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Principles of Inheritance and Variation

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The principle of inheritance and variation in the class 12 biology syllabus is a very important topic with a good weightage in the competitive exams. This chapter explains the basic concepts of inheritance and theories put forward by biologists. In this chapter, students will learn the laws of heredity, incomplete dominance, the chromosomal theory of inheritance, Sex determination, mutation, Mendel’s law of inheritance, and much more. The idea of inheritance pattern was developed after Mendel’s work and his contributions in the field of genetics. This chapter provides an insight into the essence of modern biology - which is genetics.


Let’s have a look at the notes of chapter principles of inheritance and variation in this article.


An elephant gives birth to only an elephant baby, a mango seed only gives rise to a mango tree, human beings give birth to only young human beings. This kind of continuation of species by giving birth to individuals with similar heredity or traits is because of inheritance. Thus, heredity is a process of transmitting heritable traits to young ones by their parents. The process of heredity can easily be described as the process of transferring characteristics or traits from parents to offspring or their unique individuals. This transfer can be conducted through either asexual reproduction or sexual reproduction. The traits that continue the legacy are found in genes on the chromosomes within the body of an individual. Genes are chemicals that carry genetic information about how to code proteins. Genes are the carriers of instructions that could have a similarity to one of their two parents.


Sir Gregor Mendel performed various experiments to study and know more about inheritance, heredity, genetics, and variation. Also known as the Father of Genetics was the first one in his field to conduct experiments on Inheritance, Genetics, Heredity, and Variations.


What is Variation?

The difference in DNA among individuals is called a variation in genetics. Mutation, genetic recombination, crossing over, environmental changes, and other such ways lead to genetic variation. It is the impact of these on the way genes are expressed.


Mendel’s Law of Inheritance

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Mendel performed experiments on Garden pea in the back of his garden to establish the principle of heredity. For this, he took 14 true-breeding pea plants with seven distinguishable characters that had two opposite traits.


Mendel called genes “factors” that pass to the offspring from the parents.


And the factors (genes) that coded for a pair of opposite traits were called “alleles”.


Based on his observation, Mendel gave three laws of inheritance.


1. Law of Dominance

In the case of the formation of the heterozygote, one of the alleles is dominant. And, the dominant allele is expressed in the phenotype. So, when homozygous tall with (TT) alleles are crossed with dwarf (tt) plants, all the resulting offspring plants are tall and have the tall dominant trait with genotype Tt.


2.  Law of Segregation of Genes

At the time of gamete formation during meiosis, each allele separates. So, the characters are passed to gametes without blending. Also, heterozygotes produce different kinds of gametes while homozygous only have one type.


3.  Law of Independent Assortment

Mendel proposed another set of rules which was based on the observations he made on dihybrid crosses which are known as Mendel’s Law of Independent Assortment. This particular law of Mendel states that “when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters”. In simple words, this law explains the independent separation of genes. It means that alleles inherited by progeny from different traits are acquired independently. A simple way to understand this law is with the help of a Punette square when carried out with two pairs of genes during the stage of meiosis and egg-production and pollen in the F1 generation of the RrYy plant.


principles of inheritance and variation, mendel’s law of inheritance, variation, mendelian disorders, mutation



The process of Mutation is a result of alteration in DNA sequences which then results in the changes in the phenotype and genotype of an entity. The mutation also brings variations in DNA along with the recombination. The chromosomal alteration has its negative effects and causes aberrations or abnormalities. These aberrations or abnormalities are usually observed in the cancer cells, according to the studies. Another type of mutation that takes place as a result of changes in a single base pair of DN is called a point mutation. For example insertions and deletions of base pairs of DNA along with sickle cell anaemia leads to frameshift mutations.


Mendelian Disorders 

Mentioned below are some of  Mendelian Disorders:-

  • Colour blindness

  • Cystic fibrosis 

  • Thalassemia

  • Phenylketonuria

  • Sickle cell anaemia 

  • Haemophilia 

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FAQs on Principles of Inheritance and Variation

1. What is incomplete dominance?

It refers to the condition where neither of the two alleles from the parents is dominant. So, the phenotype of the heterozygote does not resemble any parent. In such cases, the heterozygote. For example - When red snapdragons cross-pollinate with the white snapdragons, it results in pink-colored snapdragons. This is because neither red nor white alleles of the flower are dominant.

When both the alleles express themselves together in an individual it is called codominance.

2. What is the chromosomal theory of inheritance?

Sutton and Boveri in support of Mendel’s observations state that the carriers of genes are chromosomes. According to their theory: Homologous pair of chromosomes has the two alleles of a gene located on these homologous pairs of chromosomes at the same site. The homologous pair separates in the process of gamete formation during meiosis. Chromosomes segregate and also, assort independently. During fertilisation, gametes combine, and also the resulting offspring has the diploid no. of chromosomes that are similar to the parent. Morgan’s extensive work on Drosophila melanogaster or fruit flies gave experimental evidence to support the chromosomal theory of inheritance.

3. How is the sex of the progeny determined?

Henking was the first scientist to use X chromosomes and he named them the ‘X’ body. Chromosomes determining the development of sexual characters in a new individual are called sex chromosomes. The rest of the chromosomes are called autosomes. When the male produces two different types of gametes. This is called male heterogamety. It occurs in many animals, like humans, fruit flies, grasshoppers, etc. In some animals, this occurs in females. That is, the females show heterogamety. For example - birds.

4. What is Genetic Mutation?

Changes in the DNA sequence lead to mutation. Viable mutations are inherited from generation to generation. A genetic mutation changes the genotype and phenotype of the organism. Not all mutations are harmful though. Some mutations occur to adapt to the changing environment. Thus, it plays a major role in the evolution and continuation of species. But some mutations are harmful and cause genetic disorders or diseases. It involves deletion, insertion, duplication, substitution, etc. in the DNA sequence. Some mutations lead to cancer. Point mutation where substitution in the single base pair of DNA occurs. Example- Sickle cell anaemia, where the 6th codon of the gene coding for the β-globin chain changes from GAG to GUG. So, glutamine is substituted by Valine. Frameshift Mutation where insertion or deletion of one or more pairs of bases in DNA happens.

5. Is it enough to study from NCERT for class 12 board exams?

The NCERT is the core textbook while studying for your class 12 board exams. NCERT’s book covers all the concepts mentioned in the syllabus provided by CBSE. You can take references from other study material and reference books but NCERT is students’ bible. Reference books other than NCERT can be used for better understanding of concepts, to practice extra numerical questions, etc. it has been observed in previous year’s exams that most of the questions come from NCERT itself. Students are advised to first read and learn the NCERT thoroughly and only then move to the reference books. The reference books are highly recommended for practising sample papers and diagrams.

6. What are some important questions that can be asked from this chapter?

This chapter is highly important from an examination point of view and students are advised to read this chapter thoroughly. Since it is an important question, short answer type questions, as well as long answer questions, are expected from this chapter. 

  • Here are some of the questions that have reoccurred in previous year’s papers:

  • What is point mutation? Give one example

  • Differentiate between homozygous and heterozygous?

  • List out three main laws proposed by Gregor Johann Mendel?

  • Define Principles of Inheritance and variation. 

Students can find more questions to practice in reference questions with a variety of types of questions such as MCQs, short answer type of questions, and long answer type of questions. Questions related to diagrams can also be expected from this chapter.

Competitive Exams after 12th Science