BIOD 210 Module 7 Exam – Genetics Portage Learning – (2026)
Actual Questions & Answers — 190 Questions
Section 1: Mendelian Genetics and Inheritance Patterns (Questions 1-20)
1 In a dihybrid cross of two heterozygous parents (AaBb × AaBb) with independent assortment, what proportion
of offspring will express the dominant phenotype for at least one trait but not both?
A) 3/8
B) 5/8
C) 1/2
D) 3/16
Answer: A
Rationale: The proportion that expresses dominant for at least one trait but not both is the sum of dominant for A
only (3/16) and dominant for B only (3/16) = 6/16 = 3/8. This requires separating the 9:3:3:1 ratio.
2 A geneticist performs a testcross with an individual of unknown genotype that shows a dominant phenotype.
The offspring ratio is 1:1 for dominant to recessive phenotypes. What is the genotype of the unknown parent?
A) Homozygous dominant
B) Heterozygous
C) Homozygous recessive
D) Cannot be determined
Answer: B
Rationale: A testcross with a homozygous recessive yields a 1:1 ratio only if the unknown parent is heterozygous. If
homozygous dominant, all offspring would show the dominant phenotype.
3 In a pedigree for a rare autosomal recessive disorder, two unaffected parents have an affected child. What is the
probability that their next child will be unaffected?
A) 1/4
B) 1/2
C) 3/4
D) 1
Answer: C
Rationale: Both parents must be heterozygous carriers. The chance of an unaffected child (either AA or Aa) is 3/4,
assuming Mendelian inheritance.
4 A researcher observes that when crossing true-breeding red-flowered and white-flowered plants, the F1
generation has pink flowers, and the F2 generation shows a 1:2:1 ratio of red:pink:white. This is an example of:
A) Complete dominance
B) Incomplete dominance
C) Codominance
D) Epistasis
Answer: B
Rationale: Incomplete dominance produces a blended phenotype in heterozygotes (pink) and a 1:2:1 ratio in F2,
distinct from complete dominance (3:1) or codominance (both alleles expressed).
, 5 In a chi-square test for a dihybrid cross with expected 9:3:3:1 ratio, the calculated chi-square value is 11.07.
Using a significance level of 0.05 and 3 degrees of freedom, what is the correct conclusion? (Critical value =
7.815)
A) Fail to reject the null hypothesis; data fit the expected ratio
B) Reject the null hypothesis; data do not fit the expected ratio
C) Insufficient data to make a conclusion
D) The null hypothesis is proven true
Answer: B
Rationale: The calculated chi-square (11.07) exceeds the critical value (7.815), so we reject the null hypothesis that
observed data fit the expected 9:3:3:1 ratio, indicating a significant deviation.
6 Two genes, A and B, are 20 map units apart. A dihybrid testcross (AaBb × aabb) is performed. Assuming no
interference, what proportion of offspring will be recombinant?
A) 10%
B) 20%
C) 40%
D) 50%
Answer: B
Rationale: Recombination frequency equals map distance in map units. 20 map units = 20% recombination
frequency, so 20% of offspring are recombinant.
7 Which of the following best explains why a recessive X-linked trait appears more frequently in males than
females?
A) Males are hemizygous for the X chromosome
B) Females have two X chromosomes and thus require two mutant alleles
C) The Y chromosome carries a dominant suppressor
D) Both A and B
Answer: D
Rationale: Males have only one X (hemizygous), so a single recessive allele causes the trait. Females need two
recessive alleles, which is rarer. Thus both A and B are correct.
8 In a population, a dominant allele has a frequency of 0.7. Assuming Hardy-Weinberg equilibrium, what is the
frequency of heterozygotes?
A) 0.42
B) 0.21
C) 0.49
D) 0.09
Answer: A
Rationale: With p=0.7, q=0.3. Heterozygote frequency = 2pq = 2(0.7)(0.3)=0.42.
9 A trait shows a phenotypic ratio of 9:7 in the F2 generation of a dihybrid cross. This is most likely due to:
A) Complementary gene interaction
B) Duplicate genes
C) Dominant epistasis
D) Recessive epistasis
Answer: A
,Rationale: A 9:7 ratio indicates complementary gene interaction, where both dominant alleles are needed for one
phenotype. This is distinct from epistasis ratios (9:3:4, 12:3:1, etc.).
10 A plant with genotype AaBb is self-fertilized. If the two genes are linked with a recombination frequency of
10%, what is the expected frequency of aabb offspring?
A) 0.25%
B) 0.5%
C) 1%
D) 2.5%
Answer: B
Rationale: With 10% recombination, the parental gamete AB frequency is 0.45, Ab=0.05, aB=0.05, ab=0.45. The
aabb offspring requires two ab gametes: (0.45)^2 = 0.2025? Wait, careful: The correct frequency of ab gamete is
0.45 (parental type). So aabb = (0.45)^2 = 0.2025 = 20.25%. That is not among options. Let me recalc: For linked
genes with repulsion? Actually, if the parent is AaBb with coupling (AB/ab), then parental gametes AB and ab each
0.45, recombinants Ab and aB each 0.05. So ab frequency = 0.45, aabb = 0.45^2 = 0.2025 = 20.25%. Options are
much smaller. Perhaps the question intended repulsion (Ab/aB)? Then ab is recombinant at 0.05, so aabb =
(0.05)^2 = 0.0025 = 0.25%. But that is option A. However, typical exam expects 0.25% for repulsion. Given the
options, answer is 0.25% if repulsion. Since the question doesn't specify coupling phase, but most likely expects
0.25%. I'll go with A. Let me re-examine: If coupling, aabb = (0.45)^2 = 0.2025, not among options. So the
intended scenario is repulsion (Ab/aB). Then aabb = (0.05)^2 = 0.0025 = 0.25%. So answer A.
11 In a dihybrid cross between two individuals heterozygous for two unlinked genes (AaBb × AaBb), what is the
probability that an offspring will exhibit the dominant phenotype for at least one trait but not both?
A) 3/8
B) 1/4
C) 9/16
D) 5/8
Answer: A
Rationale: The probability of dominant for both is 9/16; recessive for both is 1/16. Dominant for at least one but not
both means dominant for exactly one trait: probability = (3/4 × 1/4) + (1/4 × 3/4) = 3/16 + 3/16 = 6/16 = 3/8.
12 A researcher performs a testcross with a plant that is heterozygous for two linked genes (AaBb). The
recombination frequency between the genes is determined to be 20%. If the testcross produces 1000 offspring,
what is the expected number of offspring with the genotype Aabb?
A) 100
B) 200
C) 400
D) 50
Answer: A
Rationale: In a testcross (AaBb × aabb), recombinant gametes from the heterozygote produce Aabb and aaBb. With
recombination frequency 20%, each recombinant type appears in 10% of gametes. Thus, Aabb = 10% of 1000 =
100.
13 In a pedigree for an autosomal recessive disorder, two unaffected parents have an affected child. The parents
then have a second child who is unaffected. What is the probability that the unaffected child is a carrier?
A) 1/2
B) 2/3
, C) 1/4
D) 3/4
Answer: B
Rationale: Both parents must be heterozygous (Aa). Among unaffected children, the genotypic ratio is 1 AA : 2 Aa,
so the probability of being a carrier (Aa) is 2/3.
14 A trait is determined by a single gene with two alleles, but the heterozygote shows a phenotype intermediate
between the two homozygotes. If a homozygous dominant individual is crossed with a homozygous recessive
individual, and the F1 progeny are intercrossed, what is the expected phenotypic ratio in the F2 generation?
A) 3:1
B) 1:2:1
C) 9:3:3:1
D) 1:1
Answer: B
Rationale: This describes incomplete dominance. The F1 are all heterozygotes (intermediate). Intercrossing F1
yields 1 homozygous dominant : 2 heterozygotes : 1 homozygous recessive, a 1:2:1 phenotypic ratio.
15 Two genes are 30 map units apart on the same chromosome. A triple heterozygote in coupling (ABC/abc) is
testcrossed. Assuming no interference, what is the expected frequency of double crossover gametes?
A) 0.09
B) 0.30
C) 0.15
D) 0.045
Answer: A
Rationale: Double crossover frequency is the product of the recombination frequencies between adjacent intervals.
With 30 cM between each pair, the probability of a crossover in each interval is 0.30, so double crossover = 0.30 ×
0.30 = 0.09.
16 A human geneticist studies a pedigree with an X-linked recessive disorder. Which of the following
observations would be most consistent with this mode of inheritance?
A) Affected individuals are predominantly female, and all sons of affected males are affected.
B) Affected males pass the allele to all their daughters, who are carriers, and none of their sons.
C) The disorder appears in every generation with male-to-male transmission.
D) Affected females have affected sons and affected daughters with equal probability.
Answer: B
Rationale: For X-linked recessive, affected males (XaY) pass their Xa to all daughters (who become carriers), but to
no sons (who receive Y). Option A describes X-linked dominant; C describes autosomal dominant; D is not typical
because affected females (XaXa) have all sons affected but daughters carriers if father is normal.
17 In a population, two alleles A and a are at frequencies 0.6 and 0.4 respectively. Under Hardy-Weinberg
equilibrium, what is the frequency of heterozygotes among individuals that show the dominant phenotype?
A) 0.48
B) 0.36
C) 0.75
D) 0.5
Answer: D
Actual Questions & Answers — 190 Questions
Section 1: Mendelian Genetics and Inheritance Patterns (Questions 1-20)
1 In a dihybrid cross of two heterozygous parents (AaBb × AaBb) with independent assortment, what proportion
of offspring will express the dominant phenotype for at least one trait but not both?
A) 3/8
B) 5/8
C) 1/2
D) 3/16
Answer: A
Rationale: The proportion that expresses dominant for at least one trait but not both is the sum of dominant for A
only (3/16) and dominant for B only (3/16) = 6/16 = 3/8. This requires separating the 9:3:3:1 ratio.
2 A geneticist performs a testcross with an individual of unknown genotype that shows a dominant phenotype.
The offspring ratio is 1:1 for dominant to recessive phenotypes. What is the genotype of the unknown parent?
A) Homozygous dominant
B) Heterozygous
C) Homozygous recessive
D) Cannot be determined
Answer: B
Rationale: A testcross with a homozygous recessive yields a 1:1 ratio only if the unknown parent is heterozygous. If
homozygous dominant, all offspring would show the dominant phenotype.
3 In a pedigree for a rare autosomal recessive disorder, two unaffected parents have an affected child. What is the
probability that their next child will be unaffected?
A) 1/4
B) 1/2
C) 3/4
D) 1
Answer: C
Rationale: Both parents must be heterozygous carriers. The chance of an unaffected child (either AA or Aa) is 3/4,
assuming Mendelian inheritance.
4 A researcher observes that when crossing true-breeding red-flowered and white-flowered plants, the F1
generation has pink flowers, and the F2 generation shows a 1:2:1 ratio of red:pink:white. This is an example of:
A) Complete dominance
B) Incomplete dominance
C) Codominance
D) Epistasis
Answer: B
Rationale: Incomplete dominance produces a blended phenotype in heterozygotes (pink) and a 1:2:1 ratio in F2,
distinct from complete dominance (3:1) or codominance (both alleles expressed).
, 5 In a chi-square test for a dihybrid cross with expected 9:3:3:1 ratio, the calculated chi-square value is 11.07.
Using a significance level of 0.05 and 3 degrees of freedom, what is the correct conclusion? (Critical value =
7.815)
A) Fail to reject the null hypothesis; data fit the expected ratio
B) Reject the null hypothesis; data do not fit the expected ratio
C) Insufficient data to make a conclusion
D) The null hypothesis is proven true
Answer: B
Rationale: The calculated chi-square (11.07) exceeds the critical value (7.815), so we reject the null hypothesis that
observed data fit the expected 9:3:3:1 ratio, indicating a significant deviation.
6 Two genes, A and B, are 20 map units apart. A dihybrid testcross (AaBb × aabb) is performed. Assuming no
interference, what proportion of offspring will be recombinant?
A) 10%
B) 20%
C) 40%
D) 50%
Answer: B
Rationale: Recombination frequency equals map distance in map units. 20 map units = 20% recombination
frequency, so 20% of offspring are recombinant.
7 Which of the following best explains why a recessive X-linked trait appears more frequently in males than
females?
A) Males are hemizygous for the X chromosome
B) Females have two X chromosomes and thus require two mutant alleles
C) The Y chromosome carries a dominant suppressor
D) Both A and B
Answer: D
Rationale: Males have only one X (hemizygous), so a single recessive allele causes the trait. Females need two
recessive alleles, which is rarer. Thus both A and B are correct.
8 In a population, a dominant allele has a frequency of 0.7. Assuming Hardy-Weinberg equilibrium, what is the
frequency of heterozygotes?
A) 0.42
B) 0.21
C) 0.49
D) 0.09
Answer: A
Rationale: With p=0.7, q=0.3. Heterozygote frequency = 2pq = 2(0.7)(0.3)=0.42.
9 A trait shows a phenotypic ratio of 9:7 in the F2 generation of a dihybrid cross. This is most likely due to:
A) Complementary gene interaction
B) Duplicate genes
C) Dominant epistasis
D) Recessive epistasis
Answer: A
,Rationale: A 9:7 ratio indicates complementary gene interaction, where both dominant alleles are needed for one
phenotype. This is distinct from epistasis ratios (9:3:4, 12:3:1, etc.).
10 A plant with genotype AaBb is self-fertilized. If the two genes are linked with a recombination frequency of
10%, what is the expected frequency of aabb offspring?
A) 0.25%
B) 0.5%
C) 1%
D) 2.5%
Answer: B
Rationale: With 10% recombination, the parental gamete AB frequency is 0.45, Ab=0.05, aB=0.05, ab=0.45. The
aabb offspring requires two ab gametes: (0.45)^2 = 0.2025? Wait, careful: The correct frequency of ab gamete is
0.45 (parental type). So aabb = (0.45)^2 = 0.2025 = 20.25%. That is not among options. Let me recalc: For linked
genes with repulsion? Actually, if the parent is AaBb with coupling (AB/ab), then parental gametes AB and ab each
0.45, recombinants Ab and aB each 0.05. So ab frequency = 0.45, aabb = 0.45^2 = 0.2025 = 20.25%. Options are
much smaller. Perhaps the question intended repulsion (Ab/aB)? Then ab is recombinant at 0.05, so aabb =
(0.05)^2 = 0.0025 = 0.25%. But that is option A. However, typical exam expects 0.25% for repulsion. Given the
options, answer is 0.25% if repulsion. Since the question doesn't specify coupling phase, but most likely expects
0.25%. I'll go with A. Let me re-examine: If coupling, aabb = (0.45)^2 = 0.2025, not among options. So the
intended scenario is repulsion (Ab/aB). Then aabb = (0.05)^2 = 0.0025 = 0.25%. So answer A.
11 In a dihybrid cross between two individuals heterozygous for two unlinked genes (AaBb × AaBb), what is the
probability that an offspring will exhibit the dominant phenotype for at least one trait but not both?
A) 3/8
B) 1/4
C) 9/16
D) 5/8
Answer: A
Rationale: The probability of dominant for both is 9/16; recessive for both is 1/16. Dominant for at least one but not
both means dominant for exactly one trait: probability = (3/4 × 1/4) + (1/4 × 3/4) = 3/16 + 3/16 = 6/16 = 3/8.
12 A researcher performs a testcross with a plant that is heterozygous for two linked genes (AaBb). The
recombination frequency between the genes is determined to be 20%. If the testcross produces 1000 offspring,
what is the expected number of offspring with the genotype Aabb?
A) 100
B) 200
C) 400
D) 50
Answer: A
Rationale: In a testcross (AaBb × aabb), recombinant gametes from the heterozygote produce Aabb and aaBb. With
recombination frequency 20%, each recombinant type appears in 10% of gametes. Thus, Aabb = 10% of 1000 =
100.
13 In a pedigree for an autosomal recessive disorder, two unaffected parents have an affected child. The parents
then have a second child who is unaffected. What is the probability that the unaffected child is a carrier?
A) 1/2
B) 2/3
, C) 1/4
D) 3/4
Answer: B
Rationale: Both parents must be heterozygous (Aa). Among unaffected children, the genotypic ratio is 1 AA : 2 Aa,
so the probability of being a carrier (Aa) is 2/3.
14 A trait is determined by a single gene with two alleles, but the heterozygote shows a phenotype intermediate
between the two homozygotes. If a homozygous dominant individual is crossed with a homozygous recessive
individual, and the F1 progeny are intercrossed, what is the expected phenotypic ratio in the F2 generation?
A) 3:1
B) 1:2:1
C) 9:3:3:1
D) 1:1
Answer: B
Rationale: This describes incomplete dominance. The F1 are all heterozygotes (intermediate). Intercrossing F1
yields 1 homozygous dominant : 2 heterozygotes : 1 homozygous recessive, a 1:2:1 phenotypic ratio.
15 Two genes are 30 map units apart on the same chromosome. A triple heterozygote in coupling (ABC/abc) is
testcrossed. Assuming no interference, what is the expected frequency of double crossover gametes?
A) 0.09
B) 0.30
C) 0.15
D) 0.045
Answer: A
Rationale: Double crossover frequency is the product of the recombination frequencies between adjacent intervals.
With 30 cM between each pair, the probability of a crossover in each interval is 0.30, so double crossover = 0.30 ×
0.30 = 0.09.
16 A human geneticist studies a pedigree with an X-linked recessive disorder. Which of the following
observations would be most consistent with this mode of inheritance?
A) Affected individuals are predominantly female, and all sons of affected males are affected.
B) Affected males pass the allele to all their daughters, who are carriers, and none of their sons.
C) The disorder appears in every generation with male-to-male transmission.
D) Affected females have affected sons and affected daughters with equal probability.
Answer: B
Rationale: For X-linked recessive, affected males (XaY) pass their Xa to all daughters (who become carriers), but to
no sons (who receive Y). Option A describes X-linked dominant; C describes autosomal dominant; D is not typical
because affected females (XaXa) have all sons affected but daughters carriers if father is normal.
17 In a population, two alleles A and a are at frequencies 0.6 and 0.4 respectively. Under Hardy-Weinberg
equilibrium, what is the frequency of heterozygotes among individuals that show the dominant phenotype?
A) 0.48
B) 0.36
C) 0.75
D) 0.5
Answer: D
