biology : class12 Test Cross : Dihybrid Cross, Independent Assortment : Law of Independent Assortment MCQ Questions and Answers
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Test Cross
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MCQ Questions
1. A test cross is performed by crossing an individual showing a dominant phenotype with which type of individual?
A.
A homozygous dominant individual
B.
A heterozygous dominant individual
C.
A homozygous recessive individual
D.
An individual showing any phenotype
Correct Answer:
A homozygous recessive individual
Explanation:
In a test cross, an individual with a dominant phenotype is crossed with a homozygous recessive individual to determine its genotype.
2. What is the main purpose of a test cross?
A.
To determine the genotype of an individual showing a dominant trait
B.
To produce only dominant offspring
C.
To increase the number of chromosomes
D.
To determine the sex of the offspring
Correct Answer:
To determine the genotype of an individual showing a dominant trait
Explanation:
A test cross reveals whether the dominant-phenotype parent is homozygous or heterozygous.
3. In a monohybrid test cross, the cross Tt × tt produces what phenotypic ratio?
A.
3 tall : 1 dwarf
B.
1 tall : 1 dwarf
C.
1 tall : 3 dwarf
D.
All tall
Correct Answer:
1 tall : 1 dwarf
Explanation:
The heterozygous parent produces T and t gametes, while the recessive parent produces only t gametes, giving equal numbers of tall and dwarf offspring.
4. Which cross is a dihybrid test cross?
A.
AABB × AABB
B.
AaBb × AaBb
C.
AaBb × aabb
D.
AAbb × aaBB
Correct Answer:
AaBb × aabb
Explanation:
A dihybrid test cross involves an individual heterozygous for two genes crossed with an individual homozygous recessive for both genes.
5. In the cross AaBb × aabb, what type of gametes can be produced by the AaBb parent?
A.
Only AB
B.
AB, Ab, aB, and ab
C.
Only ab
D.
AB and ab only
Correct Answer:
AB, Ab, aB, and ab
Explanation:
The heterozygous dihybrid parent can form four types of gametes when the two gene pairs assort independently.
6. Assuming independent assortment, what phenotypic ratio is expected from the dihybrid test cross AaBb × aabb?
A.
3:1
B.
9:3:3:1
C.
1:1
D.
1:1:1:1
Correct Answer:
1:1:1:1
Explanation:
The four gametes from AaBb combine with the ab gamete from aabb to produce four phenotypes in equal proportions.
7. Which parent in the cross AaBb × aabb contributes only one type of gamete?
A.
AaBb parent
B.
aabb parent
C.
Both parents
D.
Neither parent
Correct Answer:
aabb parent
Explanation:
The homozygous recessive parent aabb can produce only ab gametes, which is why it is used in a test cross.
8. If all offspring in a test cross show the dominant phenotype for a particular trait, the tested parent is most likely:
A.
Homozygous dominant for that trait
B.
Heterozygous for that trait
C.
Homozygous recessive for that trait
D.
Unable to produce gametes
Correct Answer:
Homozygous dominant for that trait
Explanation:
When crossed with a homozygous recessive individual, a homozygous dominant parent passes the dominant allele to every offspring.
9. In a test cross, the recessive parent is used because its genotype is:
A.
Known to be homozygous dominant
B.
Known to be heterozygous
C.
Known to be homozygous recessive
D.
Unknown for all traits
Correct Answer:
Known to be homozygous recessive
Explanation:
The homozygous recessive genotype ensures that the phenotype of each offspring reflects the allele contributed by the tested parent.
10. A dihybrid test cross gives four phenotypic classes in equal numbers. This result supports:
A.
Independent assortment of the two gene pairs
B.
Complete dominance at only one locus
C.
Inheritance of only recessive alleles
D.
Absence of meiosis
Correct Answer:
Independent assortment of the two gene pairs
Explanation:
Equal numbers of four phenotypic classes in a dihybrid test cross indicate that the two gene pairs assort independently.
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