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

Section–A (1 Mark Questions)

1. Give any two reasons for the selection of pea plants by Mendel for his experiments. 

Ans. The two reasons for the selection of pea plants by Mendel for his experiments are:

(i) Many varieties are present with contrasting forms of characters.

(ii) They can be easily cross-pollinated as well as self-pollinated.

2. Name the base change and the amino acid change, responsible for sickle cell anaemia. 

Ans. The change in base GAG as GUG and substitution of Glutamic acid by valine is responsible for sickle cell anaemia.

3. Name any one plant that shows the phenomenon of incomplete dominance during the inheritance of its flower colour.

Ans. Dog flower or Snapdragon (Antirrhinum sp.) is the plant that shows the phenomenon of incomplete dominance during the inheritance of its flower colour.

4. A haemophilic man marries a normal homozygous woman. What is the probability that their daughter will be haemophilic?

Ans. When a haemophilic man marries a normal homozygous woman, is the probability of their daughter being haemophilic is 0%. Haemophilia is a sex-linked recessive disorder that has its been on the X chromosome. If a man is haemophilic and the female is normal homozygous female then none of their sons will be haemophilic because they will receive one of their X chromosomes from the mother and as the mother is normal, the genes will not be transmitted.

5. What is a test cross?

Ans. A test cross is a cross in which offspring with a dominant phenotype is crossed with a homozygous recessive individual to determine the offspring’s genotype for the specific trait.

6. What is meant by aneuploidy?

Ans. Aneuploidy is the phenomenon of the existing or loss of one or more chromosomes due to failure in the separation of members of a homologous pair of chromosomes during meiosis.

7. Name the phenomena that occur when homologous chromosomes do not separate during meiosis. 

Ans. The name of the phenomenon is non – disjunction that occurs when homologous chromosomes do not separate during meiosis.

Section–B (2 Mark Questions)

8. A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?

Ans. Types of gametes produced by organism= 2n, where n= number of loci for which the organism is heterozygous. The given diploid organism is heterozygous for 4 loci, types of gametes produced =  24 = 16.

9. Name the disorder with the following chromosome complement. 

(i) 22 pairs of autosomes + XXY

(ii) 22 pairs of autosomes + 21st chromosome + XY.

Ans. (i) Klinefelter's Syndrome with chromosome complement as 22 pairs of autosomes + XXY.

(ii) Downs syndrome with chromosome complement as 22 pairs of autosomes + 21st chromosome + XY.

10. The following pedigree shows a particular trait which is absent in the parents but found in the subsequent generation irrespective of the sexes. Analyse the pedigree and draw a conclusion.

 Ans. The pedigree shows clearly that trait is absent in the parents but appears in subsequent generations. The trait in question is independent of sexes as it appears in both male and female children. Out of five children only two are showing traits so it is not a dominant trait. As it is already clear that it is not sex linked so this is an autosomal recessive trait.

11. In pea plants, yellow seeds are dominant to green. If a heterozygous yellow seeded plant is crossed with a green seeded plant, what ratio of yellow and green seeded plants would you expect in F1 generation?

Ans. The yellow seeds are dominant to green seeds. The genotype of heterozygous yellow seeded plant will be Yy and the green seeded plant will be yy. After crossing these two plants (Yy x yy), the result of F1 generation will be two Yy (yellow seeds) and two yy (green seeds). The ratio of the offspring will be 50:50.

12. How are the alleles of a gene different from each other? What is its importance?

Ans. Alleles are the alternative forms of the same gene. For an example if we take a gene for height, it comprises two alleles, one for tallness (T) and the other for the dwarfness (t). They differ in their nucleotide sequence due, which results in different phenotypes.

Importance:

• They are essential in studying the inheritance and behaviour patterns.

• They show variations in the population due to contrasting phenotypes of a character.

13. Who gave chromosomal theory of inheritance and what did it prove?

Ans. Walter Sutton and Theodor Boveri developed chromosomal theory of inheritance. According to this theory, genes are the units of heredity and are found in the chromosomes. They united the knowledge of chromosomal segregation with Mendelian principles and argued that the pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors they carried.

14. Mention four reasons why Drosophila was chosen by Morgan for his experiments in genetics.

Ans. The four reasons for which Morgan has chosen Drosophila for his experiments in genetics are as follows:

(i) Drosophila has a very short life cycle i.e. of 2-weeks.

(ii) It can be grown easily in the laboratory. 

(iii) In single mating it produces a large number of flies. 

(iv) Many hereditary variations have been shown by males and females.

(v) It has only 4 pairs of chromosomes that are distinct in size and shape.

PDF Summary - Class 12 Biology Principles of Inheritance and Variation Notes (Chapter 5)

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. 

“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. 

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.

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. 

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). 

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.

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.  

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. 

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).

Mutation 

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. 

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. 

Mutation

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

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