biology : class12 Backcross Interpretation : Dihybrid Cross, Independent Assortment : Law of Independent Assortment MCQ Questions and Answers
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Backcross Interpretation
MCQ questions and answers.
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MCQ Questions
1. In a dihybrid backcross, an individual with genotype AaBb is crossed with aabb. Assuming independent assortment and complete dominance, what phenotypic ratio is expected among the offspring?
A.
9:3:3:1
B.
1:1:1:1
C.
1:2:1
D.
3:1
Correct Answer:
1:1:1:1
Explanation:
AaBb produces AB, Ab, aB, and ab gametes in equal proportions. The aabb parent produces only ab gametes, so the four offspring phenotypes occur equally, giving a 1:1:1:1 ratio.
2. A plant of genotype AaBb is backcrossed with aabb, producing 412, 398, 96, and 94 offspring in the four phenotypic classes. Which conclusion is most appropriate?
A.
The genes assort independently because the four classes are approximately equal
B.
The genes are completely linked because two classes are larger
C.
The F1 parent was homozygous for both genes
D.
The backcross must produce a 9:3:3:1 ratio
Correct Answer:
The genes are completely linked because two classes are larger
Explanation:
The strongly unequal classes, with two large and two small categories, are inconsistent with the 1:1:1:1 expectation for independent assortment. They suggest linkage or another departure from independent assortment, rather than confirming independent assortment.
3. In a backcross AaBb × aabb, 800 progeny are obtained. If the genes assort independently, approximately how many progeny should display the phenotype corresponding to genotype aaBb?
A.
50
B.
100
C.
200
D.
400
Correct Answer:
200
Explanation:
The gamete aB from AaBb occurs with probability one-fourth, while aabb contributes only ab. Therefore, the aaBb class is expected at one-fourth of 800, or 200 progeny.
4. A true-breeding plant with dominant phenotypes for seed shape and seed colour is crossed with a double-recessive plant. The F1 plants are then backcrossed with the double-recessive parent. The backcross produces only two phenotypic classes in equal numbers. Which F1 genotype arrangement best explains this result, assuming complete linkage and no crossing over?
A.
AB/ab
B.
Ab/aB
C.
AaBB
D.
Aabb
Correct Answer:
AB/ab
Explanation:
A cis or coupling heterozygote AB/ab produces only AB and ab gametes when the genes are completely linked. Testcrossing it with ab/ab therefore yields only two phenotypic classes in equal proportions.
5. A dihybrid individual is backcrossed with aabb. The progeny phenotypes occur in the ratio 1:1 for parental combinations and no recombinant combinations are observed. What is the most defensible interpretation?
A.
The dihybrid produces four gamete types equally
B.
The two genes are completely linked in the dihybrid
C.
The dihybrid is homozygous recessive at both loci
D.
The cross follows the standard 9:3:3:1 dihybrid ratio
Correct Answer:
The two genes are completely linked in the dihybrid
Explanation:
In a backcross to aabb, progeny phenotypes directly reveal the gametes produced by the heterozygous parent. Two equally frequent parental classes and absent recombinants indicate complete linkage.
6. A backcross of AaBb with aabb gives the following progeny: 510 A-B-, 490 aabb, 250 A-bb, and 250 aaB-. Which phase of linkage in the AaBb parent is indicated by these data?
A.
Coupling phase, AB/ab
B.
Repulsion phase, Ab/aB
C.
Homozygous phase, AA/BB
D.
A phase cannot be inferred from a backcross
Correct Answer:
Coupling phase, AB/ab
Explanation:
The most frequent classes are A-B- and aabb, representing the parental gametes AB and ab. Thus, the alleles are arranged in coupling phase as AB/ab, while the less frequent classes are recombinants.
7. In a testcross of AaBb × aabb, 1,000 offspring are classified as follows: AB phenotype, 360; ab phenotype, 340; Ab phenotype, 150; aB phenotype, 150. What is the estimated recombination frequency between the two genes?
A.
15%
B.
30%
C.
34%
D.
70%
Correct Answer:
30%
Explanation:
The recombinant classes are Ab and aB, totaling 150 + 150 = 300 offspring. Recombination frequency is 300/1,000 × 100 = 30%, indicating linkage with approximately 30 map units between the genes.
8. An unknown plant showing dominant phenotypes for both traits is backcrossed with aabb. The progeny are 498 A-B-, 502 A-bb, and no offspring of the other two phenotypic classes. Which genotype of the unknown parent is consistent with these results under complete linkage?
A.
AB/ab
B.
Ab/aB
C.
AABB
D.
aabb
Correct Answer:
Ab/aB
Explanation:
The backcross progeny reveal gametes from the unknown parent. The two observed classes correspond to Ab and aB gametes, produced equally by the repulsion-phase genotype Ab/aB under complete linkage.
9. A backcross involving two genes gives 1,000 offspring. The observed classes are 462, 458, 42, and 38. Which statistical interpretation is most suitable before claiming independent assortment?
A.
The data strongly support a 1:1:1:1 ratio
B.
The two large classes suggest linkage, but a chi-square test should evaluate departure from 1:1:1:1
C.
The data prove that both genes are homozygous in the heterozygous parent
D.
The data must be interpreted using a 9:3:3:1 ratio
Correct Answer:
The two large classes suggest linkage, but a chi-square test should evaluate departure from 1:1:1:1
Explanation:
A dihybrid testcross under independent assortment predicts four equal classes. The observed excess of two classes suggests linkage, but a goodness-of-fit test is required to determine whether the deviation is statistically significant.
10. A plant with genotype AaBb is backcrossed with aabb. If the offspring include 25% A-B-, 25% A-bb, 25% aaB-, and 25% aabb, what can be inferred about the gametes of the AaBb parent?
A.
Only AB and ab gametes were produced
B.
AB, Ab, aB, and ab gametes were produced equally
C.
Only Ab and aB gametes were produced
D.
The parent produced gametes in the ratio 9:3:3:1
Correct Answer:
AB, Ab, aB, and ab gametes were produced equally
Explanation:
Because the aabb parent contributes only ab gametes, each offspring phenotype corresponds directly to one gamete from AaBb. Equal proportions of the four classes demonstrate equal production of AB, Ab, aB, and ab gametes, as expected under independent assortment.
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