biology : class12 Dihybrid Cross Design : Dihybrid Cross, Independent Assortment : Law of Independent Assortment MCQ Questions and Answers
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Dihybrid Cross Design
MCQ questions and answers.
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MCQ Questions
1. In a dihybrid cross involving genes A/a and B/b, which parental cross is best designed to produce F1 individuals that are uniformly heterozygous for both traits?
A.
AABB × aabb
B.
AAbb × aaBB
C.
AABB × aaBb
D.
AaBB × Aabb
Correct Answer:
AABB × aabb
Explanation:
The cross AABB × aabb produces F1 offspring with the genotype AaBb in every case. These F1 plants are heterozygous at both loci and can therefore be used to demonstrate independent assortment in a subsequent dihybrid cross.
2. An individual with genotype AaBb is crossed with aabb. Assuming independent assortment and complete dominance, which offspring ratio is expected?
A.
9:3:3:1
B.
1:1:1:1
C.
3:1
D.
1:2:1
Correct Answer:
1:1:1:1
Explanation:
AaBb produces four gamete types—AB, Ab, aB, and ab—in equal proportions. The aabb parent produces only ab gametes, so the four offspring phenotypes occur equally, giving a 1:1:1:1 testcross ratio.
3. Which gamete set must be used when designing a cross with a parent having genotype AaBb under independent assortment?
A.
Only AB and ab
B.
AB, Ab, aB, and ab
C.
Only Aa and Bb
D.
AABB, AAbb, aaBB, and aabb
Correct Answer:
AB, Ab, aB, and ab
Explanation:
A heterozygous dihybrid, AaBb, contributes one allele from each gene to every gamete. Independent assortment allows all four allele combinations—AB, Ab, aB, and ab—to be formed.
4. A researcher wants to distinguish whether an unknown individual showing both dominant traits is AABB, AABb, AaBB, or AaBb. Which cross is the most informative design?
A.
Cross the individual with AABB
B.
Cross the individual with AaBb
C.
Cross the individual with aabb
D.
Cross the individual with aaBB
Correct Answer:
Cross the individual with aabb
Explanation:
A testcross with aabb reveals the gametes produced by the unknown individual because the tester contributes only ab. The resulting offspring patterns can distinguish homozygous and heterozygous states at both loci.
5. For the cross AaBb × AaBb, which Punnett-square design correctly represents the gametes contributed by each parent?
A.
A 2 × 2 square using AB and ab from each parent
B.
A 4 × 4 square using AB, Ab, aB, and ab from each parent
C.
A 4 × 2 square using Aa, Bb, AB, and ab
D.
An 8 × 8 square using all possible diploid genotypes
Correct Answer:
A 4 × 4 square using AB, Ab, aB, and ab from each parent
Explanation:
Each AaBb parent forms four types of haploid gametes under independent assortment. Combining the four gametes from one parent with the four from the other requires a 4 × 4 Punnett square containing 16 equally probable combinations.
6. A dihybrid F1 individual is produced by crossing AAbb with aaBB. Which statement correctly describes the F1 generation and its suitable self-cross?
A.
F1 is AABB and should be crossed with aabb
B.
F1 is AaBb and should be crossed with another AaBb individual
C.
F1 consists of AAbb and aaBB and should be crossed with AaBb
D.
F1 is aaBb and should be crossed with AABb
Correct Answer:
F1 is AaBb and should be crossed with another AaBb individual
Explanation:
The AAbb parent contributes only Ab gametes, while aaBB contributes only aB gametes. Their offspring are uniformly AaBb, and self-crossing AaBb × AaBb is the standard design for observing the dihybrid assortment pattern.
7. In a testcross of AaBb, a student records 48 offspring. If the cross is properly designed and the genes assort independently, what is the expected number in each of the four phenotypic classes?
A.
4
B.
8
C.
12
D.
24
Correct Answer:
12
Explanation:
The testcross AaBb × aabb produces four equally probable classes in a 1:1:1:1 ratio. Dividing 48 offspring equally among four classes gives 48 ÷ 4 = 12 offspring per class.
8. Which cross would produce only one phenotypic class while still involving two independently assorting genes?
A.
AaBb × AaBb
B.
AaBb × aabb
C.
AABB × aabb
D.
Aabb × aaBb
Correct Answer:
AABB × aabb
Explanation:
AABB produces only AB gametes and aabb produces only ab gametes. Every offspring is therefore AaBb and displays the same dominant phenotype for both traits, even though two genes are included in the cross.
9. A plant with genotype AaBb is to be crossed so that its offspring reveal the allele combination carried in each gamete. Which parental genotype should be selected for the other parent?
A.
AABB
B.
AaBb
C.
aabb
D.
aaBb
Correct Answer:
aabb
Explanation:
The recessive double-homozygous tester, aabb, contributes only ab gametes. Consequently, each offspring phenotype directly reflects whether the AaBb parent contributed AB, Ab, aB, or ab.
10. To obtain the classical 9:3:3:1 phenotypic ratio, which complete cross design is required under independent assortment and complete dominance?
A.
AABB × aabb followed by selfing the F1
B.
AAbb × aaBB followed by testcrossing the F1
C.
AaBb × AaBb
D.
AaBb × aabb followed by selfing the recessive offspring
Correct Answer:
AaBb × AaBb
Explanation:
The 9:3:3:1 ratio results when two individuals heterozygous at both loci, AaBb, are crossed. Each parent produces AB, Ab, aB, and ab gametes, and independent assortment generates the four phenotypic categories in the classical ratio.
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