NCERT Class 12 Biology Chapter 5 Notes Principles of Inheritance and Variation

Class 12 Principles of Inheritance and Variation class 12 Notes Biology are available in PDF format for free download at Careers360. CBSE Guide Principles of Inheritance and Variation class 12 Notes Biology chapter wise notes are available for quick revision and preparation for Class 12th Board exam. Class 12 NCERT Notes for Principles of Inheritance and Variation are the detailed notes which covers the 2023-24 syllabus of CBSE and NCERT. It includes all the topics given in NCERT class 12 Biology textbook. Through the NCERT notes for CBSE Class 12 Principles of Inheritance and Variation biology notes, you will be able to solve all the doubts and questions of this chapter.

Chapter-wise Biology Notes For Class 12

NCERT Class 12 Biology notes are provided for 16 chapters. Along with NCERT notes, students can use NCERT solutions to exercise problems and CBSE previous year papers for board exam preparation.

Following are the links to the Biology class 12 notes PDF download for all Chapters:

• Chapter 1 - Sexual Reproduction in Flowering Plants
• Chapter 2 - Human Reproduction
• Chapter 3 - Reproductive Health
• Chapter 4 - Principles of Inheritance and Variation
• 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

Principles of Inheritance and Variation Class 12 Notes CBSE Biology

Use the extensive Chapter 5 Revision Notes on Principles of Inheritance and Variation to effectively prepare for your CBSE Class 12 Biology final exams. Careers360 provides a concise summary of the chapter, along with diagrams, table, concepts, and examples for better understanding. alll in-text and exercise questions are solved in the NCERT Solutions.

Law of Inheritance or Mendel’s Laws of Inheritance

Who is the Father of Genetics? The answer is Gregor Johann Mendel.

• Genetics is the study of principles and mechanisms of heredity and variation.
• Inheritance is the process by which characters are passed on from parent to progeny. It is the basis of heredity.

• Variation is the degree to which progeny differ from their parents. Variation may be in terms of morphology, physiology, cytology, and behavioristic traits of individuals belonging to the same species.

• Variation arises due to

  • Reshuffling of gene/chromosomes.

  • Crossing over or recombination

  • Mutation and effect of environment.

In class 12 Biology chapter 5 notes, you will study: Mendel’s Law of Inheritance: Mendel conducted a seven-year experiment on pea hybrids (Pisum sativum) and used the results to develop his theory of inheritance.

Principles of Inheritance and Variation Class 12th Notes:
Selection of pea plant: The main reasons for adopting garden pea (Pisum sativum) for experiments by Mendel were –

• Pea has many distinct and contrasting characters.

• The life span of pea plants is short.

• Pea plant flowers show self-pollination and reproductive whorls being enclosed by corolla.

• It is easy to artificially cross-pollinate the pea flowers and they were fertile too.

NCERT Class 12 Biology Chapter 5 Notes: How is Mendel’s method working?

Mendel’s success was also due to his meticulous planning and method of work –

• Mendel studied only one character at a time for a deep evaluation.

• He used all available techniques to avoid cross-pollination by undesirable pollen grains.

• He applied mathematics and statistics as well to analyze the results obtained by him and this is mentioned in Principles of Inheritance and Variation class 12 notes.

• Mendel selected 7 contrasting characters of a garden pea for his hybridization experiments and studied their contrasting characters.

• Mendel conducted artificial hybridization/cross-pollination using true-breeding pea lines. True breeding lines are those that undergo continuous self-pollination and shows stable trait inheritance.

• Hybridization experiment includes emasculation (removal of anther) and transfer of pollen (pollination).

Principles of Inheritance and Variation Class 12th Notes: Inheritance of one gene (Monohybrid cross)

Mendel crossed tall and dwarf pea plants and collected all the seeds obtained from this cross, then he grew all the seeds to generate plants of the first hybrid generation which will be called F1 generation and then he observed that all the plants are having tall characteristics. A similar observation was also found in other pairs of traits.

Mendel self-pollinated the F1 plants and found that in F2 generation some plants are also dwarf. The proportion of dwarf plants is 1/4th and tall plants of 3/4th.

• Mendel called these factors 'factors' that are passed from one generation to the next. Nowadays it is called a gene (unit of inheritance).

• Genes that code for a pair of contrasting traits and then they will be called alleles.

• He used alphabetical symbols to represent each gene, for example, capital letter (TT) for gene expressed in the F1 generation and small letter (tt) for other genes.

• Mendel also proposed that in true-breeding tall and dwarf variety allelic pair of genes for height is homozygous (TT or tt). TT, Tt or tt are called genotypes and characteristics like tall and dwarf are called the phenotype.

• The hybrids that contain alleles that express contrasting traits are called heterozygous (Tt).

• The monohybrid ratio of F2 hybrid is 3:1(phenotypic) and 1:2:1(genotypic).

Class 12th Biology chapter 5 notes: Test cross

It is the cross between an individual with a dominant trait and a recessive organism in order to know whether the dominant trait is homozygous or heterozygous.

CBSE Class 12 Biology ch 4 Notes: INHERITANCE OF ONE GENE

This is based on observations of a monohybrid cross, Mendel proposed two laws of inheritance-

1. Law of dominance: This law states;

1. Characters are controlled by discrete units called factors.

2. Factors always occur in pairs.

3. In a dissimilar pair of factors, one member of the pair dominates the other.

2. Law of Segregation:

According to this law, alleles do not blends and both the characters are recovered during gametes formation as in F2 generation and it is mentioned in Principles of Inheritance and Variation Class 12th Notes.

NCERT class 12 Biology chapter 5 notes you will also study that in gamete formation traits segregate (separate) from each other and produce different kinds of gametes. Homozygous gametes produce similar kinds of gametes, whereas heterozygous gametes produce different kinds of gametes with different traits.

3. Incomplete dominance

• It is a post-Mendelian discovery. The phenomenon of incomplete dominance occurs when neither of the two alleles is dominant, so the hybrid expression is a fine mixture or intermediate expression between the two alleles.

• In snapdragon (Mirabilis jalapa), there are two types of pure breeding plants, red-flowered and white-flowered. On crossing the two, F1 plants possess pink flowers. On selfing them, F2 generation has 1red: 2 pink: 1white. The pink flower is due to incomplete dominance.

4. Co-dominance

• Notes for class 12 Biology chapter 5 will make you aware that It is the phenomenon of two alleles lacking a dominance-recessive relationships and both expressing themselves in the organism.

• Human beings, ABO blood grouping are controlled by gene I. The gene has three alleles IA, IB and i. Any person contains any two of three allele IA, IB are dominant over i.

• The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene.

• When IA and IB are present together, both express their own types of sugars because of co-dominance. NCERT Class 12 Biology Chapter 5 Notes

Multiple Alleles

They are multiple forms of a mendelian factor or gene that occur on the same gene locus distributed in different organisms in the gene pool with an organism carrying only two alleles and a gamete only one allele. ABO blood grouping also provides a good example of multiple alleles.

Principles of Inheritance and Variation Notes Class 12: INHERITANCE OF TWO GENES (Dihybrid Cross)

When a cross was made to study the simultaneous inheritance of two pairs of mendelian factors of genes.

Law of Independent Assortment:

In NCERT notes for class 12 Biology chapter 5 you will sudy that the law 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 the Dihybrid cross two new combinations, round green & wrinkled yellow are formed due to an independent assortment of traits for seed shape i.e round, wrinkled, and seed color i.e, yellow and green.

The ratio of 9:3:3:1 can be derived as a combination series of:

3 yellow : 1 green, with 3 round : 1 wrinkled.

In class 12 Biology chapter 5 notes pdf you will read that this derivation can be written as follows:

(3 Round : 1 Wrinkled) (3 Yellow : 1 Green) = 9 Round, Yellow : 3 Wrinkled, Yellow: 3 Round, Green : 1 Wrinkled, Green

NCERT Class 12 Biology Chapter 5 Notes:
Chromosomal Theory of Inheritance

• Chromosome, as well as gene both, occurs in pairs. The two alleles of a gene pair are located on the same locus on homologous chromosomes.

• Sutton and Boveri argued that the pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors (gene) they carried.

• Sutton united the knowledge of chromosomal segregation with mendelian principles and called it the chromosomal theory of inheritance.

Linkage and Recombination

• When two genes in a Dihybrid cross were situated on the same chromosome, the proportion of parental gene combination was much higher than the non-parental type. Morgan attributed this due to the physical association or the linkage of the two genes and coined the linkage to describe the physical association of genes on the same chromosome.

• In class 12 Principles of Inheritance and Variation notes you will read that the generation of non-parental gene combinations during Dihybrid cross is called recombination. When genes are located on the same chromosome, they are tightly linked and show very low recombination.

Difference between crossing over and linkage

CBSE Class 12 Biology Revision Notes Chapter 5 Principles of Inheritance and Variation : Sex Determination

Henking in 1891 observed a trace of specific nuclear structure in few insects. Furthermore, he observed that this specific nuclear structure is only found in 50% of sperms. He called this as ‘x’ body. He was not able to explain its significance.

NCERT Class 12 Biology Chapter 5 Notes: Later it was observed that the ovum that receives the sperms with x body becomes female and those not become males, so this x body was called a sex chromosome and other chromosomes are called autosomes.

• In humans and other organisms XY types of sex determination is seen but in some insects like Drosophila XO type of sex determination is present.

• In both types of sex determination, male produce two different types of gametes either with or without X chromosome or some with X chromosome and some with Y chromosomes. Such types of sex determination are called male heterogamety. Principles of Inheritance and Variation class 12 notes pdf download.

• In birds ZW type of sex determination is present., two different types of gametes are produced by females in terms of sex chromosomes; this type of sex determination is called female heterogamety.

• Sex determination in human beings XY type. Out of 23 pairs of chromosomes, 22 pairs are exactly the same in males and females called autosomes. A pair of X chromosomes is present in females and XY in males. During spermatogenesis, male produce two types of gametes (sperms), 50% carries Y chromosome and remaining 50% contain X chromosome. Females, produce only one kind of gamete (ovum) having X chromosomes only.

• When sperm having a Y chromosome the sex of baby is male and when sperm carrying the X chromosome fertilise the egg, the sex of the baby is female.

A Mutation is a phenomenon that results in an alternation of DNA sequence and consequently results in the change in the genotype and phenotype of an organism. The mutations that arise due to due to change in single base pair of DNA are called point mutation e.g Sickle cell anemia.

Principles of Inheritance and Variation Biology Class 12 Notes: Genetic Disorders

Pedigree Analysis

• The analysis of traits in several generations of a family is called the pedigree analysis. The inheritance of a particular trait is represented in the family tree over several generations. It is used to trace the inheritance of a particular traits, abnormalities and diseases.

Broadly, genetic disorders may be grouped into two categories –

1. Mendelian disorders

2. Chromosomal disorders.

These disorders are transmitted as the affected individual is sterile. This is always dominant in nature.

NCERT Class 12 Biology Chapter 5 Notes: Mendelian disorder includes

1. Haemophilia-

A sex-linked recessive disease in which, in an infected individual, a minor cut leads to non-stop bleeding. Heterozygous females (carriers) can transmit the disease to their son. Whether or not a female will become hemophilic is extremely unlikely because the mother would have to be at least carrier, while the father would have to be hemophilic (unviable in later years).

2. Sickle cell anemia-

It is an autosomal recessive trait in which mutant hemoglobin molecules undergo polymerization under low oxygen tension, changing the shape of the red blood cell from a biconvex disc to an elongated sickle-like structure. It results from the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta-globin chain of the hemoglobin molecule. The single base substitution of GAG to GUG occurs at the sixth codon of the beta-globin gene.

3. Phenylketonuria-

It is a inborn error of metabolism inherited as an autosomal recessive trait. The affected individual lacks an enzyme that converts the amino acids phenylalanine to tyrosine. As a result of this phenylalanine is accumulated and converted into phenyl pyruvic acid and other derivatives that result in mental retardation.

CBSE Class 12 Principles of Inheritance and Variation Notes Biology : Chromosomal Disorders

Failure of segregation of chromatids during cell division results in loss or gain of chromosome called aneuploidy.

The failure of cytokinesis leads to two sets of chromosomes called polyploidy.

1. Down’s Syndrome–

This syndrome occurs due to the presence of additional copy of chromosome number 21. The affected individual is short-statured with a small rounded head, furrowed tongue, and partially opened mouth. Mental development is retarded.

2. Klinefelter’s Syndrome–

This syndrome occurs due to the presence of an additional copy of X-chromosome (XXY). Such persons have overall masculine development however, the feminine development (development of breast, i.e., Gynaecomastia) is also expressed, and also they are sterile.

3. Turner’s Syndrome-

This syndrome occurs due to the absence of one of the X chromosomes. So an individual will have 45 with XO, such females are sterile as ovaries are rudimentary. They lack secondary sexual characters.

You can also read this Principles of Inheritance and Variation class 12 notes pdf and download them for free.

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