorder, and
his grandfather also displayed similar symptoms in his late fifties. Genetic testing reveals an expanded
CAG trinucleotide repeat in the HTT gene. Which phenomenon best explains why this patient's children
may develop symptoms at an earlier age than he did?
A) Genetic imprinting
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B) Locus heterogeneity
C) Anticipation
D) Anticipation RATIONALE: Anticipation refers to the phenomenon where a genetic disorder
becomes more severe or manifests at an earlier age in successive generations. This occurs with dynamic
mutations such as trinucleotide repeat expansions, where the repeat length tends to increase during
transmission from parent to child. In Huntington disease, the CAG repeat expansion in the HTT gene
demonstrates anticipation, with larger repeat lengths typically associated with earlier onset and more
severe disease. The father's later onset at age 58 and the patient's earlier onset at age 45, with the potential
for even earlier onset in subsequent generations, exemplifies this phenomenon. This pattern is not
consistent with genetic imprinting, which involves parent-of-origin specific gene expression, nor locus
heterogeneity, which refers to mutations at different genetic loci causing similar phenotypes.
Question 2: A researcher is studying a family in which a rare disorder appears to affect only male
members of the family across multiple generations. The disorder is transmitted from an affected father to
all of his sons but never to his daughters. Which inheritance pattern is most likely responsible for this
disorder?
A) X-linked recessive
B) X-linked dominant
C) Autosomal dominant with sex-limited expression
D) Y-linked RATIONALE: Y-linked inheritance is characterized by transmission of a genetic variant
from father to all sons, since males inherit their Y chromosome exclusively from their fathers. Females do
not possess a Y chromosome and therefore cannot inherit or express Y-linked disorders. The pattern
described—affected fathers transmitting the condition to all sons but no daughters—is the hallmark of Y-
linked inheritance. This inheritance pattern is rare because the Y chromosome contains relatively few
genes, most of which are involved in sex determination and spermatogenesis. X-linked recessive
inheritance would show affected males but with no male-to-male transmission, as fathers pass their X
chromosome to daughters only. X-linked dominant would affect both males and females. Autosomal
dominant with sex-limited expression would show transmission to both sexes but expression limited to
one sex, which does not match the described transmission pattern.
Question 3: A couple presents for genetic counseling. The woman is a carrier for an X-linked recessive
disorder, and the man is unaffected. What is the probability that their male children will be affected?
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A) 0%
B) 25%
C) 50%
D) 100%
RATIONALE: In X-linked recessive inheritance, a female carrier has one normal X chromosome and
one X chromosome carrying the pathogenic variant. Male offspring receive their X chromosome from
their mother and their Y chromosome from their father. Each male child has a 50% chance of inheriting
the mother's X chromosome that carries the pathogenic variant and a 50% chance of inheriting her normal
X chromosome. If a male inherits the affected X chromosome, he will be affected because males are
hemizygous for X-linked genes—they have only one copy of the X chromosome and no second allele to
compensate for the pathogenic variant. Female children would have a 50% chance of being carriers but
would not be affected unless they inherited the variant from both parents (which is not possible here as
the father is unaffected). Thus, the correct probability for affected male children is 50%.
Question 4: Which of the following DNA repair mechanisms would be most critical for correcting
damage caused by ultraviolet radiation exposure resulting in the formation of pyrimidine dimers?
A) Base excision repair
B) Mismatch repair
C) Homologous recombination
D) Nucleotide excision repair RATIONALE: Nucleotide excision repair (NER) is the primary DNA
repair pathway responsible for removing bulky DNA lesions that distort the double helix, including
pyrimidine dimers caused by ultraviolet radiation. UV radiation induces the formation of cyclobutane
pyrimidine dimers and 6-4 photoproducts between adjacent pyrimidine bases, creating significant
distortion in the DNA structure. NER recognizes these distortions, excises a short single-stranded DNA
segment containing the damage, and uses the undamaged strand as a template for repair synthesis.
Xeroderma pigmentosum, a disorder characterized by extreme UV sensitivity and predisposition to skin
cancer, results from defects in the NER pathway. Base excision repair primarily handles small base
modifications such as oxidation or alkylation. Mismatch repair corrects replication errors such as base-
base mismatches and insertion-deletion loops. Homologous recombination repairs double-strand breaks
using a homologous template.
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Question 5: A 6-year-old male is evaluated for developmental delay, large ears, macroorchidism, and
autistic-like behaviors. The family history reveals that the mother's brother had similar features and died
from complications of seizures. Molecular analysis shows an expanded CGG repeat in the FMR1 gene.
What is the most likely diagnosis and mechanism of disease?
A) Huntington disease with anticipation
B) Myotonic dystrophy with trinucleotide expansion
C) Fragile X syndrome with unstable repeat expansion
D) Friedrich ataxia with GAA expansion
RATIONALE: Fragile X syndrome is the most common inherited cause of intellectual disability and is
caused by an unstable expansion of a CGG trinucleotide repeat in the 5' untranslated region of the FMR1
gene on the X chromosome. In affected individuals, the repeat expansion exceeds 200 copies (full
mutation), leading to hypermethylation of the promoter region and subsequent transcriptional silencing of
FMR1, resulting in absence of the fragile X mental retardation protein (FMRP). The classic features
include developmental delay, intellectual disability, characteristic facial features (long face, large ears),
macroorchidism, and autistic behaviors. The unstable nature of the repeat expansion leads to anticipation,
with increasing repeat length and more severe phenotypes in successive generations. Huntington disease
involves a CAG repeat expansion in the HTT gene causing a polyglutamine tract. Myotonic dystrophy
involves a CTG expansion in DMPK or CCTG expansion in CNBP. Friedrich ataxia involves a GAA
repeat expansion in the FXN gene.
Question 6: A researcher is studying a transcription factor that regulates gene expression by binding to a
specific DNA sequence located several thousand base pairs upstream of the target gene. When this
transcription factor binds, it increases transcription of the target gene. What type of regulatory element is
this transcription factor binding to?
A) Promoter
B) Silencer
C) Enhancer
D) Insulator
RATIONALE: Enhancers are cis-acting regulatory DNA sequences that can increase transcription of
genes from which they may be located at considerable distances—sometimes hundreds of thousands of
base pairs away from the promoter. They function by binding specific transcription factors that interact
with the basal transcriptional machinery at the promoter through DNA looping mechanisms, bringing the
enhancer and promoter into close physical proximity. Enhancers can be located upstream, downstream, or
MMSC491 Exam 2 – Comprehensive Study Guide Covering
Core Course Concepts, Advanced Theoretical Principles,
Clinical Applications, Case-Based Learning, Critical Thinking
Strategies, Key Terminology, Practice Questions, and Exam
Preparation for Academic Success
EXAM COVERAGE SUMMARY
This examination comprehensively covers core concepts in medical genetics and genomics including:
fundamental principles of inheritance patterns (autosomal dominant, autosomal recessive, X-linked
recessive, X-linked dominant, Y-linked, and mitochondrial), pedigree analysis and symbol interpretation,
genotype-phenotype relationships, dominance hierarchies including codominance, chromosomal
abnormalities and their clinical consequences, mutation classification and mechanisms (including point
mutations, frameshifts, nonsense, missense, silent, and dynamic mutations), genetic heterogeneity
including allelic, locus, and phenotypic heterogeneity, penetrance and expressivity concepts, mosaicism
(somatic and germ-line), genomic imprinting and epigenetic mechanisms, X-chromosome inactivation
and lyonization, noncoding RNA function including miRNAs and RNA interference, transcriptional and
post-transcriptional regulation, cis-acting and trans-acting regulatory elements, DNA repair mechanisms
and their clinical significance, pharmacogenetics and personalized medicine, gene therapy approaches
including viral and non-viral vectors, clinical trial phases and drug development, treatment strategies
including augmentation therapy and metabolic management, molecular pathogenesis of specific disorders
including cystic fibrosis, Huntington disease, fragile X syndrome, and mitochondrial disorders,
chromosomal nomenclature and karyotype interpretation, and advanced concepts in genomic medicine
including CRISPR and genome editing technologies.
Question 1: A 45-year-old male presents with progressive choreiform movements, cognitive decline, and
psychiatric symptoms. His father died at age 58 from complications of a neurodegenerative disorder, and
his grandfather also displayed similar symptoms in his late fifties. Genetic testing reveals an expanded
CAG trinucleotide repeat in the HTT gene. Which phenomenon best explains why this patient's children
may develop symptoms at an earlier age than he did?
A) Genetic imprinting
, Page |2
B) Locus heterogeneity
C) Anticipation
D) Anticipation RATIONALE: Anticipation refers to the phenomenon where a genetic disorder
becomes more severe or manifests at an earlier age in successive generations. This occurs with dynamic
mutations such as trinucleotide repeat expansions, where the repeat length tends to increase during
transmission from parent to child. In Huntington disease, the CAG repeat expansion in the HTT gene
demonstrates anticipation, with larger repeat lengths typically associated with earlier onset and more
severe disease. The father's later onset at age 58 and the patient's earlier onset at age 45, with the potential
for even earlier onset in subsequent generations, exemplifies this phenomenon. This pattern is not
consistent with genetic imprinting, which involves parent-of-origin specific gene expression, nor locus
heterogeneity, which refers to mutations at different genetic loci causing similar phenotypes.
Question 2: A researcher is studying a family in which a rare disorder appears to affect only male
members of the family across multiple generations. The disorder is transmitted from an affected father to
all of his sons but never to his daughters. Which inheritance pattern is most likely responsible for this
disorder?
A) X-linked recessive
B) X-linked dominant
C) Autosomal dominant with sex-limited expression
D) Y-linked RATIONALE: Y-linked inheritance is characterized by transmission of a genetic variant
from father to all sons, since males inherit their Y chromosome exclusively from their fathers. Females do
not possess a Y chromosome and therefore cannot inherit or express Y-linked disorders. The pattern
described—affected fathers transmitting the condition to all sons but no daughters—is the hallmark of Y-
linked inheritance. This inheritance pattern is rare because the Y chromosome contains relatively few
genes, most of which are involved in sex determination and spermatogenesis. X-linked recessive
inheritance would show affected males but with no male-to-male transmission, as fathers pass their X
chromosome to daughters only. X-linked dominant would affect both males and females. Autosomal
dominant with sex-limited expression would show transmission to both sexes but expression limited to
one sex, which does not match the described transmission pattern.
Question 3: A couple presents for genetic counseling. The woman is a carrier for an X-linked recessive
disorder, and the man is unaffected. What is the probability that their male children will be affected?
, Page |3
A) 0%
B) 25%
C) 50%
D) 100%
RATIONALE: In X-linked recessive inheritance, a female carrier has one normal X chromosome and
one X chromosome carrying the pathogenic variant. Male offspring receive their X chromosome from
their mother and their Y chromosome from their father. Each male child has a 50% chance of inheriting
the mother's X chromosome that carries the pathogenic variant and a 50% chance of inheriting her normal
X chromosome. If a male inherits the affected X chromosome, he will be affected because males are
hemizygous for X-linked genes—they have only one copy of the X chromosome and no second allele to
compensate for the pathogenic variant. Female children would have a 50% chance of being carriers but
would not be affected unless they inherited the variant from both parents (which is not possible here as
the father is unaffected). Thus, the correct probability for affected male children is 50%.
Question 4: Which of the following DNA repair mechanisms would be most critical for correcting
damage caused by ultraviolet radiation exposure resulting in the formation of pyrimidine dimers?
A) Base excision repair
B) Mismatch repair
C) Homologous recombination
D) Nucleotide excision repair RATIONALE: Nucleotide excision repair (NER) is the primary DNA
repair pathway responsible for removing bulky DNA lesions that distort the double helix, including
pyrimidine dimers caused by ultraviolet radiation. UV radiation induces the formation of cyclobutane
pyrimidine dimers and 6-4 photoproducts between adjacent pyrimidine bases, creating significant
distortion in the DNA structure. NER recognizes these distortions, excises a short single-stranded DNA
segment containing the damage, and uses the undamaged strand as a template for repair synthesis.
Xeroderma pigmentosum, a disorder characterized by extreme UV sensitivity and predisposition to skin
cancer, results from defects in the NER pathway. Base excision repair primarily handles small base
modifications such as oxidation or alkylation. Mismatch repair corrects replication errors such as base-
base mismatches and insertion-deletion loops. Homologous recombination repairs double-strand breaks
using a homologous template.
, Page |4
Question 5: A 6-year-old male is evaluated for developmental delay, large ears, macroorchidism, and
autistic-like behaviors. The family history reveals that the mother's brother had similar features and died
from complications of seizures. Molecular analysis shows an expanded CGG repeat in the FMR1 gene.
What is the most likely diagnosis and mechanism of disease?
A) Huntington disease with anticipation
B) Myotonic dystrophy with trinucleotide expansion
C) Fragile X syndrome with unstable repeat expansion
D) Friedrich ataxia with GAA expansion
RATIONALE: Fragile X syndrome is the most common inherited cause of intellectual disability and is
caused by an unstable expansion of a CGG trinucleotide repeat in the 5' untranslated region of the FMR1
gene on the X chromosome. In affected individuals, the repeat expansion exceeds 200 copies (full
mutation), leading to hypermethylation of the promoter region and subsequent transcriptional silencing of
FMR1, resulting in absence of the fragile X mental retardation protein (FMRP). The classic features
include developmental delay, intellectual disability, characteristic facial features (long face, large ears),
macroorchidism, and autistic behaviors. The unstable nature of the repeat expansion leads to anticipation,
with increasing repeat length and more severe phenotypes in successive generations. Huntington disease
involves a CAG repeat expansion in the HTT gene causing a polyglutamine tract. Myotonic dystrophy
involves a CTG expansion in DMPK or CCTG expansion in CNBP. Friedrich ataxia involves a GAA
repeat expansion in the FXN gene.
Question 6: A researcher is studying a transcription factor that regulates gene expression by binding to a
specific DNA sequence located several thousand base pairs upstream of the target gene. When this
transcription factor binds, it increases transcription of the target gene. What type of regulatory element is
this transcription factor binding to?
A) Promoter
B) Silencer
C) Enhancer
D) Insulator
RATIONALE: Enhancers are cis-acting regulatory DNA sequences that can increase transcription of
genes from which they may be located at considerable distances—sometimes hundreds of thousands of
base pairs away from the promoter. They function by binding specific transcription factors that interact
with the basal transcriptional machinery at the promoter through DNA looping mechanisms, bringing the
enhancer and promoter into close physical proximity. Enhancers can be located upstream, downstream, or
