Genetics – Inheritance of Two Genes (Dihybrid cross): NEET MCQs with Answers

15 free questions on Genetics – Inheritance of Two Genes (Dihybrid cross) for NEET (UG), starting with previous-year NEET and AIPMT questions. Try each one, then tap Show answer for the answer and a short explanation.

Syllabus covered: Inheritance of Two Genes (Dihybrid cross)

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Question 1

Previous-year question
In order to find out the different types of gametes produced by a pea plant having the genotype AaBb, it should be crossed to a plant with the genotype [AIPMT 2005]
  1. aaBB
  2. AaBb
  3. AABB
  4. aabb
Show answer
Answer: (D) aabb
Explanation: To reveal the gamete types of a dihybrid, a test cross with the double recessive aabb is performed. Since aabb supplies only ab gametes, the phenotypes of the offspring directly reflect the four gamete types produced by AaBb.

Question 2

Previous-year question
Two linked genes a and b show 20% recombination. The individuals of a dihybrid cross between ++/++ × ab/ab shall show gametes [AIPMT 1989]
  1. ++ 80 : ab 20
  2. ++ 50 : ab 50
  3. ++ 40 : ab 40 : + a 10 : + b 10
  4. ++ 30 : ab 30 : + a 20 : + b 20
Show answer
Answer: (C) ++ 40 : ab 40 : + a 10 : + b 10
Explanation: With 20% recombination the F₁ dihybrid produces 20% recombinant gametes (10% +a and 10% +b) and 80% parental gametes (40% ++ and 40% ab). Hence the gametic proportion is ++ 40 : ab 40 : +a 10 : +b 10.

Question 3

Previous-year question
Bateson used the terms coupling and repulsion for linkage and crossing over. Name the correct parental of coupling type alongwith its crossover or repulsion. [AIPMT 1990]
  1. Coupling AABB, aabb; Repulsion AABB, aabb
  2. Coupling AAbb, aaBB; Repulsion AaBb, aabb
  3. Coupling aaBB, aabb; Repulsion AABB, aabb
  4. Coupling AABB, aabb; Repulsion AAbb, aaBB
Show answer
Answer: (D) Coupling AABB, aabb; Repulsion AAbb, aaBB
Explanation: Bateson and Punnett (1906) found that when two dominants enter from the same parent they tend to remain together — coupling (AABB, aabb). When the two dominants come from different parents they tend to remain apart — repulsion (AAbb, aaBB).

Question 4

Previous-year question
Mendel's last law is [AIPMT 1991]
  1. segregation
  2. dominance
  3. independent assortment
  4. polygenic inheritance
Show answer
Answer: (C) independent assortment
Explanation: Mendel's third and last law is the law of independent assortment, which states that when two or more pairs of characters are considered together, the distribution of one pair into the gametes is independent of the other.

Question 5

Previous-year question
A dihybrid condition is [AIPMT 1991]
  1. tt Rr
  2. Tt rr
  3. tt rr
  4. Tt Rr
Show answer
Answer: (D) Tt Rr
Explanation: A dihybrid is heterozygous for two pairs of genes simultaneously. Only Tt Rr is heterozygous for both loci; the other genotypes are homozygous for at least one of them.

Question 6

Previous-year question
Out of 8 ascospores formed in Neurospora the arrangement is 2a : 4a : 2a showing [AIPMT 1992]
  1. no crossing over
  2. some meiosis
  3. second generation division
  4. first generation division
Show answer
Answer: (C) second generation division
Explanation: In Neurospora, if crossing over occurs between the gene and the centromere the ascospores are arranged as 2a : 4a : 2a. This pattern is known as second division segregation.

Question 7

Previous-year question
Two dominant non-allelic genes are 50 map units apart. The linkage is [AIPMT 1993]
  1. cis type
  2. trans type
  3. complete
  4. absent/incomplete
Show answer
Answer: (D) absent/incomplete
Explanation: A distance of 50 map units means a recombination frequency of 50%, which is indistinguishable from independent assortment. Linkage is therefore effectively absent or incomplete between such genes.
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Question 8

Previous-year question
Mendel studied inheritance of seven pairs of traits in pea which can have 21 possible combinations. If you are told that in one of these combinations, independent assortment is not observed in later studies, your reaction will be [AIPMT 1993]
  1. independent assortment principle may be wrong
  2. Mendel might not have studied all the combinations
  3. it is impossible
  4. later studies may be wrong
Show answer
Answer: (B) Mendel might not have studied all the combinations
Explanation: Mendel's law of independent assortment applies only to genes located on different chromosomes. Since his seven characters lie on only four chromosomes, some pairs must be linked; the reasonable conclusion is that Mendel did not test all 21 combinations.

Question 9

Previous-year question
Which of the following is suitable for experiment on linkage? [AIPMT 1993]
  1. aaBB × aaBB
  2. AABB × aabb
  3. AaBb × AaBb
  4. AAbb × AaBB
Show answer
Answer: (B) AABB × aabb
Explanation: For a linkage experiment the two parents must be pure-breeding for contrasting combinations, i.e. AABB × aabb, so that the F₁ dihybrid can be test-crossed and parental versus recombinant types counted.

Question 10

Previous-year question
A cross between pure tall pea plant with green pods and dwarf pea plant with yellow pods will produce dwarf F₂-plants out of 16 [AIPMT 1994]
  1. 9
  2. 3
  3. 4
  4. 1
Show answer
Answer: (C) 4
Explanation: This is a dihybrid cross in which dwarfness is recessive. In the standard 9 : 3 : 3 : 1 F₂-ratio the recessive character for one gene appears in 4 out of 16 individuals (3 + 1), so 4 dwarf plants are expected.

Question 11

Previous-year question
In a dihybrid cross AABB × aabb, F₂-progeny of AABB, AABb, AaBB and AaBb occurs in the ratio of [AIPMT 1994]
  1. 1 : 1 : 1 : 1
  2. 9 : 3 : 3 : 1
  3. 1 : 2 : 2 : 1
  4. 1 : 2 : 2 : 4
Show answer
Answer: (D) 1 : 2 : 2 : 4
Explanation: AABB × aabb gives F₁ AaBb; selfing gives the standard 16-square dihybrid F₂. Counting the boxes, AABB occurs once, AABb twice, AaBB twice and AaBb four times, i.e. in the ratio 1 : 2 : 2 : 4.

Question 12

Previous-year question
The phenomenon, in which an allele of one gene suppresses the activity of an allele of another gene, is known as [AIPMT 1995]
  1. epistasis
  2. dominance
  3. suppression
  4. inactivation
Show answer
Answer: (A) epistasis
Explanation: Epistasis is the suppression of the phenotypic expression of one gene by a non-allelic gene. The gene that masks is the epistatic gene and the one that is suppressed is the hypostatic gene.

Question 13

Previous-year question
A fruit fly heterozygous for sex-linked genes, is mated with normal female fruit fly. Male specific chromosome will enter egg cell in the proportion [AIPMT 1997]
  1. 1 : 1
  2. 2 : 1
  3. 3 : 1
  4. 7 : 1
Show answer
Answer: (A) 1 : 1
Explanation: The male Drosophila is XY and produces X-bearing and Y-bearing sperms in equal numbers, while the female produces only X-bearing eggs. Hence the male-specific (Y) chromosome enters the egg in a 1 : 1 proportion with the X.

Question 14

Previous-year question
How many types of genetically different gametes will be produced by a heterozygous plant having genotype AABbCc? [AIPMT 1998]
  1. Two
  2. Four
  3. Six
  4. Nine
Show answer
Answer: (B) Four
Explanation: AA is homozygous and gives one type of gamete, while Bb and Cc each give two. Hence 1 × 2 × 2 = 4 kinds of gametes are formed — ABC, ABc, AbC and Abc.

Question 15

Previous-year question
Crossing over in diploid organism is responsible for [AIPMT 1998]
  1. dominance of genes
  2. linkage between genes
  3. segregation of alleles
  4. recombination of linked alleles
Show answer
Answer: (D) recombination of linked alleles
Explanation: Crossing over is the exchange of corresponding segments between non-sister chromatids of homologous chromosomes during prophase I. It breaks the existing linkage and produces new combinations, i.e. recombination of linked alleles.

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