CBSE Class 12 Biology Revision Notes Chapter 4 Principles of Inheritance and Variation

Inheritance explains how biological characters pass from parents to offspring, while variation explains the differences observed among individuals.

The principles of genetics connect Mendel’s pea-plant experiments with chromosome behaviour, inheritance patterns, sex determination, mutations and genetic disorders. Understanding these connections helps students predict the possible genotypes and phenotypes produced in a cross.

These CBSE Class 12 Biology Revision Notes Chapter 4 cover Mendelian inheritance, deviations from complete dominance, chromosomal theory, linkage, recombination, polygenic traits and human genetic disorders.

Key Takeaways

• Monohybrid inheritance: Complete dominance produces a 3:1 F₂ phenotypic ratio and a 1:2:1 genotypic ratio.
• Dihybrid inheritance: Independent assortment produces a 9:3:3:1 F₂ phenotypic ratio.
• Linkage: Genes located on the same chromosome may be inherited together.
• Genetic disorders: Mutations may affect single genes or the number and structure of chromosomes.

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Access Class 12 Biology Chapter 4 Principles of Inheritance and Variation Notes in 30 Minutes

These Principles of Inheritance and Variation Notes organise genetics from Mendel’s crosses to human disorders.

This sequence makes the Class 12 Biology Chapter 4 Notes useful for conceptual, numerical and pedigree-based questions.

Genetics, Inheritance and Variation

Genetics is the branch of biology that studies inheritance and variation.

Inheritance is the transmission of characters from parents to offspring.

Variation refers to the differences between offspring and their parents or among members of the same species.

Inheritance maintains continuity between generations, while variation produces diversity within a population.

Essential Genetics Terms

Mendel’s Experiments on Pea Plants

Gregor Mendel conducted hybridisation experiments on garden peas from 1856 to 1863.

He used large sample sizes, analysed offspring statistically and confirmed his findings across successive generations.

His work established the basic Mendel’s laws of inheritance.

Why Mendel Selected Garden Pea

Mendel selected Pisum sativum because:

• It had several easily distinguishable contrasting traits.
• Its life cycle was relatively short.
• It normally self-pollinated.
• Cross-pollination could be performed manually.
• True-breeding varieties were available.
• It produced many seeds.
• The offspring could be studied across several generations.

Seven Contrasting Traits Studied by Mendel

Inheritance of One Gene

Inheritance involving one pair of contrasting traits is studied through a monohybrid cross.

Monohybrid Cross

Mendel crossed a true-breeding tall plant with a true-breeding dwarf plant.

Parental generation:

TT × tt

Gametes:

T and t

F₁ generation:

All Tt and tall

The dwarf trait was not expressed in F₁, but its allele remained present.

When the F₁ plants were self-pollinated:

Tt × Tt

The F₂ generation contained:

• TT
• Tt
• Tt
• tt

Genotypic and Phenotypic Ratios

The F₂ genotypic ratio is:

1 TT : 2 Tt : 1 tt

The F₂ phenotypic ratio is:

3 tall : 1 dwarf

The recessive trait reappears unchanged in the F₂ generation, showing that inheritance does not involve permanent blending.

Punnett Square

A Punnett square represents all possible combinations of parental gametes.

For Tt × Tt:

The square predicts the probability of each genotype rather than guaranteeing the exact result in a small number of offspring.

Test Cross

A test cross is performed to identify the genotype of an organism showing a dominant phenotype.

The organism is crossed with a homozygous recessive parent.

When the Unknown Parent Is Homozygous Dominant

TT × tt

All offspring are Tt and show the dominant phenotype.

When the Unknown Parent Is Heterozygous

Tt × tt

The offspring appear in a:

1 dominant : 1 recessive ratio

Therefore:

• All dominant offspring indicate a likely homozygous dominant parent.
• A 1:1 ratio indicates a heterozygous parent.

Law of Dominance

The law of dominance states:

• Characters are controlled by discrete factors.
• Factors occur in pairs.
• In a dissimilar pair, one factor may dominate the other.

The dominant trait appears in the F₁ generation, while the recessive trait is masked.

This law explains:

• Uniformity of the F₁ generation
• Expression of one parental trait in F₁
• The 3:1 phenotypic ratio under complete dominance

Law of Segregation

The law of segregation states that two alleles of a gene separate during gamete formation.

Therefore:

• A gamete receives only one allele from each pair.
• A homozygous individual produces one type of gamete.
• A heterozygous individual produces two types of gametes in equal proportions.

The alleles reunite randomly during fertilisation.

This law is also called the law of purity of gametes because alleles do not blend.

Deviations from Complete Dominance

Mendel’s pea traits showed complete dominance, but all genes do not follow the same expression pattern.

Incomplete Dominance

In incomplete dominance, neither allele completely dominates the other.

The heterozygote shows an intermediate phenotype.

Snapdragon Flower Colour

Red-flowered plant:

RR

White-flowered plant:

rr

F₁:

Rr, pink

On self-pollination:

Rr × Rr

F₂ ratio:

1 red : 2 pink : 1 white

In this case:

• Genotypic ratio = 1:2:1
• Phenotypic ratio = 1:2:1

The pink flower is not caused by allele blending because red and white phenotypes reappear in F₂.

Codominance

In codominance, both alleles express themselves fully in a heterozygote.

The heterozygote does not show an intermediate phenotype. It displays the products of both alleles.

The AB blood group is a standard example.

Multiple Alleles and ABO Blood Group Inheritance

When more than two alternative forms of a gene exist in a population, they are called multiple alleles.

The human ABO blood group is controlled by gene I, which has three alleles:

• Iᴬ
• Iᴮ
• i

Iᴬ and Iᴮ are codominant.

Both Iᴬ and Iᴮ are dominant over i.

ABO Blood Group Inheritance

The ABO system demonstrates:

• Codominance between Iᴬ and Iᴮ
• Complete dominance of Iᴬ and Iᴮ over i
• Multiple alleles at the population level

Pleiotropy

Pleiotropy occurs when one gene influences more than one phenotypic character.

The effect of a gene may extend across different tissues or biochemical pathways.

For example, a gene controlling starch synthesis in peas can affect:

• Starch-grain size
• Seed shape

Thus, one gene can produce multiple observable effects.

Inheritance of Two Genes

A cross involving two pairs of contrasting traits is called a dihybrid cross.

Mendel crossed pea plants differing in seed shape and seed colour.

Parental generation:

RRYY × rryy

Traits:

• R = round
• r = wrinkled
• Y = yellow
• y = green

The F₁ genotype was:

RrYy

All F₁ seeds were round and yellow.

Gametes Produced by F₁

The F₁ hybrid produces four types of gametes:

• RY
• Ry
• rY
• ry

Each type is expected in equal proportion when the two gene pairs assort independently.

F₂ Phenotypic Ratio

Self-crossing RrYy produces the F₂ ratio:

9 round yellow
3 round green
3 wrinkled yellow
1 wrinkled green

Therefore, the phenotypic ratio is:

9:3:3:1

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 is independent of the other pair.

The separation of R and r does not determine the separation of Y and y.

Independent assortment creates new combinations, such as:

• Round green
• Wrinkled yellow

The law applies most clearly when genes are:

• On different chromosomes
• Far apart on the same chromosome

Closely linked genes may not assort independently.

Chromosomal Theory of Inheritance

The chromosomal theory of inheritance was proposed by Walter Sutton and Theodore Boveri.

They observed that chromosome behaviour during meiosis closely parallels Mendelian factors.

Genes are located at specific positions called loci on chromosomes.

The two alleles of a gene occupy corresponding loci on homologous chromosomes.

Linkage and Recombination

Thomas Hunt Morgan tested the chromosomal theory through experiments on Drosophila melanogaster.

Fruit flies were useful because they:

• Had a short life cycle
• Produced many offspring
• Were easy to maintain
• Showed visible hereditary variations
• Had clearly distinguishable males and females

Linkage

Linkage is the physical association of genes located on the same chromosome.

Linked genes tend to be inherited together.

When two genes are strongly linked:

• Parental combinations are more frequent.
• Recombinant combinations are less frequent.

Linkage explains why some crosses do not produce the expected 9:3:3:1 ratio.

Recombination

Recombination is the production of non-parental gene combinations.

It usually occurs because of crossing over between homologous chromosomes during meiosis.

Morgan found that different linked genes showed different recombination frequencies.

For example:

• Tightly linked genes show low recombination.
• Genes farther apart show higher recombination.

Recombination Frequency and Gene Mapping

Alfred Sturtevant used recombination frequency to estimate relative distances between genes on a chromosome.

A higher recombination frequency generally indicates that the genes are farther apart.

A lower frequency indicates that they are more closely linked.

This principle became the basis of genetic mapping.

Polygenic Inheritance

Polygenic inheritance occurs when one character is controlled by several genes.

Each contributing allele has an additive effect.

Unlike Mendelian traits with distinct alternatives, polygenic traits show continuous variation.

Examples include:

• Human skin colour
• Human height

Human Skin Colour

Suppose three gene pairs influence skin colour:

A/a, B/b and C/c

Each dominant allele adds pigment.

Therefore:

• AABBCC produces the darkest phenotype.
• aabbcc produces the lightest phenotype.
• Intermediate combinations produce different shades.

Polygenic characters are also affected by environmental conditions.

Sex Determination

Sex determination is the biological mechanism that establishes whether an individual develops as male or female.

Different organisms use different chromosomal systems.

XX–XY Type

In humans and many other organisms:

• Female = XX
• Male = XY

Females produce only X-bearing ova.

Males produce:

• 50 per cent X-bearing sperm
• 50 per cent Y-bearing sperm

Therefore, the sperm determines the sex of the child.

The probability of either sex is approximately 50 per cent.

XX–XO Type

In some insects such as grasshoppers:

• Female = XX
• Male = XO

The male has only one X chromosome and no second sex chromosome.

The male produces:

• X-bearing sperm
• Sperm without a sex chromosome

ZZ–ZW Type

In birds:

• Male = ZZ
• Female = ZW

The female produces two types of eggs:

• Z-bearing
• W-bearing

Therefore, the female determines the sex of the offspring.

Haplodiploid Sex Determination

In honeybees:

• Females develop from fertilised eggs and are diploid.
• Males develop from unfertilised eggs and are haploid.

Male honeybees have no father but have a maternal grandfather because they arise through parthenogenesis.

Mutation

A mutation is a heritable change in genetic material.

Mutations may affect:

• A single nucleotide
• A gene
• Chromosome structure
• Chromosome number

Mutations create new genetic variation but may also produce disorders.

Gene Mutation

A gene mutation alters the nucleotide sequence of a gene.

A change affecting a single base pair is called a point mutation.

Sickle-cell anaemia results from a point mutation in the gene coding for the β-chain of haemoglobin.

The mutation replaces one amino acid:

Glutamic acid → Valine

This changes haemoglobin structure and may cause red blood cells to become sickle-shaped under low-oxygen conditions.

Chromosomal Mutation

Chromosomal mutations may involve:

• Deletion
• Duplication
• Inversion
• Translocation

Changes in chromosome number may result from nondisjunction.

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division.

It may produce:

• Monosomy
• Trisomy

Pedigree Analysis

Controlled genetic crosses cannot be performed in humans.

Pedigree analysis studies the inheritance of a character, abnormality or disease across several generations of a family.

It can help identify whether a trait is:

• Dominant or recessive
• Autosomal or sex-linked
• Transmitted through males, females or both

Common Pedigree Symbols

Genetic Disorders

Genetic disorders may be classified into:

• Mendelian disorders
• Chromosomal disorders

Mendelian Disorders

Mendelian disorders are caused mainly by mutations in a single gene.

They may be:

• Autosomal dominant
• Autosomal recessive
• Sex-linked dominant
• Sex-linked recessive

Their inheritance can be traced through pedigrees.

Haemophilia

Haemophilia is an X-linked recessive disorder.

Affected individuals have difficulty forming blood clots due to the absence or deficiency of a clotting factor.

It is more common in males because they have only one X chromosome.

A carrier female may transmit the affected allele to her sons.

An affected father does not pass the allele directly to his son because the son receives the Y chromosome from him.

Colour Blindness

Red-green colour blindness is an X-linked recessive disorder.

Affected individuals cannot clearly distinguish red from green because of defective cone-cell pigments.

It occurs more frequently in males.

A carrier mother has a 50 per cent chance of passing the affected allele to each son.

Sickle-Cell Anaemia

Sickle-cell anaemia is an autosomal recessive disorder.

Genotypes include:

• HbᴬHbᴬ: Normal
• HbᴬHbˢ: Carrier
• HbˢHbˢ: Affected

The disorder results from a point mutation in the β-globin gene.

Under low oxygen, abnormal haemoglobin molecules aggregate and distort red blood cells.

Phenylketonuria

Phenylketonuria is an autosomal recessive metabolic disorder.

The affected person lacks an enzyme required to convert phenylalanine into tyrosine.

Phenylalanine and its derivatives accumulate and may affect brain development if untreated.

Thalassemia

Thalassemia is an inherited disorder involving reduced synthesis of one of the globin chains of haemoglobin.

It is a quantitative defect because the amount of globin produced is reduced.

Depending on the affected chain, it may be:

• Alpha-thalassemia
• Beta-thalassemia

Chromosomal Disorders

Chromosomal disorders result from changes in chromosome number or structure.

Many numerical abnormalities arise through nondisjunction.

Down Syndrome

Down syndrome usually results from trisomy of chromosome 21.

The affected person has:

47 chromosomes

Common features may include:

• Short stature
• Small round head
• Furrowed tongue
• Broad palm
• Distinctive facial features
• Developmental delay

Klinefelter Syndrome

Klinefelter syndrome occurs in males with an additional X chromosome.

Chromosomal constitution:

47, XXY

Common features may include:

• Male appearance
• Underdeveloped testes
• Sterility
• Reduced masculine characteristics
• Possible breast development

Turner Syndrome

Turner syndrome occurs in females lacking one X chromosome.

Chromosomal constitution:

45, XO

Common features may include:

• Short stature
• Underdeveloped ovaries
• Sterility
• Poorly developed secondary sexual characteristics

Genetics Ratios and Crosses for Quick Revision

Important Differences for Quick Revision

Useful Links for Class 12 Biology