BIO 1201 (Gregg) Exam 4 Questions and Answers|
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1. According to Mendel's law of segregation, what happens to the two
alleles of a gene during gamete formation?
A. They assort independently of the alleles of other genes
B. They separate so that each gamete receives only one allele of the
gene
C. They blend together to form a single intermediate allele
D. They stay together so that each gamete carries both alleles
Answer: B. They separate so that each gamete receives only one
allele of the gene
Rationale: Segregation describes the separation of the two alleles of a
gene (during meiosis) so each gamete carries just one. Choice A describes
independent assortment, which concerns different genes, not the two
alleles of one gene.
2. In a dihybrid cross AaBb × AaBb in which the genes assort
independently and both show complete dominance, what phenotypic
ratio is expected in the offspring?
A. 3:1
B. 1:2:1
C. 9:3:3:1
D. 1:1:1:1
Answer: C. 9:3:3:1
Rationale: Each gene gives a 3:1 ratio on its own. Multiplying the two
independent ratios (3:1 × 3:1) gives 9:3:3:1. A 1:1:1:1 ratio is expected
from a test cross (AaBb × aabb).
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,3. Two heterozygous parents (Aa × Aa) are crossed. What is the
probability that an offspring will be homozygous recessive (aa)?
A. 1/4
B. 1/2
C. 3/4
D. 0
Answer: A. 1/4
Rationale: A Punnett square gives 1 AA : 2 Aa : 1 aa. Using the product
rule, the chance of getting a from each parent is 1/2 × 1/2 = 1/4.
4. A purple-flowered plant (dominant phenotype) is crossed with a
white-flowered plant (homozygous recessive). Half of the offspring
are purple and half are white. What is the genotype of the purple
parent?
A. Homozygous dominant (PP)
B. Homozygous recessive (pp)
C. Heterozygous (Pp)
D. It cannot be determined
Answer: C. Heterozygous (Pp)
Rationale: This is a test cross. If the purple parent were PP, all offspring
would be purple. A 1:1 ratio means the parent produced two types of
gametes (P and p), so it is Pp.
5. For a cross AaBbCc × AaBbCc (all genes unlinked), what is the
probability of an aabbcc offspring?
A. 1/16
B. 1/32
C. 1/64
D. 1/8
Answer: C. 1/64
Rationale: The probability of aa is 1/4, bb is 1/4, and cc is 1/4. For
independent events multiply: 1/4 × 1/4 × 1/4 = 1/64.
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, 6. Red snapdragons (RR) crossed with white snapdragons (WW)
produce pink F1 plants. If two F1 plants are crossed, what phenotypic
ratio is expected in the F2?
A. 3 red : 1 white
B. All pink
C. 1 red : 1 white
D. 1 red : 2 pink : 1 white
Answer: D. 1 red : 2 pink : 1 white
Rationale: Pink flowers show that the heterozygote has an intermediate
phenotype, which is incomplete dominance. RW × RW gives 1 RR : 2 RW :
1 WW, so the genotypic and phenotypic ratios are both 1:2:1.
7. A cross between red-coated and white-coated cattle produces roan
offspring whose coats contain both distinct red hairs and distinct
white hairs. This is an example of:
A. Incomplete dominance
B. Codominance
C. Epistasis
D. Complete dominance
Answer: B. Codominance
Rationale: In codominance both alleles are fully and separately expressed
in the heterozygote. In incomplete dominance the heterozygote shows a
blended, intermediate phenotype (such as pink flowers) rather than both
parental traits side by side.
8. The ABO blood group gene has three alleles (IA, IB, and i). How
many different genotypes are possible?
A. 6
B. 3
C. 4
D. 8
Answer: A. 6
Rationale: The six genotypes are IAIA, IAi (type A), IBIB, IBi (type B), IAIB
(type AB), and ii (type O). IA and IB are codominant, and both are dominant
to i.
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Updated| Pass Guaranteed| LSU
1. According to Mendel's law of segregation, what happens to the two
alleles of a gene during gamete formation?
A. They assort independently of the alleles of other genes
B. They separate so that each gamete receives only one allele of the
gene
C. They blend together to form a single intermediate allele
D. They stay together so that each gamete carries both alleles
Answer: B. They separate so that each gamete receives only one
allele of the gene
Rationale: Segregation describes the separation of the two alleles of a
gene (during meiosis) so each gamete carries just one. Choice A describes
independent assortment, which concerns different genes, not the two
alleles of one gene.
2. In a dihybrid cross AaBb × AaBb in which the genes assort
independently and both show complete dominance, what phenotypic
ratio is expected in the offspring?
A. 3:1
B. 1:2:1
C. 9:3:3:1
D. 1:1:1:1
Answer: C. 9:3:3:1
Rationale: Each gene gives a 3:1 ratio on its own. Multiplying the two
independent ratios (3:1 × 3:1) gives 9:3:3:1. A 1:1:1:1 ratio is expected
from a test cross (AaBb × aabb).
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,3. Two heterozygous parents (Aa × Aa) are crossed. What is the
probability that an offspring will be homozygous recessive (aa)?
A. 1/4
B. 1/2
C. 3/4
D. 0
Answer: A. 1/4
Rationale: A Punnett square gives 1 AA : 2 Aa : 1 aa. Using the product
rule, the chance of getting a from each parent is 1/2 × 1/2 = 1/4.
4. A purple-flowered plant (dominant phenotype) is crossed with a
white-flowered plant (homozygous recessive). Half of the offspring
are purple and half are white. What is the genotype of the purple
parent?
A. Homozygous dominant (PP)
B. Homozygous recessive (pp)
C. Heterozygous (Pp)
D. It cannot be determined
Answer: C. Heterozygous (Pp)
Rationale: This is a test cross. If the purple parent were PP, all offspring
would be purple. A 1:1 ratio means the parent produced two types of
gametes (P and p), so it is Pp.
5. For a cross AaBbCc × AaBbCc (all genes unlinked), what is the
probability of an aabbcc offspring?
A. 1/16
B. 1/32
C. 1/64
D. 1/8
Answer: C. 1/64
Rationale: The probability of aa is 1/4, bb is 1/4, and cc is 1/4. For
independent events multiply: 1/4 × 1/4 × 1/4 = 1/64.
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, 6. Red snapdragons (RR) crossed with white snapdragons (WW)
produce pink F1 plants. If two F1 plants are crossed, what phenotypic
ratio is expected in the F2?
A. 3 red : 1 white
B. All pink
C. 1 red : 1 white
D. 1 red : 2 pink : 1 white
Answer: D. 1 red : 2 pink : 1 white
Rationale: Pink flowers show that the heterozygote has an intermediate
phenotype, which is incomplete dominance. RW × RW gives 1 RR : 2 RW :
1 WW, so the genotypic and phenotypic ratios are both 1:2:1.
7. A cross between red-coated and white-coated cattle produces roan
offspring whose coats contain both distinct red hairs and distinct
white hairs. This is an example of:
A. Incomplete dominance
B. Codominance
C. Epistasis
D. Complete dominance
Answer: B. Codominance
Rationale: In codominance both alleles are fully and separately expressed
in the heterozygote. In incomplete dominance the heterozygote shows a
blended, intermediate phenotype (such as pink flowers) rather than both
parental traits side by side.
8. The ABO blood group gene has three alleles (IA, IB, and i). How
many different genotypes are possible?
A. 6
B. 3
C. 4
D. 8
Answer: A. 6
Rationale: The six genotypes are IAIA, IAi (type A), IBIB, IBi (type B), IAIB
(type AB), and ii (type O). IA and IB are codominant, and both are dominant
to i.
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