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Chapter 11 · Class 12 Biology

Principles of Inheritance and Variation — Important Questions

38 questions With answers CBSE format

SUMMARY: This chapter explores the fundamental principles of genetics, including how traits are inherited and the variations that occur in organisms.
KEY TOPICS: Mendel's laws of inheritance, monohybrid and dihybrid crosses, chromosomal theory of inheritance, sex determination, genetic disorders, pedigree analysis, mutation, genetic variation, linkage and recombination.

Previous chapter Organisms and Populations Next chapter Reproductive Health
Multiple Choice Questions (MCQs) 5 questions
Q1 1 Mark

Mendel performed his experiments on:

ASweet pea
BGarden pea
CMaize
DWheat
Check answerHide answer
Correct answer: Option 2 — Garden pea
Q2 1 Mark

The law of segregation deals with:

AOne gene
BTwo genes
CThree genes
DMany genes
Check answerHide answer
Correct answer: Option 1 — One gene
Q3 1 Mark

The phenotypic ratio in a typical dihybrid cross is:

A3:1
B1:2:1
C9:3:3:1
D1:1
Check answerHide answer
Correct answer: Option 3 — 9:3:3:1
Q4 1 Mark

A condition in which a gene has more than two alleles is called:

ACodominance
BMultiple allelism
CPolyploidy
DLinkage
Check answerHide answer
Correct answer: Option 2 — Multiple allelism
Q5 1 Mark

Down syndrome is due to:

ATrisomy of chromosome 21
BMonosomy of chromosome X
CTrisomy of chromosome 18
DTrisomy of chromosome 13
Check answerHide answer
Correct answer: Option 1 — Trisomy of chromosome 21
Short Answer Questions 5 questions
Q6 3 Marks

State Mendel's laws of inheritance.

Q7 3 Marks

Differentiate between incomplete dominance and codominance with examples.

Q8 3 Marks

Explain the inheritance of ABO blood group in humans.

Q9 3 Marks

What is sex linkage? Give an example.

Q10 3 Marks

What is pedigree analysis? Why is it important?

Long Answer Questions 6 questions
Q11 6 Marks

Discuss Mendel's monohybrid and dihybrid cross experiments with examples.

Q12 6 Marks

Explain the chromosomal theory of inheritance and its experimental basis.

Q13 6 Marks

Describe the inheritance of ABO blood group as an example of multiple allelism and codominance.

Q14 6 Marks

Explain the sex determination in humans, birds, and honey bees.

Q15 6 Marks

Discuss the various chromosomal disorders in humans with their causes and symptoms.

Q16 6 Marks

Compare complete dominance and incomplete dominance with the help of a table.

Assertion–Reason Questions 5 questions
Q17 1 Mark

Assertion (A): Mendel's laws are universal.

Reason (R): They apply to all sexually reproducing organisms.

Show explanationHide explanation
Correct answer: Option 1 — Both A and R are true, and R is the correct explanation of A.
Q18 1 Mark

Assertion (A): Hemophilia is a sex-linked disorder.

Reason (R): The gene for hemophilia is present on the X chromosome.

Show explanationHide explanation
Correct answer: Option 1 — Both A and R are true, and R is the correct explanation of A.
Q19 1 Mark

Assertion (A): Mendel chose pea plant for his experiments.

Reason (R): Pea plants have many easily distinguishable characters and short life cycle.

Show explanationHide explanation
Correct answer: Option 1 — Both A and R are true, and R is the correct explanation of A.
Q20 1 Mark

Assertion (A): Punnet square shows possible genotypes from a cross.

Reason (R): It helps in predicting the offspring of a genetic cross.

Show explanationHide explanation
Correct answer: Option 1 — Both A and R are true, and R is the correct explanation of A.
Q21 1 Mark

Assertion (A): Sickle cell anaemia is an autosomal recessive disorder.

Reason (R): Two copies of the defective gene are needed for the disease to manifest.

Show explanationHide explanation
Correct answer: Option 1 — Both A and R are true, and R is the correct explanation of A.
Statement-Based Questions 5 questions
Q22 1 Mark

Statement 1: F1 generation in a monohybrid cross shows the dominant phenotype.

Statement 2: The recessive trait reappears in F2 generation in 1:3 ratio.

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Correct answer: Option 1 — Both statements are true.
Q23 1 Mark

Statement 1: Linked genes do not show independent assortment.

Statement 2: Linkage was first studied by T. H. Morgan in Drosophila.

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Correct answer: Option 1 — Both statements are true.
Q24 1 Mark

Statement 1: Klinefelter syndrome has 47 XXY chromosomes.

Statement 2: Such individuals show physical and mental abnormalities.

Show answerHide answer
Correct answer: Option 1 — Both statements are true.
Q25 1 Mark

Statement 1: Colour blindness is more common in males than females.

Statement 2: It is a recessive sex-linked trait carried on X chromosome.

Show answerHide answer
Correct answer: Option 1 — Both statements are true.
Q26 1 Mark

Statement 1: Crossing over leads to recombination of linked genes.

Statement 2: It contributes to genetic variation in offspring.

Show answerHide answer
Correct answer: Option 1 — Both statements are true.
Case Study / Passage Questions 4 questions
Q27 3 Marks
Mendel crossed a true-breeding tall pea plant (TT) with a true-breeding dwarf plant (tt). The F1 generation was all tall (Tt). When F1 plants were self-pollinated the F2 generation showed both tall and dwarf plants in a 3:1 ratio. He concluded that traits are controlled by discrete factors (genes) inherited in pairs.
  1. The phenotypic ratio in F2 generation is:
    A3 tall : 1 dwarf
    B1 tall : 1 dwarf
    CAll tall
    DAll dwarf
  2. The genotype of F1 generation is:
    ATt
    BTT
    Ctt
    DTt and TT
  3. Explain Mendel's monohybrid cross and the laws derived from it.
Show answersHide answers
1. Option 1 — 3 tall : 1 dwarf
2. Option 1 — Tt
3. Mendel's monohybrid cross illustrates two laws — the law of dominance and the law of segregation. The law of dominance states that one allele of a gene (T) is dominant over the other (t) so the heterozygote (Tt) shows the dominant trait. The law of segregation states that during gamete formation the two alleles of a gene separate so each gamete carries only one allele. Self-pollination of F1 (Tt × Tt) gives a 1:2:1 genotypic ratio (TT:Tt:tt) corresponding to a 3:1 phenotypic ratio.
Q28 3 Marks
A genetic counsellor explains the ABO blood group system to a couple. The wife is blood group A (genotype IA i) and the husband is blood group B (genotype IB i). The counsellor uses a Punnett square to predict the possible blood groups of their children.
  1. The possible blood groups of children from this cross are:
    AA and B only
    BO only
    CA B AB and O
    DAB only
  2. The ABO system illustrates:
    AMultiple allelism
    BCodominance
    CBoth
    DNeither
  3. Explain how the ABO blood group system illustrates multiple allelism and codominance.
Show answersHide answers
1. Option 3 — A B AB and O
2. Option 3 — Both
3. The ABO system has three alleles — IA, IB and i. IA and IB are codominant (both expressed in heterozygote AB) while i is recessive to both. A cross between IAi and IBi gives four possible offspring genotypes — IAIB (AB), IAi (A), IBi (B) and ii (O). Each occurs with 25 percent probability. The system illustrates two important genetic concepts — multiple allelism (more than two alleles for one gene) and codominance.
Q29 3 Marks
Mendel's experiments with pea plants laid the foundation for the principles of inheritance. He observed that certain traits, such as height, flower color, and seed shape, were passed from one generation to the next in predictable patterns. By performing monohybrid crosses, he established the concept of dominant and recessive alleles. For instance, when he crossed a tall plant with a dwarf plant, all offspring in the F1 generation exhibited the tall phenotype. Upon self-pollination of the F1 generation, a phenotypic ratio of 3:1 was observed in the F2 generation, confirming the segregation of alleles. This led to the formulation of Mendel's first law, the Law of Segregation, which states that allele pairs separate during gamete formation, ensuring that offspring inherit one allele from each parent.
  1. What is the phenotypic ratio observed in the F2 generation of Mendel's monohybrid cross?
    A1:2:1
    B3:1
    C9:3:3:1
    D1:1
  2. Explain Mendel's Law of Segregation in your own words.
  3. Which trait was dominant in Mendel's pea plant experiments?
    ADwarf height
    BTall height
    CGreen seed color
    DWrinkled seed shape
Show answersHide answers
1. Option 2 — 3:1
2. Mendel's Law of Segregation states that allele pairs separate during gamete formation, so each gamete carries only one allele for each gene.
3. Option 2 — Tall height
Q30 3 Marks
In a dihybrid cross, Mendel studied the inheritance of two different traits simultaneously. For example, he crossed plants that were round yellow seeds (RRYY) with those that had wrinkled green seeds (rryy). The F1 generation produced all round yellow seeds (RrYy), indicating that both traits were dominant. When the F1 plants were self-pollinated, the F2 generation exhibited a phenotypic ratio of 9:3:3:1. This result demonstrated the principle of independent assortment, which states that alleles for different traits segregate independently of one another during gamete formation. This principle is crucial for understanding genetic variation in offspring.
  1. What is the phenotypic ratio of the F2 generation in a dihybrid cross?
    A3:1
    B9:3:3:1
    C1:1:1:1
    D1:2:1
  2. Define the principle of independent assortment.
  3. What were the parental genotypes used in Mendel's dihybrid cross example?
    ARrYy and RrYy
    BRRYY and rryy
    CRr and rr
    DRR and rr
Show answersHide answers
1. Option 2 — 9:3:3:1
2. The principle of independent assortment states that alleles for different traits segregate independently during gamete formation.
3. Option 2 — RRYY and rryy
Table-Based Questions 4 questions
Q31 3 Marks

Study the table of human genetic disorders and answer:

DisorderTypeCause
Sickle cell anaemiaAutosomal recessiveSubstitution mutation in beta-globin
HemophiliaX-linked recessiveDefective clotting factor VIII
Down syndromeChromosomalTrisomy of chromosome 21
Klinefelter syndromeChromosomal47 XXY
Turner syndromeChromosomal45 XO
  1. Down syndrome is caused by:
    ATrisomy 21
    BTrisomy 18
    CXXY
    DXO
  2. Hemophilia is inherited as:
    AAutosomal dominant
    BAutosomal recessive
    CX-linked recessive
    DY-linked
  3. Differentiate between Mendelian and chromosomal disorders with examples.
Show answersHide answers
1. Option 1 — Trisomy 21
2. Option 3 — X-linked recessive
3. Genetic disorders may be Mendelian (caused by single gene mutations) or chromosomal (involving abnormal chromosome number or structure). Mendelian disorders include autosomal recessive (sickle cell anaemia), autosomal dominant and sex-linked (hemophilia, colour blindness). Chromosomal disorders include aneuploidies like Down syndrome (trisomy 21), Klinefelter syndrome (XXY) and Turner syndrome (XO). Pedigree analysis helps trace the inheritance pattern.
Q32 6 Marks

Predict the F2 phenotypic ratio for each Mendelian cross type.

Cross typeF2 ratio
Monohybrid?
Test cross?
Incomplete dominance?
Dihybrid?
Q33 6 Marks

Identify the type and cause of each human genetic disorder.

DisorderTypeCause
Sickle cell anaemia??
Hemophilia??
Colour blindness??
Down syndrome??
Klinefelter syndrome??
Turner syndrome??
Q34 6 Marks

Predict the possible blood groups of children for each parental ABO genotype combination.

Parent 1Parent 2Possible blood groups
IAIAIAIA?
IAiIBi?
IBIBii?
IAIBii?
Picture-Based Questions 4 questions
Q35 3 Marks

Study the Punnett square of a dihybrid cross and answer:

Principles of Inheritance and Variation figure
  1. The F2 phenotypic ratio in a typical Mendelian dihybrid cross is:
    A3:1
    B1:2:1
    C9:3:3:1
    D1:1:1:1
  2. The 9:3:3:1 ratio illustrates Mendel's:
    ALaw of dominance
    BLaw of segregation
    CLaw of independent assortment
    DLaw of linkage
  3. Explain how the law of independent assortment leads to the 9:3:3:1 phenotypic ratio in F2.
Show answersHide answers
1. Option None
2. Option None
3. Mendel's law of independent assortment states that during gamete formation alleles of different genes segregate independently. In a dihybrid cross between RrYy x RrYy each parent produces four types of gametes (RY Ry rY ry). The Punnett square gives 16 possible combinations leading to a 9:3:3:1 phenotypic ratio.
Q36 2 Marks

Based on the given diagram of a monohybrid cross, answer the following:

Principles of Inheritance and Variation figure
  1. What is the phenotypic ratio of the offspring in the F2 generation?
    A1:2:1
    B3:1
    C1:1
    D9:3:3:1
  2. Identify the dominant trait in the cross shown.
Show answersHide answers
1. Option 2 — 3:1
2. dominant trait name
Q37 2 Marks

Based on the given flowchart, answer the following:

Principles of Inheritance and Variation figure
  1. Which law states that allele pairs separate during gamete formation?
    ALaw of Segregation
    BLaw of Independent Assortment
    CLaw of Dominance
    DNone of the above
  2. Explain the significance of the Law of Independent Assortment.
Show answersHide answers
1. Option 1 — Law of Segregation
2. answer text
Q38 2 Marks

Based on the given diagram of a dihybrid cross, answer the following:

Principles of Inheritance and Variation figure
  1. What is the expected phenotypic ratio in the F2 generation of a dihybrid cross?
    A3:1
    B9:3:3:1
    C1:1:1:1
    D1:2:1
  2. Identify the traits being studied in this dihybrid cross.
Show answersHide answers
1. Option 2 — 9:3:3:1
2. trait names
Previous chapter Organisms and Populations Next chapter Reproductive Health

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