Principles of Inheritance and Variation Class 12
NCERT Biology Notes for NEET – Mendelian Genetics, Crosses & Important Concepts
📚 Quick Summary
Principles of Inheritance and Variation Class 12 is one of the most important genetics topics in NCERT Biology and NEET preparation.
The chapter explains how characters are inherited from parents to offspring and how variations arise. Major topics include Mendel's experiments, laws of inheritance, monohybrid and dihybrid crosses, incomplete dominance, codominance, linkage, sex determination, pedigree analysis and genetic disorders.
NEET Strategy: Learn the NCERT terminology first and then practise genetic crosses and pedigree-based questions.
🧬 What Is Inheritance?
Inheritance is the transmission of genetic characters from parents to offspring.
These hereditary characters are controlled by genes present on chromosomes and are transmitted from one generation to the next.
Examples: Blood group, flower colour, seed shape and several other inherited characteristics.
🔬 What Is Variation?
Variation refers to differences in characteristics among individuals of the same species.
Variation is biologically important because it contributes to diversity within populations and provides the raw material for evolution.
Inheritance → similarities between generations
Variation → differences among individuals
🌱 Gregor Mendel – Father of Genetics
Gregor Johann Mendel performed hybridisation experiments on garden pea, Pisum sativum, and formulated fundamental principles explaining inheritance.
Why Did Mendel Select Pea Plants?
- Many clearly distinguishable contrasting characters
- Short life cycle
- Produces many seeds
- Normally self-pollinating
- Artificial cross-pollination can be performed easily
🌿 Seven Contrasting Traits Studied by Mendel
| Character | Dominant Trait | Recessive Trait |
|---|---|---|
| Stem height | Tall | Dwarf |
| Flower colour | Violet | White |
| Flower position | Axial | Terminal |
| Pod shape | Inflated | Constricted |
| Pod colour | Green | Yellow |
| Seed shape | Round | Wrinkled |
| Seed colour | Yellow | Green |
1. Monohybrid Cross
A monohybrid cross studies the inheritance of one pair of contrasting characters.
Example:
TT (Tall) × tt (Dwarf)
The F1 generation consists of heterozygous tall plants:
Tt × Tt
The F2 generation produces:
Genotypic ratio = 1 TT : 2 Tt : 1 tt
2. Law of Dominance
When two contrasting alleles are present together in a heterozygote, one allele expresses itself while the other does not express its phenotype.
The expressed allele is called the dominant allele, while the other is called the recessive allele.
3. Law of Segregation
The two alleles of a gene separate during gamete formation so that each gamete receives only one allele.
This law is also known as the law of purity of gametes.
4. Dihybrid Cross
A dihybrid cross studies two pairs of contrasting characters simultaneously.
Mendel studied seed shape and seed colour:
RRYY × rryy
The F1 plants are:
RrYy
Selfing the F1 hybrid produces the classic F2 phenotypic ratio:
5. Law of Independent Assortment
When two pairs of traits are considered together, segregation of one pair of factors can occur independently of the other pair during gamete formation, provided the genes assort independently.
The classic Mendelian dihybrid ratio 9:3:3:1 demonstrates independent assortment.
🧪 Test Cross
A test cross is a cross between an individual showing a dominant phenotype and a homozygous recessive individual.
It can be used to determine the genotype of an individual showing a dominant phenotype.
Monohybrid test cross of a heterozygote:
Tt × tt → 1 Tall : 1 Dwarf
🌸 Incomplete Dominance
In incomplete dominance, neither allele is completely dominant over the other, and the heterozygote shows an intermediate phenotype.
Classic NCERT example: Snapdragon flower colour
- RR → Red
- rr → White
- Rr → Pink
In the F2 generation:
Phenotypic ratio = 1 Red : 2 Pink : 1 White
Genotypic ratio = 1 : 2 : 1
🩸 Codominance
In codominance, both alleles express themselves in a heterozygous individual.
The ABO blood-group system provides an important example.
- IA and IB are codominant.
- Both IA and IB are dominant over i.
- IAIB produces blood group AB.
🧬 Multiple Alleles
When a gene has more than two alternative forms in a population, the phenomenon is called multiple allelism.
The ABO blood-group system has three alleles:
IA, IB and i
🔗 Linkage and Recombination
Genes located on the same chromosome tend to be inherited together and are described as linked genes.
Recombination can generate new combinations of alleles, particularly through crossing over during meiosis.
The experimental work of Thomas Hunt Morgan using Drosophila contributed greatly to our understanding of linkage and recombination.
♂️♀️ Sex Determination
Different organisms use different mechanisms for determining sex.
| System | Example |
|---|---|
| XX–XY | Humans, Drosophila |
| XX–XO | Some insects |
| ZZ–ZW | Birds |
| Haplodiploidy | Honey bee |
👨👩👧 Pedigree Analysis
A pedigree is a diagrammatic representation of the inheritance of a particular trait through several generations of a family.
Pedigree analysis is useful for studying the inheritance pattern of genetic traits and disorders.
Important Pedigree Symbols
- Square → Male
- Circle → Female
- Unshaded symbol → Unaffected individual
- Shaded symbol → Affected individual
- Horizontal line → Mating
- Vertical line → Offspring
⚕️ Mendelian Disorders
Mendelian disorders result from alteration or mutation in a single gene and follow recognizable inheritance patterns.
| Disorder | Inheritance |
|---|---|
| Haemophilia | X-linked recessive |
| Colour blindness | X-linked recessive |
| Sickle-cell anaemia | Autosomal recessive |
| Phenylketonuria | Autosomal recessive |
| Thalassemia | Autosomal recessive |
🧬 Chromosomal Disorders
Chromosomal disorders can arise due to abnormalities in chromosome number or structure.
- Down syndrome: Trisomy of chromosome 21
- Klinefelter syndrome: 47, XXY
- Turner syndrome: 45, XO
🎯 High-Yield NEET Revision Table
| Concept | Remember |
|---|---|
| Monohybrid F2 phenotype | 3:1 |
| Monohybrid genotype | 1:2:1 |
| Dihybrid F2 phenotype | 9:3:3:1 |
| Incomplete dominance | 1:2:1 phenotype |
| Codominance | Both alleles express |
| Test cross | Dominant phenotype × homozygous recessive |
| Morgan | Drosophila, linkage & recombination |
❓ Frequently Asked Questions
What are the three laws of Mendel?
The three commonly discussed Mendelian principles are the law of dominance, law of segregation and law of independent assortment.
What is the monohybrid phenotypic ratio?
Under complete dominance, a Mendelian monohybrid cross produces an F2 phenotypic ratio of 3:1.
What is the dihybrid phenotypic ratio?
For two independently assorting gene pairs showing complete dominance, the F2 phenotypic ratio is 9:3:3:1.
What is the difference between incomplete dominance and codominance?
In incomplete dominance, the heterozygote has an intermediate phenotype. In codominance, both alleles express themselves in the heterozygote.
Why is Mendel called the Father of Genetics?
Mendel established fundamental principles of inheritance through controlled hybridisation experiments on pea plants.
⚡ One-Minute NEET Revision
Mendel: Pea plant experiments
Monohybrid: 3:1 phenotype; 1:2:1 genotype
Dihybrid: 9:3:3:1
Incomplete dominance: Intermediate heterozygote
Codominance: Both alleles express
AB blood group: IAIB
Morgan: Drosophila, linkage and recombination
Down syndrome: Trisomy 21
Turner syndrome: 45, XO
Klinefelter syndrome: 47, XXY
🎯 Preparing for NEET Biology?
Use NCERT as your primary source, revise important genetics ratios repeatedly and practise inheritance-based MCQs and PYQs.
Explore NEET Biology Resources →📌 Final Takeaway
Principles of Inheritance and Variation Class 12 becomes much easier when you connect Mendel's experiments with the ratios and laws they demonstrate.
For NEET, pay special attention to NCERT examples, Mendelian ratios, incomplete dominance, codominance, ABO blood groups, linkage, pedigree analysis and genetic disorders.
NCERT-focused Biology notes, NEET revision, MCQs and exam-preparation resources.