Principles of Inheritance and Variation — Class 12 Biology NCERT Solutions (Free)

Free step-by-step NCERT solutions for Class 12 Biology chapter "Principles of Inheritance and Variation" — 8 important questions with detailed answers for CBSE board exam preparation.

TL;DR: Free step-by-step NCERT solutions for Class 12 Biology chapter "Principles of Inheritance and Variation" — 8 important questions with detailed answers…

By Syllab.in · Updated Jun 14, 2026

Key Questions Covered:

Define gene, allele, phenotype, and genotype.
State Mendel's law of segregation with explanation and example.
State and explain Mendel's law of independent assortment with an example dihy…
Explain test cross and back cross. Distinguish between them.
Explain the concept of linkage and crossing over. How do they affect genetic …
Define mutation and explain different types of mutations with examples.
+ 2 more questions in the full chapter

Solutions Summary:

Question	Status
Define gene, allele, phenotype, and genotype.	✓ Solved
State Mendel's law of segregation with explanation and ex…	✓ Solved
State and explain Mendel's law of independent assortment …	✓ Solved
Explain test cross and back cross. Distinguish between them.	✓ Solved
Explain the concept of linkage and crossing over. How do …	✓ Solved
Define mutation and explain different types of mutations …	✓ Solved

Showing 6 of 8 questions

Q1: Define gene, allele, phenotype, and genotype.

Key genetic terms: Gene: Segment of DNA coding for a specific trait or protein. Basic unit of heredity. Located on chromosomes at fixed positions (loci). Genes control traits through protein synthesis. Allele: Alternative forms of a gene at the same locus on homologous chromosomes. Example: gene for flower color has alleles for red, white, pink. Organisms are diploid, carrying two alleles for each gene (one from each parent). Genotype: Genetic composition of an organism; allele combination at...

Q2: State Mendel's law of segregation with explanation and example.

Law of Segregation (First Law of Inheritance): Statement: When gametes are formed during meiosis, allele pairs separate so that each gamete receives only one allele of each gene. During fertilization, alleles from both parents combine to restore the pair in offspring. Explanation: - Diploid organisms carry two alleles for each trait (one from each parent) - During meiosis, homologous chromosomes separate, carrying different alleles to different gametes - Each gamete receives only ONE allele - ...

Q3: State and explain Mendel's law of independent assortment with an example dihybrid cross.

Law of Independent Assortment (Second Law of Inheritance): Statement: Allele pairs of different genes assort independently during gamete formation. The inheritance of one trait is independent of inheritance of another trait (for genes on different chromosomes). Explanation: - Applies to genes on non-homologous chromosomes - During meiosis, distribution of alleles for one gene does not affect distribution of alleles for another gene - Each gene segregates independently - Results in new combinat...

Q4: Explain test cross and back cross. Distinguish between them.

Test cross and back cross are controlled matings used in genetics to determine genotypes and study inheritance: Test cross: - Definition: Cross between individual with dominant phenotype (unknown genotype) and homozygous recessive individual - Purpose: Determine whether dominant individual is homozygous (AA) or heterozygous (Aa) - Parental cross: Aa (or AA) × aa - Results: * If dominant parent is AA: All offspring show dominant phenotype (Aa) * If dominant parent is Aa: 1:1 ratio of dominan...

Q5: Explain the concept of linkage and crossing over. How do they affect genetic variation?

Linkage and crossing over are phenomena affecting inheritance of genes on same chromosome: Linkage: - Definition: Genes located close together on same chromosome tend to be inherited together as a unit - Concept: Linked genes do not assort independently; violates Mendel's law of independent assortment - Reason: Physical proximity on chromosome; separated only by crossing over - Strength: Linkage strength depends on distance between genes (map distance in centimorgans) - Example: In Drosophila, ...

Q6: Define mutation and explain different types of mutations with examples.

Mutation: Sudden, random change in DNA sequence causing alteration in genetic information. Source of genetic variation and evolution. Types of mutations: 1. Gene (Point) mutations: Change in single nucleotide - Substitution: One base replaced by another * Silent mutation: no change in protein (codon redundancy); example: CCU→CCC (both code leucine) * Missense mutation: changes amino acid; example: sickle cell anemia (glutamic acid→valine in β-globin) * Nonsense mutation: crea...

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