Principles of Inheritance and Variation Class 12 Biology Notes | MCQs, Q&A, FAQs

The study of Principles of Inheritance and Variation Class 12 is one of the most fascinating aspects of biology. It deals with the transfer of genetic information from one generation to the next and the variations that arise in living organisms. The term inheritance refers to the transmission of traits from parents to offspring, while variation refers to the differences among individuals of the same species.

The principles of inheritance and Variation Class 12 were first studied systematically by Gregor Johann Mendel, who is regarded as the “Father of Genetics.” His experiments on pea plants (Pisum sativum) laid the foundation of modern genetics. This chapter explains Mendel’s laws, deviations from them, chromosomal basis of inheritance, genetic disorders, and the importance of variation in evolution.

Principles of Inheritance and Variation Class 12 Biology Notes | MCQs, Q&A, FAQs

What is Inheritance?

Inheritance is the process by which genetic material is passed from parents to offspring. It ensures continuity of traits in a species. For example, eye color, blood group, or height are inherited traits determined by genes.

Key Features of Inheritance:

• Traits are controlled by genes located on chromosomes.

• Genes exist in pairs (alleles), one inherited from each parent.

• The expression of traits depends on dominant and recessive alleles.

What is Variation?

Variation is the occurrence of differences among individuals of the same species. It is essential for evolution, as it provides the raw material for natural selection.

Types of Variation:

1. Somatic Variation – Arises in body cells, not inherited.

2. Germinal Variation – Occurs in reproductive cells, passed on to offspring.

3. Continuous Variation – Traits that show a range (e.g., height, skin color).

4. Discontinuous Variation – Traits with clear categories (e.g., blood groups).

Mendel – The Father of Genetics

Mendel’s Experiment

• Mendel performed hybridization experiments on pea plants for 7 years.

• He selected 7 contrasting traits, such as tall vs. dwarf plants, round vs. wrinkled seeds, etc.

• He cross-pollinated pea plants and studied inheritance patterns.

Why Pea Plant (Pisum sativum)?

• Short life cycle.

• Easy to cultivate.

• Naturally self-pollinating but can be cross-pollinated manually.

• Produced large number of offspring.

• Presence of contrasting traits.

Mendel’s Laws of Inheritance

1. Law of Dominance

• When two contrasting alleles are present, one dominates over the other.

• Example: In tall (TT) × dwarf (tt) cross, all F1 plants were tall (Tt).

• Dominant allele masks the recessive allele.

2. Law of Segregation (Law of Purity of Gametes)

• The two alleles of a gene separate during gamete formation.

• Each gamete carries only one allele for a trait.

• Example: In Tt, gametes will carry either T or t.

3. Law of Independent Assortment

• When two pairs of contrasting traits are inherited together, they assort independently during gamete formation.

• Example: Round yellow (RRYY) × Wrinkled green (rryy) → produces gametes independently, leading to new combinations like RrYy, Rryy, etc.

Mendel’s Experiments in Detail

Monohybrid Cross

• Cross involving a single trait.

• Example: Tall (TT) × Dwarf (tt).

• F1 Generation: All tall (Tt).

• F2 Generation: Ratio → 3 Tall : 1 Dwarf.

• Genotypic Ratio: 1 (TT) : 2 (Tt) : 1 (tt).

Dihybrid Cross

• Cross involving two traits.

• Example: Round Yellow (RRYY) × Wrinkled Green (rryy).

• F1 Generation: All Round Yellow (RrYy).

• F2 Generation: Ratio → 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green.

• Phenotypic Ratio: 9:3:3:1.

Mendelian Terminology

• Gene: Basic unit of heredity.

• Allele: Different forms of a gene.

• Homozygous: Same alleles (TT, tt).

• Heterozygous: Different alleles (Tt).

• Genotype: Genetic constitution of an organism.

• Phenotype: Observable traits.

• Dominant Trait: Expressed when one or both alleles are present.

• Recessive Trait: Expressed only when both alleles are recessive.

Deviations from Mendelian Ratios

1. Incomplete Dominance

• Neither allele is completely dominant.

• Example: In Snapdragon (Mirabilis jalapa), Red (RR) × White (rr) → Pink (Rr).

2. Codominance

• Both alleles express equally.

• Example: Human blood group AB (IAIB).

3. Multiple Alleles

• More than two alleles exist for a trait.

• Example: ABO blood group system (IA, IB, i).

4. Polygenic Inheritance

• Trait controlled by multiple genes.

• Example: Human skin color, height.

5. Pleiotropy

• Single gene influences multiple traits.

• Example: Sickle cell anemia.

Chromosomal Theory of Inheritance

• Proposed by Sutton and Boveri (1902).

• Genes are located on chromosomes.

• Chromosomes segregate and assort independently, just like Mendel’s factors.

• Confirmed Mendel’s findings at the cellular level.

Evidence from Drosophila (Fruit Fly)

• Thomas Hunt Morgan studied eye color in Drosophila.

• Proved linkage and sex-linked inheritance.

Linkage and Recombination

Linkage

• Genes located close together on the same chromosome tend to be inherited together.

• Example: Morgan’s experiment with fruit flies.

Recombination

• New combinations of traits due to crossing over during meiosis.

• Increases variation in offspring.

Sex Determination

Different mechanisms:

1. XY Type – Humans, Drosophila (male XY, female XX).

2. XO Type – Grasshopper (male XO, female XX).

3. ZW Type – Birds (male ZZ, female ZW).

4. Haplodiploidy – Honeybees (males haploid, females diploid).

Genetic Disorders

Mendelian Disorders (Single Gene Defects)

1. Thalassemia – Defective hemoglobin synthesis.

2. Phenylketonuria – Absence of enzyme phenylalanine hydroxylase.

3. Sickle Cell Anemia – Mutation in hemoglobin gene.

Chromosomal Disorders

1. Down’s Syndrome – Trisomy of chromosome 21.

2. Turner’s Syndrome – Monosomy (XO).

3. Klinefelter’s Syndrome – Extra X chromosome (XXY).

Importance of Variation

• Provides raw material for evolution.

• Increases adaptability to changing environments.

• Helps in survival of species.

• Basis of natural selection and speciation.

Multiple Choice Questions (MCQs with Answers) Principles of Inheritance and Variation Class 12 

Q1. Who is regarded as the “Father of Genetics”?

a) Charles Darwin
b) Gregor Johann Mendel 
c) Hugo de Vries
d) Thomas Hunt Morgan

Answer: b) Gregor Johann Mendel

Q2. The law of segregation is also known as:

a) Law of dominance
b) Law of purity of gametes 
c) Law of linkage
d) Law of evolution

Answer: b) Law of purity of gametes

Q3. In a monohybrid cross, the phenotypic ratio in F2 generation is:

a) 1:2:1
b) 9:3:3:1
c) 3:1 
d) 2:1

Answer: c) 3:1

Q4. Which plant did Mendel use in his hybridization experiments?

a) Sunflower
b) Pisum sativum 
c) Arabidopsis
d) Tobacco

Answer: b) Pisum sativum

Q5. Which of the following shows incomplete dominance?

a) ABO blood group
b) Snapdragon flower color 
c) Human eye color
d) Tall and dwarf pea plants

Answer: b) Snapdragon flower color

Q6. Which phenomenon occurs when both alleles express equally in the heterozygote?

a) Codominance 
b) Incomplete dominance
c) Mutation
d) Segregation

Answer: a) Codominance

Q7. ABO blood group system in humans is an example of:

a) Multiple alleles 
b) Codominance
c) Polygenic inheritance
d) Both a & b

Answer: d) Both a & b

Q8. The chromosomal theory of inheritance was proposed by:

a) Mendel
b) Sutton and Boveri 
c) Morgan
d) Darwin

Answer: b) Sutton and Boveri

Q9. Which is a sex-linked disorder?

a) Down’s syndrome
b) Turner’s syndrome
c) Hemophilia 
d) Thalassemia

Answer: c) Hemophilia

Q10. Which of the following is a polygenic trait?

a) Blood group
b) Sickle cell anemia
c) Skin color 
d) Hemophilia

Answer: c) Skin color

 Short Answer Questions (with Answers)

Principles of Inheritance and Variation Class 12 

Q1. Define inheritance and variation.

Answer:

• Inheritance is the transmission of genetic traits from parents to offspring.

• Variation refers to the differences observed among individuals of the same species.

Q2. Why did Mendel choose pea plants for his experiments?

Answer:

• Short life cycle.

• Easy to grow and cross-pollinate.

• Naturally self-pollinating.

• Presence of contrasting traits.

• Produced large number of seeds.

Q3. What is codominance? Give an example.

Answer:
Codominance is the phenomenon in which both alleles of a gene express themselves equally in the heterozygote.
Example: Human blood group AB (IAIB).

Q4. Write the phenotypic ratio of a dihybrid cross.

Answer:
The phenotypic ratio of F2 generation in a dihybrid cross is 9:3:3:1.

Q5. What is linkage?

Answer:
Linkage is the tendency of genes located close together on the same chromosome to be inherited together.

Q6. Name two Mendelian disorders.

Answer:

• Thalassemia

• Phenylketonuria

Q7. Differentiate between genotype and phenotype.

Answer:

• Genotype: The genetic constitution of an organism (e.g., Tt).

• Phenotype: Observable traits (e.g., Tall plant).

Q8. What is pleiotropy? Give an example.

Answer:
Pleiotropy is the phenomenon where a single gene influences multiple traits.
Example: Sickle cell anemia.

Q9. Name two chromosomal disorders in humans.

Answer:

• Down’s syndrome

• Turner’s syndrome

Q10. Define multiple alleles. Give an example.

Answer:
Multiple alleles occur when a gene has more than two alternative forms.
Example: ABO blood group system (IA, IB, i).

 Long Answer Questions (with Answers)

Principles of Inheritance and Variation Class 12 

Q1. Explain Mendel’s three laws of inheritance with examples.

Answer:

1. Law of Dominance – When two contrasting alleles are present, one expresses itself (dominant) while the other remains masked (recessive).
   Example: Tall allele (T) dominates over dwarf allele (t).

2. Law of Segregation – Two alleles of a gene separate during gamete formation and each gamete receives only one allele.
   Example: A heterozygote Tt produces gametes T and t.

3. Law of Independent Assortment – Alleles of different genes assort independently of each other during gamete formation.
   Example: Dihybrid cross (9:3:3:1 ratio).

Q2. Explain the chromosomal theory of inheritance.

Answer:

• Proposed by Sutton and Boveri (1902).

• Stated that genes are located on chromosomes.

• Chromosomes segregate and assort independently during meiosis, similar to Mendel’s factors.

• Later confirmed by Thomas Hunt Morgan in Drosophila (fruit flies).

Q3. Differentiate between incomplete dominance and codominance with examples.

Answer:

• Incomplete Dominance: Neither allele is completely dominant. The heterozygote shows an intermediate character.
  Example: Snapdragon flower color – Red (RR) × White (rr) → Pink (Rr).

• Codominance: Both alleles express equally in heterozygote.
  Example: Human blood group AB (IAIB).

Q4. Explain dihybrid cross and its phenotypic ratio.

Answer:

• A cross involving two traits, e.g., seed shape and color.

• Parental cross: Round Yellow (RRYY) × Wrinkled Green (rryy).

• F1 generation: All Round Yellow (RrYy).

• F2 generation: Ratio = 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green.

• Demonstrates the Law of Independent Assortment.

Q5. Write a note on genetic disorders in humans.

Answer:

1. Mendelian Disorders (Single gene defects):

   • Thalassemia (defective hemoglobin).

   • Phenylketonuria (enzyme deficiency).

   • Sickle Cell Anemia (mutation in hemoglobin gene).

2. Chromosomal Disorders:

   • Down’s Syndrome (trisomy 21).

   • Turner’s Syndrome (XO).

   • Klinefelter’s Syndrome (XXY).

Frequently Asked Questions (FAQs)

Q1. Who is called the Father of Genetics?

Gregor Johann Mendel is called the Father of Genetics because of his classical experiments on pea plants that explained how traits are inherited from one generation to the next.

Q2. What is the difference between inheritance and variation?

Inheritance is the process through which traits are passed from parents to offspring, ensuring continuity of characters within a species. Variation, on the other hand, refers to the differences among individuals of the same species, and it plays a crucial role in evolution and survival.

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Q3. What are Mendel’s three laws of inheritance?

Mendel proposed three fundamental laws: the Law of Dominance, which states that one allele can mask the expression of another; the Law of Segregation, which explains that alleles separate during gamete formation; and the Law of Independent Assortment, which states that alleles of different traits assort independently of each other during inheritance.

Q4. What is the phenotypic ratio of a monohybrid and dihybrid cross?

In a monohybrid cross, the phenotypic ratio observed in the F2 generation is 3:1, meaning three plants with the dominant trait and one with the recessive trait. In a dihybrid cross, the F2 generation shows a phenotypic ratio of 9:3:3:1, representing different combinations of two traits.

Q5. Name two chromosomal disorders in humans.

Two common chromosomal disorders in humans are Down’s Syndrome, caused by trisomy of chromosome 21, and Turner’s Syndrome, which occurs when a female has only one X chromosome (XO).

Q6. What is the difference between incomplete dominance and codominance?

In incomplete dominance, neither allele is completely dominant over the other, and the heterozygous condition shows an intermediate phenotype, such as pink flowers in Snapdragon. In codominance, both alleles express themselves equally and simultaneously, such as in the human AB blood group.

Q7. What is pleiotropy? Give an example.

Pleiotropy refers to a situation where a single gene influences multiple traits. A well-known example is sickle cell anemia, where a single gene mutation not only affects hemoglobin structure but also causes several physiological effects like anemia and resistance to malaria.

Q8. What is the chromosomal theory of inheritance?

The chromosomal theory of inheritance, proposed by Sutton and Boveri, states that genes are located on chromosomes, and the behavior of chromosomes during meiosis explains Mendel’s laws of inheritance. This theory provided a cytological basis for Mendel’s observations.

Q9. What are Mendelian disorders?

Mendelian disorders are genetic diseases caused due to alterations in a single gene. Examples include Thalassemia, Phenylketonuria, and Sickle Cell Anemia. These disorders follow Mendelian patterns of inheritance.

Q10. Why is variation important in living organisms?

Variation is important because it ensures survival and adaptability in changing environments. It provides the raw material for natural selection and evolution, helping species to develop new traits and adapt to environmental challenges.

Conclusion

The Principles of Inheritance and Variation Class 12 form the foundation of genetics. Mendel’s laws explained how traits are passed on, while later studies showed exceptions and complexities like incomplete dominance, codominance, linkage, and polygenic inheritance. The chromosomal theory linked genetics with cytology, making genetics a modern science. Understanding inheritance and variation not only helps in biology but also in fields like agriculture, medicine, and evolutionary studies.

We are Principles of Inheritance and Variation Class 12 Notes And Questions. 

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