Principles of Inheritance and Variation — Class 12 MCQs with Answers

Principles of Inheritance and Variation — important questions & MCQs with answers (Class 12 Biology)

14 solved questions from this chapter's difficulty levels, each with its answer and explanation. The other 78 are timed and scored when you sign in.

1. Q1Easy

   The father of modern genetics is:

   A.Charles Darwin

   B.Watson and Crick

   C.Gregor Mendel✓ Correct

   D.Thomas Morgan

   Answer: C. Gregor Mendel

   Explanation: Gregor Mendel is called the father of genetics for discovering the laws of inheritance using garden pea (Pisum sativum) between 1856 and 1863, decades before chromosomes or DNA were known; Darwin studied evolution, not inheritance mechanics.

2. Q2Easy

   In a heterozygous organism (Tt), the trait that appears in the phenotype is:

   A.Dominant (T)✓ Correct

   B.Recessive (t)

   C.Both equally

   D.Neither

   Answer: A. Dominant (T)

   Explanation: In a heterozygous Tt individual, the dominant allele T is expressed and masks the recessive allele t, so the phenotype matches the homozygous dominant TT — students often think the traits blend instead of one being masked.

3. Q3Easy

   In humans, the sex chromosome makeup of a normal male is:

   A.XY✓ Correct

   B.XX

   C.XO

   D.YY

   Answer: A. XY

   Explanation: A normal human male carries the sex chromosome combination XY, with the Y chromosome triggering male development; a normal female carries XX — the sex chromosome pair, not the number of autosomes, determines sex.

4. Q4Easy

   Mendel chose the pea plant for his experiments primarily because:

   A.It is the largest plant

   B.It has many visible traits with clear contrasting forms✓ Correct

   C.It grows only in winter

   D.It has no flowers

   Answer: B. It has many visible traits with clear contrasting forms

   Explanation: Mendel picked garden pea because it shows sharply contrasting traits like tall versus dwarf and yellow versus green seeds, is naturally self-pollinating for pure lines, and has a short life cycle — not because of plant size or season.

5. Q5Easy

   A test cross involves crossing an individual with the:

   A.Homozygous dominant parent

   B.Homozygous recessive parent✓ Correct

   C.Heterozygote

   D.F2 generation

   Answer: B. Homozygous recessive parent

   Explanation: A test cross mates an individual of unknown genotype with a homozygous recessive partner: an all-dominant offspring set reveals a homozygous parent, while a 1:1 dominant-to-recessive split reveals a heterozygous parent.

6. Q6Easy

   Haemophilia is an example of a disorder that is:

   A.Chromosomal

   B.Autosomal dominant

   C.Y-linked

   D.X-linked recessive✓ Correct

   Answer: D. X-linked recessive

   Explanation: Haemophilia is carried on the X chromosome and is recessive, so a male needs only one copy of the faulty allele (he is hemizygous) to be affected, while a female needs two copies — making the disorder far more common in males.

7. Q7Medium

   A test cross of a heterozygous tall plant (Tt) gives offspring in the ratio:

   A.All tall

   B.3 tall : 1 dwarf

   C.1 tall : 1 dwarf✓ Correct

   D.1:2:1

   Answer: C. 1 tall : 1 dwarf

   Explanation: A test cross of a heterozygous Tt tall plant with a homozygous recessive tt dwarf plant gives gametes T and t from the tall parent, so offspring split 1 Tt (tall) : 1 tt (dwarf) — a 1:1 ratio, not the 3:1 seen when selfing Tt.

8. Q8Medium

   In a monohybrid cross between two heterozygotes (Tt × Tt), the F₂ phenotypic ratio is:

   A.3 : 1✓ Correct

   B.1 : 1

   C.9 : 3 : 3 : 1

   D.1 : 2 : 1

   Answer: A. 3 : 1

   Explanation: Crossing two Tt heterozygotes gives a 1 TT : 2 Tt : 1 tt genotypic ratio, but since T is dominant, TT and Tt both look tall — so the F₂ phenotypic ratio compresses to 3 tall : 1 dwarf, not the genotypic 1:2:1.

9. Q9Medium

   Haemophilia is an example of:

   A.Autosomal dominant disorder

   B.X-linked recessive disorder✓ Correct

   C.Autosomal recessive disorder

   D.Y-linked disorder

   Answer: B. X-linked recessive disorder

   Explanation: Haemophilia is X-linked recessive: the faulty allele sits on the X chromosome, so a hemizygous male (one X) shows the disease from a single copy, while a female needs two copies — far rarer than in males.

10. Q10Medium

    A dihybrid plant (RrYy) can produce how many types of gametes?

    A.One

    B.Two

    C.Eight

    D.Four✓ Correct

    Answer: D. Four

    Explanation: A dihybrid RrYy individual produces gamete types RY, Ry, rY and ry through independent assortment of the two gene pairs — four combinations, following the 2ⁿ rule for n heterozygous genes (2² = 4).

11. Q11Medium

    The F₂ phenotypic ratio in a typical Mendelian dihybrid cross is:

    A.3 : 1

    B.1 : 1 : 1 : 1

    C.9 : 3 : 3 : 1✓ Correct

    D.1 : 2 : 1

    Answer: C. 9 : 3 : 3 : 1

    Explanation: A dihybrid cross like RrYy × RrYy produces 16 combinations that sort into a 9:3:3:1 phenotypic ratio only because the two genes assort independently on separate chromosomes — linkage would distort this ratio.

12. Q12Hard

    Mendel's law of segregation is based on which cellular event?

    A.Mitosis

    B.Crossing over

    C.Separation of homologous chromosomes during meiosis I✓ Correct

    D.Random fertilisation alone

    Answer: C. Separation of homologous chromosomes during meiosis I

    Explanation: Segregation happens when homologous chromosomes, each carrying one allele of a pair, separate at anaphase I of meiosis, so every gamete gets only one allele; this is distinct from crossing over, which exchanges segments between them.

13. Q13Hard

    ABO blood-group inheritance is an example of:

    A.Complete dominance only

    B.Polygenic inheritance

    C.Pleiotropy

    D.Codominance and multiple alleles✓ Correct

    Answer: D. Codominance and multiple alleles

    Explanation: ABO blood groups involve three alleles — Iᴬ, Iᴮ and i — at one locus (multiple alleles); Iᴬ and Iᴮ are both fully expressed together in the AB phenotype (codominance), while i is recessive to both.

14. Q14Hard

    A haemophilic son is born to phenotypically normal parents. The genotypes of the parents must be:

    A.Both affected

    B.Carrier mother (XᴴXʰ) and normal father (XᴴY)✓ Correct

    C.Normal homozygous mother and affected father

    D.Affected mother and normal father

    Answer: B. Carrier mother (XᴴXʰ) and normal father (XᴴY)

    Explanation: A haemophilic son born to phenotypically normal parents means the affected X came from his mother, who must be a carrier while appearing normal because her second X is unaffected; the father, being normal, contributes only a normal X and Y.

Principles of Inheritance and Variation — FAQs

More Class 12 Biology chapters

• 01Sexual Reproduction in Flowering Plants
• 02Human Reproduction
• 03Reproductive Health
• 05Molecular Basis of Inheritance
• 06Evolution
• 07Human Health and Disease

Class 12 — other subjects

• Class 12 Chemistry chapters
• Class 12 Computer Science chapters
• Class 12 English chapters
• Class 12 Mathematics chapters
• Class 12 Physics chapters