COMSAE PHASE 1 GENETICS PRACTICE
EXAM WITH ACTUAL QUESTIONS AND
VERIFIED ANSWERS, PLUS EXPLAINED
RATIONALES/EXPERT VERIFIED FOR
GUARANTEED 100% PASS 2026/LATEST
UPDATE/INSTANT DOWNLOAD PDF
Question 1
A 24-year-old woman with no significant medical history has a brother
with cystic fibrosis. Her parents are clinically unaffected. She is
planning a pregnancy and asks about her probability of being a carrier
for the familial CFTR mutation. Assume cystic fibrosis is inherited in an
autosomal recessive manner and that the woman's parents are both
obligate carriers. What is the probability that the woman is a carrier?
A. 0%
B. 25%
C. 50%
D. 67%
E. 100%
Answer: D. 67%
Rationale: Because the woman is unaffected and both parents are
obligate carriers, the possible genotypes are AA, Aa, Aa, and aa. Since
she is unaffected, the aa possibility is excluded. Among the remaining
three possibilities, two are carriers, giving a carrier probability of 2/3,
or approximately 67%. This is a classic application of conditional
probability in autosomal recessive inheritance.
Question 2
1
,A man with Marfan syndrome has a pathogenic variant in the FBN1
gene. His partner has no clinical features of Marfan syndrome and no
family history of the disorder. Assuming the affected man is
heterozygous, what is the probability that each child will inherit the
pathogenic allele?
A. 0%
B. 25%
C. 50%
D. 75%
E. 100%
Answer: C. 50%
Rationale: Marfan syndrome is usually inherited in an autosomal
dominant pattern. A heterozygous affected parent produces gametes
containing either the normal or pathogenic allele with equal
probability. Therefore, each pregnancy has a 50% chance of inheriting
the pathogenic allele.
Question 3
A newborn has hypotonia, characteristic facial features, a single
transverse palmar crease, and an atrioventricular septal defect.
Chromosomal analysis demonstrates 47 chromosomes with three copies
of chromosome 21. Which mechanism most commonly produces this
chromosomal abnormality?
A. Mitotic recombination
B. Meiotic nondisjunction
C. Uniparental disomy
D. Robertsonian translocation in all cases
E. Unequal crossing over
Answer: B. Meiotic nondisjunction
2
,Rationale: Trisomy 21 most commonly results from meiotic
nondisjunction, particularly during maternal meiosis I. Failure of
homologous chromosomes to separate produces a gamete containing
two copies of chromosome 21; fertilization by a normal gamete
produces a zygote with three copies.
Anaphase I: homologs fail to separate.
Nondisjunction occurs during
Meiosis IMeiosis II
Meiosis IMeiosis II
Move through the process
4.0
Move through the process
Give feedback
Question 4
3
, A 6-year-old boy has developmental delay, a long narrow face, large
ears, and macroorchidism. His maternal uncle has similar findings.
Molecular testing demonstrates expansion of a CGG trinucleotide repeat
in the FMR1 gene. Which genetic phenomenon best explains why the
disease may become more severe in subsequent generations?
A. Genomic imprinting
B. Anticipation
C. Codominance
D. Variable expressivity
E. Loss of heterozygosity
Answer: B. Anticipation
Rationale: Fragile X syndrome results from CGG repeat expansion in
FMR1. Repeat length can increase during transmission, particularly
through maternal transmission, producing earlier or more severe
disease in successive generations. This phenomenon is called
anticipation.
Question 5
A woman with phenotypically normal parents has a child with an
autosomal recessive metabolic disorder. Her husband is also an
unaffected carrier. What is the probability that their next child will be
affected?
A. 0%
B. 12.5%
C. 25%
D. 50%
E. 75%
Answer: C. 25%
4
EXAM WITH ACTUAL QUESTIONS AND
VERIFIED ANSWERS, PLUS EXPLAINED
RATIONALES/EXPERT VERIFIED FOR
GUARANTEED 100% PASS 2026/LATEST
UPDATE/INSTANT DOWNLOAD PDF
Question 1
A 24-year-old woman with no significant medical history has a brother
with cystic fibrosis. Her parents are clinically unaffected. She is
planning a pregnancy and asks about her probability of being a carrier
for the familial CFTR mutation. Assume cystic fibrosis is inherited in an
autosomal recessive manner and that the woman's parents are both
obligate carriers. What is the probability that the woman is a carrier?
A. 0%
B. 25%
C. 50%
D. 67%
E. 100%
Answer: D. 67%
Rationale: Because the woman is unaffected and both parents are
obligate carriers, the possible genotypes are AA, Aa, Aa, and aa. Since
she is unaffected, the aa possibility is excluded. Among the remaining
three possibilities, two are carriers, giving a carrier probability of 2/3,
or approximately 67%. This is a classic application of conditional
probability in autosomal recessive inheritance.
Question 2
1
,A man with Marfan syndrome has a pathogenic variant in the FBN1
gene. His partner has no clinical features of Marfan syndrome and no
family history of the disorder. Assuming the affected man is
heterozygous, what is the probability that each child will inherit the
pathogenic allele?
A. 0%
B. 25%
C. 50%
D. 75%
E. 100%
Answer: C. 50%
Rationale: Marfan syndrome is usually inherited in an autosomal
dominant pattern. A heterozygous affected parent produces gametes
containing either the normal or pathogenic allele with equal
probability. Therefore, each pregnancy has a 50% chance of inheriting
the pathogenic allele.
Question 3
A newborn has hypotonia, characteristic facial features, a single
transverse palmar crease, and an atrioventricular septal defect.
Chromosomal analysis demonstrates 47 chromosomes with three copies
of chromosome 21. Which mechanism most commonly produces this
chromosomal abnormality?
A. Mitotic recombination
B. Meiotic nondisjunction
C. Uniparental disomy
D. Robertsonian translocation in all cases
E. Unequal crossing over
Answer: B. Meiotic nondisjunction
2
,Rationale: Trisomy 21 most commonly results from meiotic
nondisjunction, particularly during maternal meiosis I. Failure of
homologous chromosomes to separate produces a gamete containing
two copies of chromosome 21; fertilization by a normal gamete
produces a zygote with three copies.
Anaphase I: homologs fail to separate.
Nondisjunction occurs during
Meiosis IMeiosis II
Meiosis IMeiosis II
Move through the process
4.0
Move through the process
Give feedback
Question 4
3
, A 6-year-old boy has developmental delay, a long narrow face, large
ears, and macroorchidism. His maternal uncle has similar findings.
Molecular testing demonstrates expansion of a CGG trinucleotide repeat
in the FMR1 gene. Which genetic phenomenon best explains why the
disease may become more severe in subsequent generations?
A. Genomic imprinting
B. Anticipation
C. Codominance
D. Variable expressivity
E. Loss of heterozygosity
Answer: B. Anticipation
Rationale: Fragile X syndrome results from CGG repeat expansion in
FMR1. Repeat length can increase during transmission, particularly
through maternal transmission, producing earlier or more severe
disease in successive generations. This phenomenon is called
anticipation.
Question 5
A woman with phenotypically normal parents has a child with an
autosomal recessive metabolic disorder. Her husband is also an
unaffected carrier. What is the probability that their next child will be
affected?
A. 0%
B. 12.5%
C. 25%
D. 50%
E. 75%
Answer: C. 25%
4
