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

NCERT Class 12 Biology Chapter 4 Notes: Download PDF

These notes make it easier to understand how traits are inherited and how variations arise in organisms. The NCERT Class 12 Biology Chapter 4 Notes Principles of Inheritance and Variation, PDF can be downloaded for convenient study anytime, helping students revise efficiently. With clear explanations and examples, these NCERT Notes for Class 12 Biology break down the complex concepts and support quick and effective exam preparation.

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Class 12 Biology Chapter 4 Principles of Inheritance and Variation Notes

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 characteristics 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 behavioral traits of individuals belonging to the same species.

• Variation arises due to

  • Reshuffling of genes/chromosomes.

  • Crossing over or recombination

  • Mutation and the effect of the environment.

Selection of pea plant:

The main reasons for adopting the 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 the corolla.

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

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.

• 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 show stable trait inheritance.

• A hybridization experiment includes emasculation (removal of anthers) and transfer of pollen (pollination).

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 the F₁ generation, and then he observed that all the plants had tall characteristics. A similar observation was also found in other pairs of traits.

Mendel self-pollinated the F₁ plants and found that in the F₂ generation, some plants were also dwarf. The proportion of dwarf plants is 1/4th, and tall plants are 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, a capital letter (TT) for a gene expressed in the F₁ generation and a small letter (tt) for other genes.

• Mendel also proposed that in true-breeding tall and dwarf varieties, an 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 phenotypes.

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

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

Test cross

A test cross 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.

Laws Based on Monohybrid Cross

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 the Law of Segregation, alleles do not blend, and both characters are recovered during gamete formation, as in the F₂ generation.

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.

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, the F₂ generation has 1 red: 2 pink: 1white. The pink flower is due to incomplete dominance.

Co-dominance

• It is the phenomenon of two alleles lacking a dominance-recessive relationship and both expressing themselves in the organism.

• Human beings, ABO blood grouping is controlled by gene I. The gene has three alleles: I^A, I^B, and i. Any person contains any two of the three alleles I^A, I^B, which 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 I^A and I^B are present together, both express their own types of sugars because of co-dominance.

Difference Between Incomplete and Co-Dominance

Multiple Alleles

There 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.

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:

The Law of Independent Assortment 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:

• 9 round yellow

• 3 round green

• 3 wrinkled yellow

• 1 wrinkled green

Chromosomal Theory of Inheritance

• Chromosome, as well as genes, 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 (genes) they carried.

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

Linkage and Recombination

The concept of linkage and recombination is explained below:

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

• The generation of non-parental gene combinations during a Dihybrid cross is called recombination. When genes are located on the same chromosome, they are tightly linked and show very low recombination.

The difference between crossing over and linkage

Sex Determination

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

Later, it was observed that the ovum that receives the sperm with X body becomes female and those that do 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 are seen, but in some insects, like Drosophila, the XO type of sex determination is present.

• In both types of sex determination, males produce two different types of gametes, either with or without an X chromosome, or some with an X chromosome and some with a Y chromosome. Such types of sex determination are called male heterogamety.

• In birds, the 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 is of the 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, males produce two types of gametes (sperms): 50% carry the Y chromosome, and the remaining 50% contain the X chromosome. Females produce only one kind of gamete (ovum) with X chromosomes only.

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

Mutation

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

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 trait, abnormalities, and diseases.

Genetic Disorders

Broadly, human 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.

Mendelian disorders include

1. Haemophilia-

Haemophilia is 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 sons. Whether or not a female will become hemophilic is extremely unlikely because the mother would have to be at least a carrier, while the father would have to be hemophilic (unviable in later years).

2. Sickle cell anemia-

Sickle cell anemia 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-

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

Chromosomal Disorders

• Failure of segregation of chromatids during cell division results in loss or gain of a 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 an additional copy of chromosome 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 the X chromosome (XXY). Such persons have overall masculine development; however, the feminine development (development of breast, i.e., Gynaecomastia) is also expressed, and they are also 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 their ovaries are rudimentary. They lack secondary sexual characters.

Also, Read

• NCERT Notes for Class 12 Physics
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Chapter 4 Principles of Inheritance and Variation: Previous Year Question and Answers

Previous years’ questions from this chapter are listed below. Students can understand important concepts of inheritance, variation, and related genetic principles by studying the Principles of Inheritance and Variation Class 12 Notes.

Question 1. Relative Biological Effectiveness (RBE) usually refers to damage caused by

Option 1. Low temperature

Option 2. High temperature

Option 3. Encephalitis

Option 4. Radiation

Answer :

Relative Biological Effectiveness (RBE) is a measure used in radiation biology to compare the biological damage caused by different types of radiation. It determines the effectiveness of a specific radiation type in causing harm compared to a reference radiation. RBE values vary for different radiation sources, with high-energy particles having higher RBE than low-energy photons. Thus, RBE is used to assess the damage caused by radiation.

Hence, the correct answer is option (4), Radiation

Question 2. In recent years, DNA sequences (nucleotide sequences) of mtDNA (Mitochondrial DNA) and Y-chromosomes were considered for the study of human evolution, because

Option 1. Their structure is known in greater detail

Option 2. They can be studied from the samples of fossil remains

Option 3. They are small and therefore, easy to study (they are uniparental in origin and do not take part in recombination

Option 4. They are uniparental in origin and do not take part in recombination

Answer :

Wilson and Sarich chose mitochondrial DNA (mtDNA) for the study of maternal line inheritance. At the same time, Y-chromosomes were considered for the study of human evolution, particularly in the male domain. It is possible because they are uniparental in origin and do not take part in recombination.

Hence, the correct answer is option (4). They are uniparental in origin and do not take part in recombination

Question 3. Inversions are considered as crossover suppressors because

Option 1. Homozygous inversions are lethal and thus do not appear in the next generation.

Option 2. Inversion heterozygotes, i.e., one copy having the normal chromosome and its homolog having an inversion, do not allow crossing over to occur as they cannot pair at all.

Option 3. Due to inversion present, four chromosomes take part in the pairing and crossing over events, making the structure difficult to separate and leading to gamete formation.

Option 4. The pairing and crossing over do occur in inversion heterozygotes, but the gametes that have crossover products are lethal.

Answer :

In inversion heterozygotes, crossing over can still occur, but it produces abnormal recombinant gametes with duplications or deletions. These gametes are usually non-viable, so effectively, inversions suppress the production of viable crossover products, preventing them from contributing to the next generation.

Hence, the correct answer is option (4). The pairing and crossing over do occur in inversion heterozygotes, but the gametes that have crossover products are lethal.

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