📘 Inheritance of two genes || Class 12 Biology
Directions (CBSE Pattern):
For each question, choose the correct option:
A. Both Assertion and Reason are true and Reason is the correct explanation of Assertion
B. Both Assertion and Reason are true but Reason is NOT the correct explanation of Assertion
C. Assertion is true but Reason is false
D. Assertion is false but Reason is true
Question 1
Assertion (A): In a dihybrid cross, the F₂ generation shows a phenotypic ratio of 9 : 3 : 3 : 1.
Reason (R): This is because alleles of different genes assort independently during gamete formation.
✅ Correct Option: A
Explanation: Independent assortment of two gene pairs results in the classic 9:3:3:1 ratio.
Question 2
Assertion (A): Mendel’s dihybrid cross provided evidence for the law of independent assortment.
Reason (R): Each pair of alleles segregates independently of other pairs during meiosis.
✅ Correct Option: A
Explanation: This definition itself explains the law of independent assortment.
Question 3
Assertion (A): The F₁ generation of a dihybrid cross is always phenotypically uniform.
Reason (R): All F₁ individuals are heterozygous for both traits.
✅ Correct Option: A
Explanation: Genotype RrYy in all F₁ plants results in uniform dominant phenotype.
Question 4
Assertion (A): Wrinkled green seeds appear least frequently in the F₂ generation.
Reason (R): They possess double recessive genotype.
✅ Correct Option: A
Explanation: Double recessive (rryy) appears 1 out of 16 times.
Question 5
Assertion (A): A dihybrid test cross produces a phenotypic ratio of 1 : 1 : 1 : 1.
Reason (R): Equal numbers of four types of gametes are produced by the heterozygous parent.
✅ Correct Option: A
Explanation: RrYy produces RY, Ry, rY, ry in equal proportion.
Question 6
Assertion (A): Recombinant phenotypes appear in F₂ generation of a dihybrid cross.
Reason (R): Crossing over occurs during mitosis.
✅ Correct Option: C
Explanation: Recombinants arise due to independent assortment, not mitosis.
Question 7
Assertion (A): Independent assortment increases genetic variation.
Reason (R): It creates new combinations of alleles in gametes.
✅ Correct Option: A
Explanation: New allele combinations lead to greater variation.
Question 8
Assertion (A): Genes located on the same chromosome always assort independently.
Reason (R): Linked genes are inherited together.
✅ Correct Option: D
Explanation: Genes on the same chromosome (linked genes) do not assort independently.
Question 9
Assertion (A): The dihybrid phenotypic ratio is the product of two monohybrid ratios.
Reason (R): Each character pair follows a 3 : 1 ratio independently.
✅ Correct Option: A
Explanation: (3:1) × (3:1) = 9:3:3:1
Question 10
Assertion (A): The genotype RrYy produces four types of gametes.
Reason (R): Heterozygous alleles segregate during meiosis.
✅ Correct Option: A
Explanation: Segregation and independent assortment lead to four gamete types.
Question 11
Assertion (A): Law of independent assortment is applicable to all genes.
Reason (R): Some genes are linked and inherited together.
✅ Correct Option: D
Explanation: Law does not apply to linked genes.
Question 12
Assertion (A): Dihybrid cross disproved the theory of blending inheritance.
Reason (R): Traits remain discrete and do not mix permanently.
✅ Correct Option: A
Explanation: Traits reappear unchanged in later generations.
Question 13
Assertion (A): Metaphase I of meiosis is important for independent assortment.
Reason (R): Random orientation of homologous chromosomes occurs at this stage.
✅ Correct Option: A
Explanation: Random alignment causes independent assortment.
Question 14
Assertion (A): Pure breeding plants produce only one type of gamete.
Reason (R): They are homozygous for the trait.
✅ Correct Option: A
Explanation: Homozygosity leads to identical gametes.
Question 15
Assertion (A): Dihybrid inheritance deals with inheritance of two contrasting traits.
Reason (R): It involves only one pair of alleles.
✅ Correct Option: C
Explanation: Dihybrid inheritance involves two pairs of alleles, not one.