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BIOD210 Exam 1: Mendelian Foundations
35 Questions | Monohybrid/Dihybrid Crosses, Probability, Pedigree Basics,
Chromosomal Inheritance
1. In garden peas, tall (T) is dominant to dwarf (t). If a heterozygous tall plant is crossed
with a dwarf plant, what is the expected phenotypic ratio of the offspring?
● A. All tall
● B. 3 tall : 1 dwarf
● C. 1 tall : 1 dwarf
● D. All dwarf
Correct Answer: C
Rationale: This is a test cross. The heterozygous tall parent (Tt) produces gametes T
and t in equal proportions (1:1). The dwarf parent (tt) produces only t gametes. The
Punnett square yields: Tt (tall) and tt (dwarf) in 1:1 ratio. Option A would result from TT
× tt; B from Tt × Tt; D from tt × tt.
,2. A true-breeding purple-flowered pea plant is crossed with a true-breeding
white-flowered plant. All F₁ offspring are purple. When F₁ plants are self-fertilized, what
proportion of F₂ plants will be true-breeding?
● A. 1/4
● B. 1/2
● C. 3/4
● D. All of them
Correct Answer: B
Rationale: P: PP × pp → F₁: all Pp. F₁ self-cross: Pp × Pp → F₂: 1 PP : 2 Pp : 1 pp.
True-breeding genotypes are PP and pp = 1/4 + 1/4 = 1/2. The 2/4 Pp are not
true-breeding.
3. In humans, the ability to taste PTC (T) is dominant to non-tasting (t). If two
heterozygous tasters have four children, what is the probability that exactly three will be
tasters?
● A. 3/4
● B. 27/64
● C. 9/64
● D. 27/256
Correct Answer: B
Rationale: Use binomial expansion: probability of taster = 3/4, non-taster = 1/4. For
exactly 3 tasters in 4 children: C(4,3) × (3/4)³ × (1/4)¹ = 4 × 27/64 × 1/4 = 108/256 =
27/64.
4. In summer squash, white fruit (W) is dominant to yellow (w), and disk-shaped (D) is
dominant to sphere-shaped (d). A WwDd plant is crossed with a wwdd plant. What
proportion of offspring will be white and sphere-shaped?
, ● A. 1/4
● B. 1/8
● C. 3/8
● D. 1/2
Correct Answer: A
Rationale: This is a test cross for two genes. WwDd produces gametes: WD, Wd, wD, wd
(1:1:1:1). wwdd produces only wd. Offspring: WwDd (white, disk), Wwdd (white, sphere),
wwDd (yellow, disk), wwdd (yellow, sphere) in 1:1:1:1 ratio. White, sphere = 1/4.
5. The probability of event A is 0.3 and event B is 0.4. If A and B are independent, what is
P(A and B)?
● A. 0.7
● B. 0.12
● C. 0.1
● D. Cannot be determined
Correct Answer: B
Rationale: For independent events, P(A and B) = P(A) × P(B) = 0.3 × 0.4 = 0.12. Option A
adds probabilities (wrong for "and"); C subtracts; D is incorrect because independence is
stated.
6. In a monohybrid cross with complete dominance, the F₂ phenotypic ratio is 3:1. What
is the expected genotypic ratio?
● A. 1:2:1
● B. 3:1
● C. 9:3:3:1
● D. 1:1:1:1
Correct Answer: A
, Rationale: F₁: Aa × Aa → F₂: 1 AA : 2 Aa : 1 aa. This is the genotypic ratio. Phenotypic 3:1
combines AA and Aa as dominant phenotype. C is dihybrid phenotypic; D is test cross
result.
7. A couple both have type A blood. Their first child has type O blood. What is the
probability their second child will also have type O blood?
● A. 0%
● B. 25%
● C. 50%
● D. 75%
Correct Answer: B
Rationale: Type A parents with type O child must both be heterozygous Iᴬi. Cross: Iᴬi × Iᴬi
→ 1 IᴬIᴬ : 2 Iᴬi : 1 ii. Each child has independent 25% chance of type O. Previous child's
blood type doesn't affect next (independent events).
8. In Drosophila, red eyes (w⁺) are dominant to white eyes (w). Which cross would
definitively determine if a red-eyed male is heterozygous or homozygous?
● A. Cross with red-eyed female
● B. Cross with white-eyed female
● C. Cross with heterozygous female
● D. Self-fertilization
Correct Answer: B
Rationale: This is a test cross. Crossing with homozygous recessive (white-eyed female,
w/w) reveals the male's genotype: if all offspring red-eyed, male is w⁺/w⁺; if 1:1 ratio,
male is w⁺/w. Other crosses give ambiguous results.
EXAM PACKAGE 2026/2027 Exams | Actual Questions &
Verified Answers | Comprehensive Genetics Assessment |
Pass Guarantee
BIOD210 Exam 1: Mendelian Foundations
35 Questions | Monohybrid/Dihybrid Crosses, Probability, Pedigree Basics,
Chromosomal Inheritance
1. In garden peas, tall (T) is dominant to dwarf (t). If a heterozygous tall plant is crossed
with a dwarf plant, what is the expected phenotypic ratio of the offspring?
● A. All tall
● B. 3 tall : 1 dwarf
● C. 1 tall : 1 dwarf
● D. All dwarf
Correct Answer: C
Rationale: This is a test cross. The heterozygous tall parent (Tt) produces gametes T
and t in equal proportions (1:1). The dwarf parent (tt) produces only t gametes. The
Punnett square yields: Tt (tall) and tt (dwarf) in 1:1 ratio. Option A would result from TT
× tt; B from Tt × Tt; D from tt × tt.
,2. A true-breeding purple-flowered pea plant is crossed with a true-breeding
white-flowered plant. All F₁ offspring are purple. When F₁ plants are self-fertilized, what
proportion of F₂ plants will be true-breeding?
● A. 1/4
● B. 1/2
● C. 3/4
● D. All of them
Correct Answer: B
Rationale: P: PP × pp → F₁: all Pp. F₁ self-cross: Pp × Pp → F₂: 1 PP : 2 Pp : 1 pp.
True-breeding genotypes are PP and pp = 1/4 + 1/4 = 1/2. The 2/4 Pp are not
true-breeding.
3. In humans, the ability to taste PTC (T) is dominant to non-tasting (t). If two
heterozygous tasters have four children, what is the probability that exactly three will be
tasters?
● A. 3/4
● B. 27/64
● C. 9/64
● D. 27/256
Correct Answer: B
Rationale: Use binomial expansion: probability of taster = 3/4, non-taster = 1/4. For
exactly 3 tasters in 4 children: C(4,3) × (3/4)³ × (1/4)¹ = 4 × 27/64 × 1/4 = 108/256 =
27/64.
4. In summer squash, white fruit (W) is dominant to yellow (w), and disk-shaped (D) is
dominant to sphere-shaped (d). A WwDd plant is crossed with a wwdd plant. What
proportion of offspring will be white and sphere-shaped?
, ● A. 1/4
● B. 1/8
● C. 3/8
● D. 1/2
Correct Answer: A
Rationale: This is a test cross for two genes. WwDd produces gametes: WD, Wd, wD, wd
(1:1:1:1). wwdd produces only wd. Offspring: WwDd (white, disk), Wwdd (white, sphere),
wwDd (yellow, disk), wwdd (yellow, sphere) in 1:1:1:1 ratio. White, sphere = 1/4.
5. The probability of event A is 0.3 and event B is 0.4. If A and B are independent, what is
P(A and B)?
● A. 0.7
● B. 0.12
● C. 0.1
● D. Cannot be determined
Correct Answer: B
Rationale: For independent events, P(A and B) = P(A) × P(B) = 0.3 × 0.4 = 0.12. Option A
adds probabilities (wrong for "and"); C subtracts; D is incorrect because independence is
stated.
6. In a monohybrid cross with complete dominance, the F₂ phenotypic ratio is 3:1. What
is the expected genotypic ratio?
● A. 1:2:1
● B. 3:1
● C. 9:3:3:1
● D. 1:1:1:1
Correct Answer: A
, Rationale: F₁: Aa × Aa → F₂: 1 AA : 2 Aa : 1 aa. This is the genotypic ratio. Phenotypic 3:1
combines AA and Aa as dominant phenotype. C is dihybrid phenotypic; D is test cross
result.
7. A couple both have type A blood. Their first child has type O blood. What is the
probability their second child will also have type O blood?
● A. 0%
● B. 25%
● C. 50%
● D. 75%
Correct Answer: B
Rationale: Type A parents with type O child must both be heterozygous Iᴬi. Cross: Iᴬi × Iᴬi
→ 1 IᴬIᴬ : 2 Iᴬi : 1 ii. Each child has independent 25% chance of type O. Previous child's
blood type doesn't affect next (independent events).
8. In Drosophila, red eyes (w⁺) are dominant to white eyes (w). Which cross would
definitively determine if a red-eyed male is heterozygous or homozygous?
● A. Cross with red-eyed female
● B. Cross with white-eyed female
● C. Cross with heterozygous female
● D. Self-fertilization
Correct Answer: B
Rationale: This is a test cross. Crossing with homozygous recessive (white-eyed female,
w/w) reveals the male's genotype: if all offspring red-eyed, male is w⁺/w⁺; if 1:1 ratio,
male is w⁺/w. Other crosses give ambiguous results.
