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BIOD 210 MODULE 1-7 GENETICS ACTUAL EXAM
PREP 2026 ALL QUESTIONS AND CORRECT
DETAILED ANSWERS WITH RATIONALES ALREAD A
GRADED WITH EXPERT FEEDBACK |NEW AND
REVISED
1. In a monohybrid cross between two heterozygous pea plants (Aa ×
Aa), what proportion of the offspring will exhibit the recessive
phenotype?
A) 0%
B) 25%
C) 50%
D) 75%
*Rationale: The recessive phenotype appears only in homozygous
recessive (aa) offspring. From Aa × Aa, the genotypic ratio is 1 AA : 2
Aa : 1 aa, so 1/4 or 25% show the recessive trait.*
2. A woman is a carrier for hemophilia, an X-linked recessive disorder.
She has a child with a normal male. What is the probability that their son
will have hemophilia?
A) 0%
B) 25%
C) 50%
D) 100%
*Rationale: The mother is X^H X^h, father is X^H Y. Sons inherit X
from mother and Y from father. Probability son gets X^h = 1/2. Thus
50% of sons (but 25% of all children) will have hemophilia. The
question asks for probability son will have hemophilia, so 50%. Wait
careful: Among sons, 50% affected. Among all offspring, 25% affected
sons. The phrasing "their son" means given that the child is a son,
probability = 1/2. Answer C) 50%.*
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3. Which DNA base pairs are held together by three hydrogen bonds?
A) Adenine–Thymine
B) Guanine–Cytosine
C) Adenine–Uracil
D) Thymine–Guanine
Rationale: G-C base pairs form three hydrogen bonds, while A-T pairs
form two hydrogen bonds. This makes G-C rich regions more stable
and requires more energy to denature.
4. In Griffith's experiment with Streptococcus pneumoniae, which
combination resulted in mouse death and demonstrated the
"transforming principle"?
A) Live rough strain alone
B) Live smooth strain alone
C) Heat-killed smooth strain alone
D) Heat-killed smooth strain mixed with live rough strain
Rationale: Heat-killed smooth (virulent) strain alone did not kill mice.
However, when mixed with live rough (avirulent) strain, the rough
strain was transformed into smooth, killing mice – demonstrating that
a substance from dead smooth bacteria could genetically transform
live rough bacteria.
5. A codon is defined as:
A) A three-base sequence on tRNA that pairs with mRNA
B) A three-base sequence on mRNA that specifies an amino acid
C) A three-base sequence on DNA that binds RNA polymerase
D) A single base that determines a stop signal
Rationale: Codons are triplets of nucleotides in mRNA that are read
by the ribosome to incorporate specific amino acids during translation.
The complementary triplet on tRNA is the anticodon.
6. A woman with blood type AB (genotype I^A I^B) and a man with
blood type O (genotype ii) have children. Which blood types are
possible in their offspring?
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A) A and B only
B) A, B, and AB
C) A, B, AB, and O
D) A only
Rationale: The woman contributes either I^A or I^B; the man
contributes i. Offspring genotypes: I^A i (type A) or I^B i (type B).
Type O requires ii and type AB requires I^A I^B – neither possible. So
only A and B.
7. During DNA replication, which enzyme synthesizes the new DNA
strand by adding nucleotides to the 3' end?
A) Helicase
B) Primase
C) DNA polymerase III
D) DNA ligase
*Rationale: DNA polymerase III (in prokaryotes) or DNA polymerase
δ/ε (in eukaryotes) catalyzes the addition of deoxynucleotides to the
growing 3' OH group. Helicase unwinds DNA, primase synthesizes
RNA primers, ligase seals nicks.*
8. Which of the following is an example of a pleiotropic gene?
A) A gene that controls flower color in peas
B) The gene responsible for Marfan syndrome, affecting skeletal,
cardiovascular, and ocular systems
C) A gene with two alleles that show incomplete dominance
D) A gene located on the X chromosome
Rationale: Pleiotropy occurs when a single gene influences multiple
phenotypic traits. Marfan syndrome (FBN1 gene) affects connective
tissue in many systems. Option A is one trait; C is allelic interaction; D
is sex linkage.
9. A DNA strand has the sequence 5'-ATGCCT-3'. What is the
complementary strand sequence (written 5' to 3')?
A) 5'-TACGGA-3'
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B) 5'-AGGCAT-3'
C) 5'-AGGCAT-3' (yes, same as B)
D) 5'-ATGCCT-3'
*Rationale: Complementary base pairing: A-T, G-C. The
complementary sequence to 5'-ATGCCT-3' is 3'-TACGGA-5'. Written
5' to 3', it is 5'-AGGCAT-3'. So correct is 5'-AGGCAT-3'.*
10. In a pedigree, a trait appears in every generation and both males and
females are affected. Affected males pass the trait to all their daughters
but none of their sons. This pattern suggests:
A) Autosomal dominant
B) Autosomal recessive
C) X-linked dominant
D) X-linked recessive
*Rationale: X-linked dominant traits are passed from affected father to
all daughters (since daughters inherit father's X) and to no sons (sons
inherit father's Y). Affected mothers have 50% chance of passing to both
sons and daughters.*
11. Which molecule carries the genetic information from the nucleus to
the ribosome?
A) tRNA
B) rRNA
C) mRNA
D) snRNA
Rationale: Messenger RNA (mRNA) is transcribed from DNA and
carries the coding sequence to ribosomes for protein synthesis. tRNA
brings amino acids, rRNA is structural, snRNA involved in splicing.
12. In the lac operon, the presence of lactose leads to:
A) Binding of the lac repressor to the operator, blocking transcription
B) Binding of allolactose to the lac repressor, causing it to release
from the operator
C) Activation of the lac repressor by cAMP
BIOD 210 MODULE 1-7 GENETICS ACTUAL EXAM
PREP 2026 ALL QUESTIONS AND CORRECT
DETAILED ANSWERS WITH RATIONALES ALREAD A
GRADED WITH EXPERT FEEDBACK |NEW AND
REVISED
1. In a monohybrid cross between two heterozygous pea plants (Aa ×
Aa), what proportion of the offspring will exhibit the recessive
phenotype?
A) 0%
B) 25%
C) 50%
D) 75%
*Rationale: The recessive phenotype appears only in homozygous
recessive (aa) offspring. From Aa × Aa, the genotypic ratio is 1 AA : 2
Aa : 1 aa, so 1/4 or 25% show the recessive trait.*
2. A woman is a carrier for hemophilia, an X-linked recessive disorder.
She has a child with a normal male. What is the probability that their son
will have hemophilia?
A) 0%
B) 25%
C) 50%
D) 100%
*Rationale: The mother is X^H X^h, father is X^H Y. Sons inherit X
from mother and Y from father. Probability son gets X^h = 1/2. Thus
50% of sons (but 25% of all children) will have hemophilia. The
question asks for probability son will have hemophilia, so 50%. Wait
careful: Among sons, 50% affected. Among all offspring, 25% affected
sons. The phrasing "their son" means given that the child is a son,
probability = 1/2. Answer C) 50%.*
,2|Page
3. Which DNA base pairs are held together by three hydrogen bonds?
A) Adenine–Thymine
B) Guanine–Cytosine
C) Adenine–Uracil
D) Thymine–Guanine
Rationale: G-C base pairs form three hydrogen bonds, while A-T pairs
form two hydrogen bonds. This makes G-C rich regions more stable
and requires more energy to denature.
4. In Griffith's experiment with Streptococcus pneumoniae, which
combination resulted in mouse death and demonstrated the
"transforming principle"?
A) Live rough strain alone
B) Live smooth strain alone
C) Heat-killed smooth strain alone
D) Heat-killed smooth strain mixed with live rough strain
Rationale: Heat-killed smooth (virulent) strain alone did not kill mice.
However, when mixed with live rough (avirulent) strain, the rough
strain was transformed into smooth, killing mice – demonstrating that
a substance from dead smooth bacteria could genetically transform
live rough bacteria.
5. A codon is defined as:
A) A three-base sequence on tRNA that pairs with mRNA
B) A three-base sequence on mRNA that specifies an amino acid
C) A three-base sequence on DNA that binds RNA polymerase
D) A single base that determines a stop signal
Rationale: Codons are triplets of nucleotides in mRNA that are read
by the ribosome to incorporate specific amino acids during translation.
The complementary triplet on tRNA is the anticodon.
6. A woman with blood type AB (genotype I^A I^B) and a man with
blood type O (genotype ii) have children. Which blood types are
possible in their offspring?
,3|Page
A) A and B only
B) A, B, and AB
C) A, B, AB, and O
D) A only
Rationale: The woman contributes either I^A or I^B; the man
contributes i. Offspring genotypes: I^A i (type A) or I^B i (type B).
Type O requires ii and type AB requires I^A I^B – neither possible. So
only A and B.
7. During DNA replication, which enzyme synthesizes the new DNA
strand by adding nucleotides to the 3' end?
A) Helicase
B) Primase
C) DNA polymerase III
D) DNA ligase
*Rationale: DNA polymerase III (in prokaryotes) or DNA polymerase
δ/ε (in eukaryotes) catalyzes the addition of deoxynucleotides to the
growing 3' OH group. Helicase unwinds DNA, primase synthesizes
RNA primers, ligase seals nicks.*
8. Which of the following is an example of a pleiotropic gene?
A) A gene that controls flower color in peas
B) The gene responsible for Marfan syndrome, affecting skeletal,
cardiovascular, and ocular systems
C) A gene with two alleles that show incomplete dominance
D) A gene located on the X chromosome
Rationale: Pleiotropy occurs when a single gene influences multiple
phenotypic traits. Marfan syndrome (FBN1 gene) affects connective
tissue in many systems. Option A is one trait; C is allelic interaction; D
is sex linkage.
9. A DNA strand has the sequence 5'-ATGCCT-3'. What is the
complementary strand sequence (written 5' to 3')?
A) 5'-TACGGA-3'
, 4|Page
B) 5'-AGGCAT-3'
C) 5'-AGGCAT-3' (yes, same as B)
D) 5'-ATGCCT-3'
*Rationale: Complementary base pairing: A-T, G-C. The
complementary sequence to 5'-ATGCCT-3' is 3'-TACGGA-5'. Written
5' to 3', it is 5'-AGGCAT-3'. So correct is 5'-AGGCAT-3'.*
10. In a pedigree, a trait appears in every generation and both males and
females are affected. Affected males pass the trait to all their daughters
but none of their sons. This pattern suggests:
A) Autosomal dominant
B) Autosomal recessive
C) X-linked dominant
D) X-linked recessive
*Rationale: X-linked dominant traits are passed from affected father to
all daughters (since daughters inherit father's X) and to no sons (sons
inherit father's Y). Affected mothers have 50% chance of passing to both
sons and daughters.*
11. Which molecule carries the genetic information from the nucleus to
the ribosome?
A) tRNA
B) rRNA
C) mRNA
D) snRNA
Rationale: Messenger RNA (mRNA) is transcribed from DNA and
carries the coding sequence to ribosomes for protein synthesis. tRNA
brings amino acids, rRNA is structural, snRNA involved in splicing.
12. In the lac operon, the presence of lactose leads to:
A) Binding of the lac repressor to the operator, blocking transcription
B) Binding of allolactose to the lac repressor, causing it to release
from the operator
C) Activation of the lac repressor by cAMP
