CBSE Class 12 Biology Exemplar NCERT Solutions Chapter 5: Principles of Inheritance and Variation

Reviewed by NCERT Help Editorial Team · Updated on 29 Jul 2026

NCERT Solutions PDF Class 12 PDF

This resource provides comprehensive NCERT Solutions for CBSE Class 12 Biology Exemplar, Chapter 5: Principles of Inheritance and Variation. It covers key genetic concepts such as linkage, chromosome variations (aneuploidy, polyploidy), recombination, and different modes of inheritance including sex-linked and autosomal patterns. The solutions offer detailed explanations for Multiple Choice Questions (MCQs), breaking down complex topics into understandable steps. This guide is designed to help students clarify doubts, reinforce their understanding of Mendelian genetics and its extensions, and prepare effectively for their board examinations by mastering the principles of heredity and variation.

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Quick info

Board	CBSE
Class	Class 12
Subject	Biology Exemplar
Session	2026
Language	English
Type	NCERT Solutions
Chapter	Chapter 5

Chapter summary

Chapter 5 of the CBSE Class 12 Biology Exemplar focuses on the Principles of Inheritance and Variation. This section provides detailed solutions to MCQs covering topics like gene linkage on chromosomes, chromosomal abnormalities such as aneuploidy and polyploidy, the relationship between gene distance and recombination frequency, and the inheritance patterns of genetic diseases (autosomal and sex-linked). The solutions aim to clarify fundamental concepts of genetics and heredity.

Learning outcomes

Understand the concept of linkage groups and how genes on the same chromosome behave.
Differentiate between aneuploidy and polyploidy, including conditions like monosomy.
Explain the direct relationship between the distance of genes on a chromosome and the percentage of recombination.
Identify and explain the inheritance pattern of sex-linked recessive genetic diseases.
Analyze genetic problems involving carrier females and their male progeny.
Clarify the definitions of autosomal dominant, autosomal recessive, and sex-linked dominant inheritance.

Topics covered

Paper topics

Linkage
Linkage Groups
Recombination
Gene Distance
Chromosome Number Variations
Aneuploidy
Polyploidy
Monosomy
Sex-linked Inheritance
Autosomal Inheritance
Carrier Female
Genetic Diseases

Important topics

Linkage and Recombination
Aneuploidy vs. Polyploidy
Sex-linked Recessive Inheritance
Relationship between Gene Distance and Recombination Frequency
Identifying Inheritance Patterns

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Questions and Solutions

Q. 1

All genes located on the same chromosome

(a) form different groups depending upon their relative distance

(b) form one linkage group

(d) form interactive groups that affect the phenotype

Solution: The correct answer is (b). All genes present on a particular chromosome are considered to form a single linkage group. The number of linkage groups in a species generally corresponds to the number of chromosomes in its haploid set. For example, in human males, there are 24 linkage groups (22 pairs of autosomes + 1 X-chromosome + 1 Y-chromosome), and in females, there are 23 linkage groups (22 pairs of autosomes + 2 X-chromosomes). Options (a), (c), and (d) are incorrect because genes on the same chromosome are linked and do not form separate groups based on distance, nor do they fail to form linkage groups or form interactive groups in the manner described.

Q. 2

Conditions of a karyotype 2n±1 and 2n±2 are called

(a) aneuploidy

(b) polyploidy

(d) monosomy

Solution: The correct answer is (a). Aneuploidy refers to numerical changes in chromosome number that involve the addition or deletion of one or more chromosomes, but not an entire set. Conditions like 2n±1 (e.g., monosomy, where one chromosome is missing) and 2n±2 (e.g., nullisomy, where a pair of chromosomes is missing, or tetrasomy, where an extra pair is present) fall under aneuploidy. Polyploidy (b) involves the addition of one or more complete sets of chromosomes (e.g., 3n, 4n). Allopolyploidy (c) is a specific type of polyploidy involving chromosome sets from different species. Monosomy (d) is a specific type of aneuploidy (2n-1) but does not encompass all conditions listed (2n±1 and 2n±2).

Q. 3

Distance between the genes and percentage of recombination shows

(a) a direct relationship

(b) an inverse relationship

(d) no relationship

Solution: The correct answer is (a). The percentage of recombination between genes is directly proportional to the distance between them on a chromosome. This is because the likelihood of a crossing-over event (which leads to recombination) occurring between two genes increases with the physical distance separating them. Genes that are close together are less likely to be separated by crossing over, resulting in lower recombination frequencies (higher linkage), while genes farther apart have a higher chance of recombination.

Q. 4

If a genetic disease is transferred from a phenotypically normal but carrier female to only some of the male progeny, the disease is

(a) autosomal dominant

(b) autosomal recessive

(d) sex-linked recessive

Solution: The correct answer is (d) sex-linked recessive. In sex-linked recessive inheritance, the gene responsible for the disease is located on the X chromosome. A carrier female has one normal X chromosome and one X chromosome with the recessive allele for the disease. She does not show the disease herself. When she reproduces, she can pass on either her normal X chromosome or the affected X chromosome to her children. Male offspring inherit one X chromosome from their mother and one Y chromosome from their father. If a male inherits the affected X chromosome from his carrier mother, he will express the sex-linked recessive disease because he only has one X chromosome. Autosomal dominant (a) and autosomal recessive (b) diseases are inherited through genes on autosomes and affect males and females equally. Sex-linked dominant (c) diseases are also expressed in females if they inherit one affected X chromosome.

Common mistakes

Confusing the number of linkage groups with the number of chromosomes.
Misinterpreting the difference between aneuploidy and polyploidy.
Assuming an inverse relationship between gene distance and recombination frequency.
Incorrectly identifying the mode of inheritance for sex-linked disorders.

Revision tips

Review the definitions of linkage and recombination thoroughly.
Draw diagrams to visualize aneuploidy and polyploidy conditions.
Work through the MCQs multiple times to solidify understanding of inheritance patterns.
Pay close attention to the 'Thinking Process' sections for detailed explanations.

Practice MCQs

Q1. All genes located on the same chromosome are considered to form:

Q2. Karyotypes described by the conditions 2n±1 and 2n±2 are classified as:

Q3. The percentage of recombination between genes and the distance between them on a chromosome show:

Q4. If a genetic disease is transmitted from a phenotypically normal but carrier female to only some of her male progeny, the disease is likely:

Frequently asked questions

What is a linkage group in genetics?

A linkage group consists of all the genes located on the same chromosome. These genes tend to be inherited together.

What is the difference between aneuploidy and polyploidy?

Aneuploidy involves the addition or deletion of individual chromosomes (e.g., 2n±1), while polyploidy involves the addition of entire sets of chromosomes (e.g., 3n, 4n).

How does the distance between genes affect recombination?

The farther apart two genes are on a chromosome, the higher the chance of a crossover event occurring between them, resulting in a higher percentage of recombination.

Why are sex-linked recessive diseases more common in males?

Males have only one X chromosome. If they inherit an X chromosome with a recessive allele for a sex-linked disease, they will express the disease, whereas females need to inherit the allele on both X chromosomes to be affected.

What does it mean for a female to be a 'carrier' of a genetic disease?

A carrier female has one copy of a recessive allele for a genetic disease on her X chromosome, but she does not express the disease herself because she also has a normal allele on her other X chromosome. However, she can pass the recessive allele to her offspring.

Are all genes on the same chromosome always inherited together?

Not necessarily. While genes on the same chromosome are linked, crossing over during meiosis can separate them, leading to recombination and independent assortment of alleles.

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