Principles of Inheritance and Variation

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Principles of Inheritance and Variation Class 12 Notes

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Principle of inheritance and variation in 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 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.

Sir Gregor Mendel performed various experiments to study and know more about inheritance, heredity, genetics and variation.

What is the 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 as “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 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.

  1. 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 one type.

  1. Law of Independent Assortment

This law explains the independent separation of genes. It means that alleles inherited by progeny from different traits are acquired independently.

FAQ (Frequently Asked Questions)

1. What is Incomplete Dominance?

Ans: 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 coloured 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?

Ans: 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 pair 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 fertilization, gametes combine and also the resulting offspring has the diploid no. of chromosomes, that is 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?

Ans: Henking was the first scientist to X chromosomes and he names 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 examples - birds.

4. What is Genetic Mutation?

Ans: 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 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.

Genetic Mutation is of Two Types:

  • 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.