Important Questions Class 12 Biology Chapter 4 Principles of Inheritance and Variation With Answers

Inheritance is the transfer of characters from parents to offspring through genetic material.
Variation explains why offspring resemble their parents but still differ in many visible or functional traits.

Genetics becomes easier when students connect crosses, ratios, chromosomes, and disorders instead of memorising isolated terms. Important Questions Class 12 Biology Chapter 4 help students revise Principles of Inheritance and Variation for CBSE 2026-27 board exams, school tests, and pre-board papers. NCERT Chapter 4 includes Mendel’s laws, monohybrid cross, dihybrid cross, test cross, incomplete dominance, co-dominance, ABO blood groups, linkage, recombination, sex determination, mutation, pedigree analysis, Mendelian disorders, and chromosomal disorders.

Key Takeaways

• Mendel’s Laws: Law of dominance, law of segregation, and law of independent assortment explain basic inheritance patterns.
• Genetic Crosses: Monohybrid cross gives 3:1 phenotypic ratio, while dihybrid cross gives 9:3:3:1 ratio.
• Human Inheritance: ABO blood group shows multiple alleles and co-dominance in Class 12 Biology.
• Genetic Disorders: Haemophilia, colour blindness, sickle-cell anaemia, thalassemia, Down’s syndrome, Turner’s syndrome, and Klinefelter’s syndrome are NCERT examples.

Important Questions Class 12 Biology Chapter 4 Structure 2026-27

Important Questions Class 12 Biology Chapter 4 for CBSE 2026-27

Most questions from this chapter test ratios, symbols, and examples. CBSE 2026-27 can frame the same idea as a Punnett square, pedigree, disorder, or reasoning answer.

1. What is genetics?

Genetics is the branch of Biology that studies inheritance and variation of characters from parents to offspring.

1. Inheritance passes characters from parents to progeny.
2. Variation makes progeny differ from parents.
3. Genes control inherited traits.

Final answer:
Genetics studies heredity and variation.

2. What is inheritance?

Inheritance is the process by which characters pass from parents to offspring.

1. It forms the basis of heredity.
2. It explains resemblance between parents and offspring.
3. It depends on genetic material.

Final fact:
Inheritance explains why a mango seed forms a mango plant.

3. What is variation?

Variation is the degree by which progeny differ from their parents.

1. Sexual reproduction creates variation.
2. Mutation also creates variation.
3. Recombination creates new gene combinations.

Final fact:
Variation helps explain differences among siblings.

Principles of Inheritance and Variation Class 12 Biology Important Questions on Mendel’s Work

Mendel used pea plants because he could control pollination and observe contrasting traits. His results became reliable because he used large sample size and mathematical analysis.

4. Why did Mendel select pea plants for his experiments?

Mendel selected pea plants because they showed clear contrasting traits and could undergo controlled breeding.

1. Pea plants had true-breeding lines.
2. They showed contrasting traits like tall and dwarf.
3. They produced many seeds.
4. They had a short life cycle.
5. Artificial cross-pollination was possible.

Final fact:
Mendel studied 14 true-breeding pea varieties.

5. What is a true-breeding line?

A true-breeding line shows stable trait inheritance for several generations after continuous self-pollination.

1. The trait remains unchanged across generations.
2. The line is usually homozygous for that trait.
3. Mendel used true-breeding pea lines for crosses.

Final answer:
A true-breeding line gives the same trait generation after generation.

6. Name the seven contrasting traits studied by Mendel in pea plants.

Mendel studied seven pairs of contrasting traits in pea plants.

1. Stem height: tall or dwarf.
2. Flower colour: violet or white.
3. Flower position: axial or terminal.
4. Pod shape: inflated or constricted.
5. Pod colour: green or yellow.
6. Seed shape: round or wrinkled.
7. Seed colour: yellow or green.

Final fact:
Mendel selected traits with two clear opposite forms.

7. What are genes and alleles?

Genes are units of inheritance, while alleles are different forms of the same gene.

1. Genes contain information for a trait.
2. Alleles control contrasting forms of a trait.
3. T and t are alleles for plant height.

Final answer:
Genes control characters, and alleles control alternate trait forms.

8. What is the difference between genotype and phenotype?

Genotype is the genetic constitution, while phenotype is the visible expression of a trait.

Final fact:
TT and Tt can both produce tall phenotype.

9. What is the difference between homozygous and heterozygous?

Homozygous means both alleles are identical, while heterozygous means both alleles are different.

Final fact:
A heterozygous tall pea plant has genotype Tt.

Mendel’s Laws Class 12 Biology Questions With Answers

Mendel’s laws explain how alleles behave during gamete formation and fertilisation. CBSE questions often ask students to explain a law using a monohybrid or dihybrid cross.

10. What is the law of dominance?

The law of dominance states that in a dissimilar pair of factors, one factor dominates the other.

Main points:

1. Characters are controlled by discrete factors.
2. Factors occur in pairs.
3. One factor can dominate another.
4. The recessive trait remains hidden in F1 generation.

Final fact:
Tallness dominates dwarfness in Mendel’s pea plant cross.

11. What is the law of segregation?

The law of segregation states that alleles of a pair separate during gamete formation.

1. Each parent has two alleles for a trait.
2. Gametes receive only one allele.
3. Alleles do not blend.
4. Recessive trait can reappear in F2 generation.

Final answer:
Law of segregation is also called purity of gametes.

12. What is the law of independent assortment?

The law of independent assortment states that segregation of one pair of characters is independent of another pair.

1. It applies to dihybrid crosses.
2. Mendel observed 9:3:3:1 ratio in F2 generation.
3. Seed colour and seed shape assorted independently.

Final fact:
This law applies when genes assort independently.

Monohybrid Cross Class 12 Biology Questions

A monohybrid cross follows one character at a time. Students should remember both phenotype and genotype ratios in F2 generation.

13. What is a monohybrid cross?

A monohybrid cross is a cross between two individuals considering one contrasting character.

Example:
Tall pea plant × dwarf pea plant

Parental genotypes:
TT × tt

F1 generation:
All Tt

F1 phenotype:
All tall

Final fact:
A monohybrid cross studies one gene pair.

14. Explain Mendel’s monohybrid cross using plant height.

Mendel crossed true-breeding tall pea plants with true-breeding dwarf pea plants. All F1 plants were tall.

Cross:
TT × tt

Gametes:
T and t

F1 genotype:
Tt

F1 phenotype:
Tall

Selfing of F1:
Tt × Tt

F2 genotypes:
TT, Tt, Tt, tt

Final result:
F2 phenotypic ratio = 3 tall : 1 dwarf
F2 genotypic ratio = 1 TT : 2 Tt : 1 tt

15. Why did dwarf trait reappear in F2 generation?

The dwarf trait reappeared because alleles segregated during gamete formation and reunited during fertilisation.

1. The dwarf allele remained hidden in F1.
2. F1 plants had genotype Tt.
3. Gametes carried either T or t.
4. Fertilisation produced tt in F2.

Final fact:
Reappearance of dwarf trait supports law of segregation.

16. What is the phenotypic and genotypic ratio of a monohybrid cross?

The F2 phenotypic ratio is 3:1, and the genotypic ratio is 1:2:1.

Cross:
Tt × Tt

Genotypes:
1 TT : 2 Tt : 1 tt

Phenotypes:
3 tall : 1 dwarf

Final answer:
Phenotypic ratio = 3:1
Genotypic ratio = 1:2:1

17. What is a test cross Class 12 Biology?

A test cross is a cross between an organism with dominant phenotype and a homozygous recessive parent.

Purpose:

1. It identifies unknown genotype.
2. It shows whether the dominant organism is homozygous or heterozygous.
3. Mendel used it to test tall F2 plants.

Final fact:
A test cross uses the recessive parent.

18. What result comes from a test cross with heterozygous tall pea plant?

A test cross with heterozygous tall plant gives 1 tall : 1 dwarf ratio.

Cross:
Tt × tt

Gametes:
T, t and t

Progeny:
Tt and tt

Final result:
Phenotypic ratio = 1 tall : 1 dwarf

19. What result comes from a test cross with homozygous tall pea plant?

A test cross with homozygous tall plant gives all tall progeny.

Cross:
TT × tt

Gametes:
T and t

Progeny:
All Tt

Final result:
100 percent tall progeny

Dihybrid Cross Class 12 Biology Questions

A dihybrid cross tracks two characters together. Mendel’s pea seed colour and seed shape cross is the most important example.

20. What is a dihybrid cross?

A dihybrid cross is a cross between two individuals considering two pairs of contrasting characters.

Example:
Round yellow seeds × wrinkled green seeds

Parental genotypes:
RRYY × rryy

F1 genotype:
RrYy

F1 phenotype:
Round yellow

Final fact:
Dihybrid cross supports independent assortment.

21. Explain Mendel’s dihybrid cross in pea plants.

Mendel crossed round yellow seeded plants with wrinkled green seeded plants. All F1 plants had round yellow seeds.

Parental cross:
RRYY × rryy

Gametes:
RY and ry

F1 genotype:
RrYy

F1 selfing:
RrYy × RrYy

F2 phenotypes:

1. Round yellow.
2. Round green.
3. Wrinkled yellow.
4. Wrinkled green.

Final result:
F2 phenotypic ratio = 9:3:3:1

22. What is the phenotypic ratio of a dihybrid cross?

The F2 phenotypic ratio of a Mendelian dihybrid cross is 9:3:3:1.

Ratio:
9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green

Final fact:
This ratio supports law of independent assortment.

23. A diploid organism is heterozygous for 4 loci. How many types of gametes can it produce?

It can produce 16 types of gametes.

Formula:
Number of gamete types = 2^n

Where:
n = number of heterozygous loci

Given:
n = 4

Calculation:
2^4 = 16

Final result:
16 types of gametes

24. In the cross TtYy × Ttyy, what proportion will be tall and green?

The proportion of tall and green offspring will be 3/8.

For height:
Tt × Tt gives 3/4 tall.

For seed colour:
Yy × yy gives 1/2 green.

Combined probability:
3/4 × 1/2 = 3/8

Final result:
Tall and green = 3/8

25. In the cross TtYy × Ttyy, what proportion will be dwarf and green?

The proportion of dwarf and green offspring will be 1/8.

For height:
Tt × Tt gives 1/4 dwarf.

For seed colour:
Yy × yy gives 1/2 green.

Combined probability:
1/4 × 1/2 = 1/8

Final result:
Dwarf and green = 1/8

Incomplete Dominance Class 12 Biology and Co-dominance Questions

Not every trait follows complete dominance. Snapdragon flower colour and ABO blood group show how inheritance can move beyond simple dominant-recessive patterns.

26. What is incomplete dominance?

Incomplete dominance occurs when the F1 phenotype is intermediate between both parents.

Example:
Red snapdragon × white snapdragon

Cross:
RR × rr

F1 genotype:
Rr

F1 phenotype:
Pink

F2 ratio:
1 red : 2 pink : 1 white

Final fact:
In incomplete dominance, phenotypic and genotypic ratios are both 1:2:1.

27. What is co-dominance?

Co-dominance occurs when both alleles express equally in the F1 generation.

Example:
AB blood group in humans

1. IA produces A sugar.
2. IB produces B sugar.
3. IAIB expresses both A and B sugars.

Final answer:
AB blood group is an example of co-dominance.

28. How does ABO blood group show multiple alleles?

ABO blood group shows multiple alleles because one gene has three alleles: IA, IB, and i.

Alleles:

1. IA
2. IB
3. i

Genotypes and blood groups:

1. IAIA or IAi = A group.
2. IBIB or IBi = B group.
3. IAIB = AB group.
4. ii = O group.

Final fact:
A person carries only two alleles, but the population has three alleles.

29. A child has blood group O. Father has blood group A and mother has blood group B. What are the parents’ genotypes?

The father must be IAi, and the mother must be IBi.

Reason:

1. Blood group O has genotype ii.
2. Child must receive i from both parents.
3. Father with A group must carry i.
4. Mother with B group must carry i.

Parents’ genotypes:
Father = IAi
Mother = IBi

Possible offspring:
IAIB, IAi, IBi, ii

Final fact:
Other children may have AB, A, B, or O blood groups.

30. What is pleiotropy?

Pleiotropy occurs when one gene produces multiple phenotypic effects.

Example:
Phenylketonuria

1. Mutation affects phenylalanine hydroxylase.
2. It causes mental retardation.
3. It reduces hair and skin pigmentation.

Final fact:
One gene can affect more than one trait.

31. What is polygenic inheritance?

Polygenic inheritance occurs when three or more genes control one trait.

Example:
Human skin colour

1. Several genes contribute to the phenotype.
2. Each allele has an additive effect.
3. Environment may also influence the trait.

Final fact:
Human height and skin colour show continuous variation.

Chromosomal Theory of Inheritance Class 12 Biology Questions

Chromosomal theory links Mendel’s factors with chromosomes. Sutton and Boveri explained that chromosome behaviour during meiosis matches gene behaviour.

32. Who proposed the chromosomal theory of inheritance?

Walter Sutton and Theodore Boveri proposed the chromosomal theory of inheritance.

1. They compared chromosome behaviour with gene behaviour.
2. Chromosomes occur in pairs.
3. Genes occur in pairs.
4. Chromosomes segregate during gamete formation.

Final fact:
Sutton united chromosomal segregation with Mendelian principles.

33. What does the chromosomal theory of inheritance state?

Chromosomal theory states that genes are located on chromosomes and chromosomes carry hereditary information.

1. Chromosomes and genes occur in pairs.
2. Homologous chromosomes separate during meiosis.
3. Alleles segregate during gamete formation.
4. Independent chromosome assortment explains independent assortment of genes.

Final answer:
Chromosomes form the physical basis of inheritance.

34. Why was Drosophila used in genetic studies?

Drosophila melanogaster was used because it grows easily and produces many progeny quickly.

Advantages:

1. It grows on simple synthetic medium.
2. It completes life cycle in about two weeks.
3. One mating produces many progeny.
4. Male and female flies are easily distinguished.
5. Hereditary variations are visible under low-power microscope.

Final fact:
Morgan used Drosophila for inheritance studies.

Linkage and Recombination Class 12 Biology Questions

Linkage explains why some genes do not assort independently. Recombination frequency helps estimate distance between genes on a chromosome.

35. What is linkage?

Linkage is the physical association of genes on the same chromosome. Linked genes tend to inherit together.

1. Morgan coined the term linkage.
2. Genes on the same chromosome may not assort independently.
3. Closely linked genes show low recombination.

Final fact:
Linkage can change expected Mendelian ratios.

36. What is recombination?

Recombination is the generation of non-parental gene combinations.

1. It occurs due to crossing over.
2. It separates linked genes in some gametes.
3. Higher recombination means genes are farther apart.

Final fact:
Morgan used recombination to explain non-parental combinations.

37. What was T.H. Morgan’s contribution to genetics?

T.H. Morgan showed linkage and recombination using Drosophila. His work supported the chromosomal theory of inheritance.

1. He studied sex-linked genes in fruit flies.
2. He observed deviation from 9:3:3:1 ratio.
3. He explained physical association of genes.
4. His student Sturtevant mapped genes using recombination frequency.

Final fact:
Morgan’s work connected genes with chromosomes.

38. Why do linked genes show low recombination?

Linked genes show low recombination when they lie close together on the same chromosome.

1. Close genes stay together during crossing over.
2. Fewer non-parental combinations form.
3. Recombination percentage remains low.

Final fact:
White and yellow genes in Drosophila showed 1.3 percent recombination.

Sex Determination Class 12 Biology Questions

Sex determination questions are important for both board exams and social awareness. NCERT clearly explains that sperm determines sex in humans.

39. What is sex determination?

Sex determination is the mechanism by which male or female sex gets decided in an organism.

1. It may depend on sex chromosomes.
2. It may depend on chromosome number.
3. Humans show XY type.
4. Honey bees show haplodiploid system.

Final fact:
Different organisms use different sex-determination systems.

40. What is male heterogamety?

Male heterogamety occurs when males produce two types of gametes.

Examples:

1. XO type in grasshopper.
2. XY type in humans.
3. XY type in Drosophila.

Final fact:
Human males produce X-bearing and Y-bearing sperms.

41. How is sex determined in humans?

Sex in humans is determined by the chromosome carried by the sperm.

1. Females have XX chromosomes.
2. Males have XY chromosomes.
3. All ova carry X chromosome.
4. Half the sperms carry X chromosome.
5. Half the sperms carry Y chromosome.
6. X sperm with ovum forms XX female.
7. Y sperm with ovum forms XY male.

Final fact:
There is 50 percent probability of male or female child in each pregnancy.

42. Why is the father responsible for the sex of the child?

The father is genetically responsible because sperm carries either X or Y chromosome.

1. Mother always contributes X chromosome.
2. Father contributes X or Y chromosome.
3. X-bearing sperm gives female child.
4. Y-bearing sperm gives male child.

Final fact:
Blaming women for the sex of a child is scientifically wrong.

43. How is sex determined in honey bees?

Honey bees show haplodiploid sex determination. Fertilised eggs become females, while unfertilised eggs become males.

1. Females are diploid.
2. Females have 32 chromosomes.
3. Males are haploid.
4. Males have 16 chromosomes.
5. Males develop by parthenogenesis.

Final fact:
Male honey bees have no father but can have a grandfather.

Mutation and Pedigree Analysis Class 12 Biology Questions

Mutation creates genetic changes, while pedigree analysis traces inheritance in human families. These topics often appear in NCERT exercise and case-based questions.

44. What is mutation?

Mutation is a change in DNA sequence that can alter genotype and phenotype.

Types include:

1. Point mutation.
2. Insertion.
3. Deletion.
4. Duplication.
5. Chromosomal aberration.

Final fact:
Mutation and recombination create genetic variation.

45. What is point mutation?

Point mutation is a change in a single base pair of DNA.

Example:
Sickle-cell anaemia

1. One base changes in the beta-globin gene.
2. GAG changes to GUG.
3. Glutamic acid changes to valine.
4. RBCs become sickle-shaped under low oxygen.

Final fact:
Sickle-cell anaemia is a classic point mutation example.

46. What are mutagens?

Mutagens are physical or chemical agents that induce mutations.

Example:
UV radiation

1. Mutagens alter DNA.
2. They can change genotype.
3. They may affect phenotype.

Final fact:
UV radiation acts as a mutagen.

47. What is pedigree analysis?

Pedigree analysis is the study of inheritance of a trait across several generations of a family.

1. It represents family history using symbols.
2. It tracks traits, abnormalities, or diseases.
3. It helps identify dominant, recessive, or sex-linked inheritance.

Final answer:
Pedigree analysis is useful in human genetics.

Genetic Disorders Class 12 Biology Questions With Answers

Genetic disorders in this chapter belong to Mendelian disorders or chromosomal disorders. Students should remember cause, inheritance type, and symptoms.

48. What are Mendelian disorders?

Mendelian disorders are genetic disorders caused by mutation in a single gene.

Examples:

1. Haemophilia.
2. Cystic fibrosis.
3. Sickle-cell anaemia.
4. Colour blindness.
5. Phenylketonuria.
6. Thalassemia.

Final fact:
Pedigree analysis can trace Mendelian disorders.

49. What is colour blindness?

Colour blindness is a sex-linked recessive disorder caused by defects in red or green cone cells of the eye.

1. It prevents red-green colour discrimination.
2. It occurs due to mutation on the X chromosome.
3. It affects about 8 percent of males.
4. It affects about 0.4 percent of females.

Final fact:
Males are more affected because they have only one X chromosome.

50. What is haemophilia?

Haemophilia is a sex-linked recessive disorder in which blood fails to clot normally.

1. One clotting protein is affected.
2. A simple cut causes continuous bleeding.
3. Carrier females may transmit it to sons.
4. Queen Victoria’s family pedigree showed haemophilia.

Final fact:
Female haemophilia is extremely rare.

51. What is sickle-cell anaemia?

Sickle-cell anaemia is an autosomal recessive disorder caused by mutation in the beta-globin gene.

1. HbA and HbS are the alleles.
2. HbSHbS individuals show disease.
3. HbAHbS individuals are carriers.
4. GAG changes to GUG.
5. Glutamic acid changes to valine.

Final fact:
RBCs become sickle-shaped under low oxygen tension.

52. What is phenylketonuria?

Phenylketonuria is an autosomal recessive inborn error of metabolism.

1. The affected person lacks phenylalanine hydroxylase.
2. Phenylalanine does not convert into tyrosine.
3. Phenylalanine accumulates.
4. Phenylpyruvic acid forms.
5. Accumulation in brain causes mental retardation.

Final fact:
Phenylpyruvic acid is excreted through urine.

53. What is thalassemia?

Thalassemia is an autosome-linked recessive blood disorder caused by reduced synthesis of globin chains.

1. Alpha thalassemia affects alpha globin chain.
2. Beta thalassemia affects beta globin chain.
3. It causes abnormal haemoglobin formation.
4. It results in anaemia.

Final fact:
Thalassemia is quantitative, while sickle-cell anaemia is qualitative.

54. What are chromosomal disorders?

Chromosomal disorders occur due to absence, excess, or abnormal arrangement of chromosomes.

Causes:

1. Aneuploidy.
2. Polyploidy.
3. Chromosomal rearrangement.

Examples:

1. Down’s syndrome.
2. Turner’s syndrome.
3. Klinefelter’s syndrome.

Final fact:
Aneuploidy results from failure of chromatid segregation.

55. What is Down’s syndrome?

Down’s syndrome is caused by an extra copy of chromosome 21. It is also called trisomy 21.

Symptoms:

1. Short stature.
2. Small round head.
3. Furrowed tongue.
4. Partially open mouth.
5. Broad palm with palm crease.
6. Retarded physical, psychomotor, and mental development.

Final fact:
Down’s syndrome gives 47 chromosomes.

56. What is Klinefelter’s syndrome?

Klinefelter’s syndrome is caused by an additional X chromosome in males. The karyotype is 47, XXY.

Features:

1. Overall masculine development.
2. Breast development or gynaecomastia.
3. Sterility.

Final fact:
Klinefelter’s syndrome is a sex chromosome disorder.

57. What is Turner’s syndrome?

Turner’s syndrome occurs due to absence of one X chromosome in females. The karyotype is 45, X0.

Features:

1. Sterile female.
2. Rudimentary ovaries.
3. Lack of secondary sexual characters.
4. Short stature.

Final fact:
Turner’s syndrome is a monosomic condition.

Class 12 Biology Important Links