NCERT Solutions Class 12 Biology Chapter 4 Principles of Inheritance and Variation

Chapter 4, Principles of Inheritance and Variation from Class 12 Biology teaches students how traits/characteristics are passed from parents to offspring and why certain differences appear among individuals of the same species. It also explains Mendel’s experiments, basic genetic terms, and the patterns of inheritance seen in different crosses. Concepts like linkage, recombination, and chromosomal basis of inheritance, etc., are introduced in this chapter.

ALLEN provides NCERT Solutions which makes the complicated genetic cross diagrams simple and easy to understand. Topics like Punnett squares and pedigree analysis are made easy so that you never get stuck. These solutions are meant to assist you in preparing for your classroom, providing you with the benefit of expert-checked answers and clear explanations. The solutions will also help students preparing for competitive exams like NEET.

1.0Key Concepts of Class 12 Biology Chapter 4 Principles of Inheritance and Variation

The chapter takes students through the basic principles of heredity and explains how traits are passed from one generation to the next. It also covers the role of chromosomes, gene interactions, and genetic variations in organisms. The major concepts include:

Mendel’s Laws of Inheritance: Understand the laws of dominance, segregation, and independent assortment and their role in the inheritance of traits.
Monohybrid and Dihybrid Crosses: Learn how to solve genetic crosses and determine phenotypic and genotypic ratios.
Chromosomal Theory of Inheritance: Study how genes are present on chromosomes and understand their behaviour during cell division and inheritance.
Linkage and Recombination: Explore how genes located on the same chromosome may be inherited together and how recombination creates new combinations of traits.
Sex Determination: Learn how chromosomes determine sex in humans and understand different mechanisms of sex determination in organisms.
Genetic Disorders: Study inherited conditions such as haemophilia, sickle-cell anaemia, and colour blindness, along with their genetic basis.
Variation: Understand the sources of genetic variation and why differences among individuals are important for populations and evolution.

2.0NCERT Class 12 Biology Chapter 4 Principles of Inheritance and Variation : Detailed Solutions

Mention the advantages of selecting pea plant for experiment by Mendel.

Ans.

Annual plant with short life cycle of 2-3 months.
It has many contrasting trait.
Pea seeds are large.
Easy to cultivate.
Pea plant is naturally self pollinating.
Artificial cross pollination can be easily performed.

Differentiate between the following - [IMP.]

(A) Dominance and Recessive (B) Homozygous and Heterozygous (C) Monohybrid and Dihybrid

Ans.

(A) Dominance and Recessive

Dominance	Recessive
Allele which can express itself both in homozygous and heterozygous condition is known as dominant allele.	Allele which can express itself only in homozygous condition is known as recessive allele.

(B)Homozygous and Heterozygous

Homozygous	Heterozygous
In diploid individual, if similar alleles of a gene is present then it is said to be homozygous.<br> - Also termed as Pure/true breeding.<br>	In diploid individual, if dissimilar alleles of a gene is present then it is said to be heterozygous.<br> - Also termed as Impure/Hybrid.<br>

(C)Monohybrid and Dihybrid.

Monohybrid	Dihybrid
Monohybrid involves cross between parents which differs in only one pair of contrasting characters.	Dihybrid involvesc ross between parents differs in two pairs of contrasting characters.
For example The cross between tall and dwarf pea plant is a monohybrid cross.	For example The cross between pea plants having yellow round seed with those having green wrinkled seeds is a dihybrid cross.

A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced? Ans. A diploid organism heterozygous for 4 loci, will have four different contrasting characters at four different loci. For example if an organism is heterozygous at four loci with four characters say AaBbCcDd then during meiosis it will segregate to form 16 separate gametes. Type of gamete $= 2^{n}$ where n is no of heterozygous pair So $n = 4$ so $2^{4}$ so 16 gametes.
Explain the Law of Dominance using a monohybrid cross. Ans. When two different factors (genes) or a pair of contrasting forms of a character are present in an organism, only one expresses itself in the $F_{1}$ generation and is termed as dominant while the other remains unexpressed and called recessive factors (gene).
Define and design a test cross. Ans. When $F_{1}$ progeny (heterozygous) is crossed with recessive parent then it is called test cross.

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Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.

Ans.

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When a cross in made between tall plant with yellow seeds (TtYy) and tall plant with green seed (Ttyy), what proportions of phenotype in the offspring could be expected to be : [IMP.] (a) Tall and green. (b) Dwarf and green.

Ans. Tall & Yellow Seeds Tall & Green Seeds

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Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in $F_{1}$ generation for a dihybrid cross? Ans. The co-existence of two or more genes in the same chromosome is termed as linkage. If the genes are located close to each other and on the same chromosome, they are inherited together and are referred to as linked genes. If two heterozygous parents exhibit linkage, then the outcome is as follows:

BbLl × BbLl Blue long Blue long So in $F_{1}$ generation the parental combination will comparatively be more than the newer combinations which are less in number.

Briefly mention the contribution of T.H. Morgan in genetics. Ans. Thomas Hunt Morgan is called father of experimental genetics.

Experimental verification of the chromosomal theory of inheritance was given by Thomas Hunt Morgan and his colleagues, led to discovering the basis for the variation that sexual reproduction produced.
Morgan worked with the tiny fruit files, Drosophila melanogaster, which were found very suitable for such studies.
Morgan carried out several dihybrid crosses in Drosophila to study genes that were sex-linked.
He stated and established that genes are located on the chromosome.
He established the principle of linkage, crossing over, sex linked inheritance and discovered the relationship between gene and chromosome.
He established the technique of chromosome mapping.
He observed and worked on mutation.

What is pedigree analysis? Suggest how such an analysis, can be useful. Ans. A pedigree is a record of inheritance of a specific genetic trait for two or more generations which is presented in the form of a diagram or family tree. Pedigree analysis is an analysis of several generations of a family which is used on human beings. Usefulness of pedigree analysis:

Serves as a powerful tool which can be used to trace the inheritance of a particular trait, disease or an abnormality
It is helpful for genetic counsellors to suggest couples about the possibility of having children with genetic abnormalities such as colour blindness, haemophilia, thalassaemia, sickle-cell anaemia etc
Helpful in reasoning why marriage between close relatives is harmful.

How is sex determined in human beings? Ans. Sex determination in human is done by XX-XY type method. In humans, females have XX chromosomes and males have two different type sex chromosomes (XY).

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Male progeny 50% female progeny - 50%

12.A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings. [IMP.] Ans. $I^{A} I^{o} \times I^{B} I^{O}$

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Possible genotype of other offsprings - $I^{A} I^{B}, I^{A} I^{O}$ , $I^{B} I^{O}, I^{O} I^{O}$

Explain the following terms with example. (a) Co-dominance (b) Incomplete dominance

Ans.

(a) Co-dominance: In this phenomenon both the alleles are able to express themselves independently when found together in a heterozygote. They are termed as co- dominant alleles. E.g., - ABO blood group (b) Incomplete dominance: Incomplete dominance may be defined as the partial /expression of both alleles in a heterozygote so that the phenotype is intermediate between those of two homozygotes. E.g., - (i) Flower colours of Mirabilis jalapa (4 o'clock plant) (ii) Snapdragon

What is point mutation? Give one example. Ans. Mutations arising due to change in single base pair of DNA is called point mutation. Eg: sickle cell anemia.
Who had proposed the chromosomal theory of the inheritance? Ans. In 1902 the chromosomal theory of inheritance was proposed by Theodore Boveri and Walter Sutton.
Mention any two autosomal genetic disorders with their symptoms. [IMP.]

Ans. Sickle cell anemia - Symptoms:-

Shape of RBCs change from biconcave to sickleshaped(curved) under the influence of low oxygen tension.
These sickle-shaped RBCs are more rapidly destructed than the normal ones causing anaemia Phenylketonuria:-
This inborn error of metabolism is also inherited as the autosomal recessive trait. The affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine. As a result of this phenylalanine is accumulated and converted into phenylpyruvic acid and other derivatives. Symptoms:-
Accumulation of these in brain results in mental retardation. These are also excreted through urine because of its poor absorption by kidney.

3.0NCERT Solutions for Class 12 Biology | Other Chapter-wise Links

Find chapter-wise NCERT Solutions for Class 12 Biology with step-by-step explanations and answers to textbook questions through the links below.

Chapter Number	Chapterwise NCERT Solutions
Chapter 1	Sexual Reproduction in Flowering Plants
Chapter 2	Human Reproduction
Chapter 3	Reproductive Health
Chapter 5	Molecular Basis of Inheritance
Chapter 6	Evolution
Chapter 7	Human Health and Disease
Chapter 8	Microbes in Human Welfare
Chapter 9	Biotechnology: Principles and Processes
Chapter 10	Biotechnology and Its Applications
Chapter 11	Organisms and Populations
Chapter 12	Ecosystem
Chapter 13	Biodiversity and Conservation

4.0Key Features and Benefits of Class 12 Biology Chapter 4 Principles of Inheritance and Variation

Step-by-Step Genetic Crosses: Understand Mendelian crosses and learn how to determine genotypic and phenotypic ratios with clear explanations.
Better Understanding of Inheritance: Strengthen your knowledge of Mendel’s laws, inheritance patterns, chromosomes, and the transmission of traits from parents to offspring.
NCERT-Based Solutions: Find solutions to the NCERT exercise questions based on the concepts covered in Class 12 Biology Chapter 4.
Effective Revision: Revise important genetic terms, principles, and inheritance patterns to improve understanding and recall.
Strong Foundation for NEET: Build a strong foundation in genetics and inheritance that supports preparation for NEET and other biology-based competitive exams.