Principles of Inheritance and Variation Class 12 Notes CBSE Biology Chapter 5 [Free PDF Download] (2022)

Important Topics Covered in Chapter 5

To excel in any exam, you should know what is asked in the exam. Here is the list of important topics that are covered in Chapter 5 of CBSE class 12 Biology - Principles of Inheritance and Variation:

  • Inheritance of One Gene - Monohybrid Cross

  • Inheritance of Two Genes - Dihybrid Cross

  • Incomplete and Co-dominance

  • Multiple Alleles

  • Chromosomal Theory of Inheritance

  • Linkage and Recombination

  • Sex Determination

  • Pedigree Chart and Analysis

  • Genetic Disorders in Humans - Mendelian Disorders and Chromosomal Disorders

  • Law of Inheritance

What is Heredity?

Heredity is the condition when the genes are transferred from one generation to the other through sexual reproduction. Various genes and many inheritable characters are passed on so that the offspring produced are better adapted to the new changing conditions. The characteristics are present in the form of genes on the chromosomes.

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“The father of genetics” is Gregor Johann Mendel. He proposed 3 main laws of inheritance which are known as Mendel’s laws.

Mendel’s Experiment

Gregor Mendel, after performing his experiments on pea plants, discovered the fundamental laws of inheritance. He proposed three laws of inheritance which we are studying to date. He has chosen pea plants having seven opposite traits of particular characters and conducted his experiment on 14 true-breeding pea plant varieties.

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Seven contrasting characters selected by Mendel

Mendel’s Laws

There were 3 laws that were proposed by Mendel

1. Law of Dominance: It is explained in this law that all of the traits, or the characters are controlled by the unit called the factors. These factors are found to be in pairs and are called alleles. If they occur in the same pair they are called homozygous, they can be either dominant or recessive and if the alleles occur in a different pair then it is called heterozygous, It will always be dominant.

“For example Allele for tallness is dominant over the allele for dwarfism”.

2. Law of Segregation of Genes: Law of segregation is based on the fact that alleles do not show any blending and that both the characters are recovered as such in the second filial generation though one of these is not seen in the first generation. The segregation of factors or a pair of alleles occurs in such a manner that the gamete receives only one of the two factors from each other.

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Examples of the law of segregation of alleles. In this R is dominant over r.

3. Law of Independent Assortment: It states that pairs of traits in the parental generation sort independently from one another when passing from one generation to the next. It is explained with the help of a dihybrid cross.

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Inheritance of One Gene

The inheritance of one gene using Mendel’s law can be explained with the help of a monohybrid cross. In this experiment, the cross between tall and dwarf plants is done which will result in all tall hybrid plants in the F1 (First Filial or First) generation. Then these progenies are self-pollinated and will result in the production of F2 generation where three tall plants and one dwarf plant will be formed. Thus, the ratio will be 3:1.

Incomplete Dominance

Incomplete dominance is a type of inheritance in which one allele for a specific trait is not completely dominant over the other allele i.e. neither allele is dominant over the other in heterozygous organisms. This results in a combined phenotype. Incomplete dominance is also called mosaic or partial dominance. Here new phenotypic characters are expressed entirely.

Mirabilis jalapa, the marvel of Peru commonly called a 4 o'clock plant. It is a very good example of incomplete dominance because of its Inheritance of flower color. It is shown in the figure given below where red flowers (dominant) were crossed with white flowers (recessive), the F1 generation contains flowers that are pink in color (intermediate).

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The phenotypic and the genotypic ratio observed will be the same, that is 1:2:1.

Multiple Allelism or Codominance

The condition in which three or more alternative forms of alleles present for a single gene on the same chromosome is known as Multiple Allelism and the alleles are known as multiple alleles.

For example, Multiple allelism is better understand with the help of the ABO blood group system in humans. The inheritance of the ABO blood group is a gene I (in which I represent isohemagglutinin) that remains in the 3 allelic expressions: IA, IB, and i which are codominant in humans. An individual can possess any two of these alleles. Gene IA is responsible for blood group A and codes for glycoprotein A while gene IB is responsible for blood group B and codes for glycoprotein B.

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The gene ‘i’ does not produce any glycoprotein and so the person who will be having these two alleles together in a homozygous condition will have O group blood. The genes IA and IB are dominant over ‘i’ but alleles IA and IB are dominant equally and produce both the glycoproteins A and B simultaneously and results in the blood group AB. Such alleles are known as co-dominant alleles.

Inheritance of Two Genes

The inheritance of two genes requires two characters of the same trait. This can be observed with the help of a dihybrid cross. Mendel has chosen two traits that involve the color and the shape of the seed to explain the inheritance of two genes. Y represents the dominant yellow color seed color, y represents a recessive green color while R represents the round shape of the seed, and r represents the wrinkled shape of the seed. The genotype of the parents can then be written as RRYY and rryy. The gametes RY and ry will unite after fertilization and will produce the F1 hybrid RrYy. The dihybrid cross is also useful in the study of the Law of Independent Assortment. After the self-pollination of the F1 hybrid, the F2 ratio was found to be 9:3:3:1.

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Chromosomal Theory of Inheritance

Chromosomal theory of inheritance was given by Walter Sutton in 1902. This theory also explains the linear structure of chromosomes with genes in particular sites which was mentioned as loci while Boveri also studied this theory separately. So, this theory is also known as the Boveri-Sutton chromosome theory. According to this theory-

(i) Genes are found at specific locations on the chromosomes.

(ii) During meiosis the homologous chromosomes separate.

(iii) After fertilization the number of chromosomes becomes diploid.

(iv) Chromosomes segregate as well as assort independently.

Linkage and Recombination

Different experiments were conducted by T.H. Morgan to understand the process of linkage and recombination. He performed several dihybrid crosses in Drosophila (fruit fly) to study genes that were sex-linked. For example, Morgan crossed the yellow-bodied, White-eyed females to brown-bodied, red-eyed males and then self-breed their F1 progeny that results in a slight variation from the 9:3:3:1 ratio in the F2 generation because the two genes did not segregate independently of each other. Thus, it signifies that the genes are linked. The genes show linkage physically and this condition is called linkage.

Recombination is the condition where the genetic material is rearranged. For example, crossing over is known as recombination. In this, the recombinants are formed which results in variation. The recombinants are the progenies that are the combination of both the parents.

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Sex Determination

Sex determination is the process where the gender of the child can be revealed. Sex chromosomes are responsible for the determination of the sex of a child. In the case of humans, females have XX types of chromosomes while males have one X and one Y type of chromosomes. Thus, when the egg (female gamete) formed will be having identical X-chromosome each but male sperms (male gametes) are not identical as they have one X-chromosome and one Y-chromosome. So it's a matter of chance that which sperm fuses with the egg (X or Y). Thus, the females are said to be homogametic (same type of chromosomes) while males are said to be heterogametic (different types of chromosomes).

In the case of insects, the mechanism of sex determination is of XO type. Here the eggs consist of the X chromosomes while the sperms may have one or none X chromosomes. Thus, the males are said to be homogametic (same type of chromosomes) while females are said to be heterogametic (different types of chromosomes).

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The sudden changes in the sequence of DNA are known as mutations. The changes in the DNA may be heritable and will be passed onto the next generations affecting both the genotype, as well as the phenotype of an individual. The different types of mutation are frameshift mutations, insertions, deletions, duplications, substitutions, etc. Mutations may be harmful or may not affect at all.

(i) The addition or deletion of DNA bases will lead to changes in the reading frame called the Frameshift mutations.

(ii) The addition of DNA bases is known as insertions.

(iii) Removal of DNA bases is known as deletions.

(iv) If a piece of DNA is copied for more than one time is known as duplication.

Thus, these mutations will lead to a change in the DNA sequence resulting in the formation of the wrong protein.

Genetic Disorders

Pedigree Analysis

The occurrence and appearance of certain phenotypes of a particular gene and organism are represented by a Pedigree chart. It results in the presentation of the family information with an easily readable chart.

Mendelian Disorders

Genetic disorders are divided into two categories- Mendelian disorders and chromosomal disorders. Mendelian disorders are those that are caused due to mutation or alteration in a single gene. The most common examples of these diseases are Hemophilia, Sickle-cell anemia, Cystic fibrosis, Color blindness, Thalassemia, Phenylketonuria, etc. The Mendelian disorders are found to be either dominant or recessive. The trait is also found to be linked to sex chromosomes called sex-linked diseases, such as hemophilia and color blindness.

Chromosomal Disorders

Chromosomal disorders are those that are caused due to addition or loss or abnormal arrangement of one or more chromosomes. The addition or subtraction of chromosomes depends upon the improper segregation of chromatids during the process of the cell division cycle, this results in the condition called aneuploidy. For example, In Down’s syndrome results there is an addition of an extra copy of chromosome 21. Similarly, in the case of Turner’s syndrome, there is a loss of an X chromosome in human females. The other condition is polyploidy where the process of cytokinesis is absent after the telophase stage of cell division will lead to an increase in a whole set of chromosomes in an organism which is often seen mostly in the case of plants.

CBSE Class 12 Biology Notes Chapter 5- Principles of Inheritance and Variation

Laws of Inheritance

Inheritance refers to a phenomenon through which traits from parent to offspring are transferred. This concept forms the base of the heredity phenomena. Some of the inheritance laws that students will study through revision notes Class 12 Biology Chapter 5 are:

First Law- Law of Dominance

It comprises the factors that control characters and refer to the discrete units. These factors occur in pairs, and in dissimilar cases, one member pair is dominant and the other is recessive.

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Second Law- Law of Segregation

Both traits are improved in the F2 generation and alleles do not show any blending. Though parents possess two alleles, however during gamete formation, they get divided from each other. A gamete attains only one of the two factors in this formation.

Laws of Independent Assortment

Based on the observations on dihybrid crosses, Mendel projected general rule, which became popular as Mendel’s Law of Independent Assortment. The law states that when two traits pairs combine in a hybrid, the separation of one pair of traits becomes independent of other traits pairs. With the help of a Punnett square carried out with two genes pairs, students can understand the concept better.

Chromosome Theory of Inheritance

Gene as well as chromosome both occur in pairs. Two alleles of any gene pair are present on the identical point on homologous chromosomes.

Linkage and Recombination

Linkage is responsible for keeping the genes together. It includes individual chromosomes and lessens variability. However, during the Dihybrid cross, the production of non-parental gene combinations refers to recombination. When genes are present on the identical chromosome, they link with each other tightly and show low recombination.


When it comes to genetic inheritance, mutation serves as one of the vital topics. A slight change or modification in the genetic material of an individual resulting in genetic diversity refers to the phenomena of mutation. The process can either affect a small portion of the gene sequence or may affect the large one.

Chromosome Disorders

During the cell division, the failure of segregation of chromatids results in gain or loss of chromosomes. This phenomenon refers to aneuploidy. Any alteration in the number or even chromosome structure fallouts in chromosome disorders. Some babies are born with either more or a smaller number of chromosomes that are required. There are various diseases caused by this phenomenon, such as down’s syndrome, turner’s syndrome and more.

Importance of Class 12 Chapter 5 Biology Notes

Students can get familiar with topics like heredity, chromosome theory of inheritance and its characteristics, genetic disorders and more by going through notes. Not only from the perspective of board exams, but students preparing for AIIMS, NEET or other competitive exams must refer to NCERT Class 12 revision notes Biology.

After studying the contents of Biology revision notes, students can design their study material in a well-planned way. They get new ideas to schedule and prepare for their final exams through Class 12 revision notes Chapter 5. Moreover, these notes have followed the latest pattern and accomplish the requirements of students.

Key Points of Revision Notes of Chapter 5 of CBSE Class 12 Biology

Revision notes are very important study materials for any student. Here are a few benefits of downloading the free PDF of revision notes of CBSE Class 12 Biology:

  • As the revision notes are short and precise, they enhance the memory of the students. The students are able to store the information for a longer period of time.

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  • The revision notes improve the attention span of the students.

  • They help in quickly revising before exams.

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