Genetics is one of the most important and scoring units of Class 12 Biology. This chapter explains how traits are inherited from parents to offspring and how variations occur. Questions from Genetics are frequently asked in board exams, NEET, CUET, and other competitive exams.
The Genetics unit mainly includes:
- Principles of inheritance
- Mendel’s experiments
- Chromosomal theory of inheritance
- Sex determination
- Linkage and recombination
- Genetic disorders
In this blog post, you will get Class 12 Biology Genetics Notes with clear concepts, examples, definitions, laws, and diagrams explanation (theoretical) for easy understanding and quick revision.
Class 12 Biology Genetics Notes | Complete Mendel Laws, Inheritance & Disorders
1. What is Genetics?
Genetics is the branch of biology that deals with:
- Inheritance (transfer of characters from parents to offspring)
- Variation (differences among individuals)
📌 The term Genetics was coined by William Bateson in 1905.
Key Terms
- Heredity: Transmission of traits from one generation to another
- Variation: Differences among individuals of the same species
- Trait: Any observable character (height, color, blood group)
2. Mendel and His Experiments
Gregor Johann Mendel – Father of Genetics
Gregor Mendel conducted experiments on pea plants (Pisum sativum) between 1856–1863.
Why Pea Plant?
- Easy to grow
- Short life cycle
- Clear contrasting traits
- Self-pollinating but cross-pollination possible
3. Mendel’s Traits in Pea Plant
| Character | Dominant Trait | Recessive Trait |
|---|---|---|
| Seed shape | Round | Wrinkled |
| Seed color | Yellow | Green |
| Flower color | Violet | White |
| Pod shape | Inflated | Constricted |
| Pod color | Green | Yellow |
| Flower position | Axial | Terminal |
| Stem height | Tall | Dwarf |
4. Important Genetic Terms
- Gene: Functional unit of inheritance
- Alleles: Alternative forms of a gene
- Homozygous: Same alleles (TT or tt)
- Heterozygous: Different alleles (Tt)
- Genotype: Genetic constitution
- Phenotype: Physical appearance
5. Monohybrid Cross
A cross involving one pair of contrasting traits.
Example: Tall (TT) × Dwarf (tt)
F₁ Generation
- All plants are Tall (Tt)
F₂ Generation
- Genotypic ratio: 1 TT : 2 Tt : 1 tt
- Phenotypic ratio: 3 Tall : 1 Dwarf
6. Mendel’s Laws of Inheritance
1. Law of Dominance
- One allele masks the expression of another allele.
2. Law of Segregation
- Allele pairs separate during gamete formation.
📌 Also called Law of Purity of Gametes
7. Dihybrid Cross
A cross involving two pairs of contrasting traits.
Example
- Seed shape (Round/Wrinkled)
- Seed color (Yellow/Green)
Phenotypic Ratio (F₂)
9 : 3 : 3 : 1
This experiment explains the Law of Independent Assortment.
8. Law of Independent Assortment
- Genes for different traits assort independently during gamete formation.
- Valid only when genes are located on different chromosomes.
9. Test Cross
- Cross between heterozygous individual and homozygous recessive.
- Helps to determine genotype.
Example:
- Tt × tt → 1 Tall : 1 Dwarf
10. Incomplete Dominance
- Neither allele is completely dominant.
Example: Snapdragon Flower Color
- Red × White → Pink (F₁)
- F₂ ratio: 1 Red : 2 Pink : 1 White
11. Co-dominance
- Both alleles express equally.
Example: Human Blood Group (AB)
- IA and IB alleles are co-dominant.
12. Multiple Alleles
- More than two alleles exist for a gene.
Example: ABO Blood Group System
- IA, IB, i
| Blood Group | Genotype |
|---|---|
| A | IAIA or IAi |
| B | IBIB or IBi |
| AB | IAIB |
| O | ii |
13. Chromosomal Theory of Inheritance
Proposed by Sutton and Boveri.
Main Points
- Genes are located on chromosomes
- Chromosomes segregate during meiosis
- Behavior of chromosomes explains Mendel’s laws
14. Linkage
- Genes located close together on the same chromosome tend to be inherited together.
- Discovered by T.H. Morgan using Drosophila melanogaster.
Types of Linkage
- Complete linkage
- Incomplete linkage
15. Recombination
- Exchange of genetic material between homologous chromosomes.
- Occurs during crossing over in meiosis.
16. Sex Determination
In Humans
- Male: XY
- Female: XX
📌 Discovered by Henking
Other Types
- XO type (Grasshopper)
- ZW type (Birds)
- Haplodiploidy (Honey bee)
17. Mutation
- Sudden change in DNA sequence.
- Causes genetic variation.
Types
- Gene mutation
- Chromosomal mutation
18. Genetic Disorders
1. Mendelian Disorders
Caused by mutation in a single gene.
Examples:
- Sickle cell anemia
- Hemophilia
- Color blindness
- Thalassemia
2. Chromosomal Disorders
Caused by abnormal chromosome number.
| Disorder | Cause |
|---|---|
| Down syndrome | Trisomy 21 |
| Turner syndrome | XO |
| Klinefelter syndrome | XXY |
19. Pedigree Analysis
- Used to study inheritance patterns in families.
- Helps identify genetic disorders.
Symbols:
- Square → Male
- Circle → Female
- Shaded → Affected individual
20. Importance of Genetics
- Understanding hereditary diseases
- Crop improvement
- Medical science
- Evolutionary studies
- Biotechnology
Conclusion
Genetics forms the foundation of modern biology and plays a crucial role in understanding life processes. A strong grip on Class 12 Biology Genetics Notes helps students score high in board exams and prepares them for competitive exams like NEET and CUET. By learning concepts step-by-step and practicing numerical and pedigree-based questions, students can master this unit easily.
FAQs
FAQ 1. What is Genetics in Class 12 Biology?
Genetics is the branch of biology that deals with heredity and variation. In Class 12 Biology, genetics explains how traits are passed from parents to offspring through genes and chromosomes.
FAQ 2. Who is known as the Father of Genetics and why?
Gregor Johann Mendel is known as the Father of Genetics because he discovered the basic laws of inheritance through his experiments on pea plants.
FAQ 3. Which Genetics chapter is most important for board exams?
The most important topics are Mendel’s Laws of Inheritance, Monohybrid and Dihybrid Cross, Blood Group Inheritance, Genetic Disorders, and Chromosomal Theory of Inheritance.
FAQ 4. What is the difference between genotype and phenotype?
Genotype refers to the genetic makeup of an organism, while phenotype refers to the physical expression of traits influenced by genotype and environment.
FAQ 5. What is a monohybrid cross with example?
A monohybrid cross is a cross between two individuals differing in one trait, such as Tall (TT) × Dwarf (tt), resulting in a 3:1 phenotypic ratio in F₂ generation.
FAQ 6. What are Mendel’s three laws of inheritance?
Mendel’s three laws are:
- Law of Dominance
- Law of Segregation
- Law of Independent Assortment
FAQ 7. What are genetic disorders explained in Class 12 Biology?
Class 12 Biology covers Mendelian disorders like Sickle Cell Anemia, Hemophilia, Color Blindness, and Chromosomal disorders like Down syndrome, Turner syndrome, and Klinefelter syndrome.
FAQ 8. Are Class 12 Genetics notes useful for NEET preparation?
Yes, Class 12 Genetics notes are extremely useful for NEET as many questions are directly based on NCERT concepts, Mendelian crosses, blood groups, and genetic disorders.