Introduction
Inheritance is the process by which characteristics are passed from parents to offspring, while variation refers to the differences observed among individuals of the same species. The study of inheritance and variation forms the basis of Genetics.
This chapter is highly important for NEET, with questions frequently asked from Mendel's experiments, inheritance patterns, chromosomal theory, and genetic disorders.
Genetics: Definition
Genetics is the branch of biology that deals with heredity and variation.
Important Terms
Heredity: Transmission of traits from parents to offspring.
Variation: Differences among individuals.
Character: A heritable feature (e.g., flower color).
Trait: Alternative forms of a character (e.g., red or white flower).
Gregor Johann Mendel – Father of Genetics
Gregor Mendel conducted experiments on garden pea plants (Pisum sativum) between 1856 and 1863.
Why Did Mendel Choose Pea Plants?
Easy to grow.
Short generation time.
Produces many seeds.
Self-pollinating and cross-pollinating.
Availability of contrasting traits.
Mendel's Seven Contrasting Characters
| 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 |
Monohybrid Cross
A cross involving one pair of contrasting traits.
Example:
Tall Plant (TT) × Dwarf Plant (tt)
F₁ Generation
All offspring are Tall (Tt).
F₂ Generation
Obtained by selfing F₁ plants.
Genotypic Ratio
1 TT : 2 Tt : 1 tt
Phenotypic Ratio
3 Tall : 1 Dwarf
Mendel's First Law – Law of Dominance
Statement
When two contrasting alleles are present together, only one expresses itself in the F₁ generation.
Example
Tallness (T) is dominant over dwarfness (t).
Significance
Explains why only one trait appears in hybrids.
Mendel's Second Law – Law of Segregation
Statement
The two alleles of a gene separate during gamete formation and pass independently into different gametes.
Also Known As
Law of Purity of Gametes
Importance
Each gamete carries only one allele of a gene.
Dihybrid Cross
A cross involving two pairs of contrasting traits.
Example
Round Yellow (RRYY) × Wrinkled Green (rryy)
F₁ Generation
All Round Yellow (RrYy)
F₂ Phenotypic Ratio
9 : 3 : 3 : 1
9 Round Yellow
3 Round Green
3 Wrinkled Yellow
1 Wrinkled Green
Mendel's Third Law – Law of Independent Assortment
Statement
Different pairs of alleles assort independently during gamete formation.
Importance
Explains the appearance of new combinations of traits.
Chromosomal Theory of Inheritance
Proposed by:
Walter Sutton
Theodore Boveri
Main Points
Genes are located on chromosomes.
Chromosomes occur in pairs.
Chromosomes segregate during meiosis.
Genes are inherited through chromosomes.
Linkage and Recombination
Linkage
The tendency of genes located on the same chromosome to be inherited together.
Discovered By
Thomas Hunt Morgan
Recombination
Formation of new gene combinations due to crossing over.
Importance
Creates genetic variation.
Sex Determination
Human Sex Determination
Humans follow the XX-XY mechanism.
Female
44 + XX
Male
44 + XY
Gametes
Female produces only X-bearing ova.
Male produces:
50% X-bearing sperms
50% Y-bearing sperms
Sex Ratio
50% Male : 50% Female
Important Fact
The father determines the sex of the child.
Mutation
Mutation is a sudden heritable change in genetic material.
Types
Gene Mutation
Change in DNA sequence.
Example:
Sickle Cell Anaemia
Chromosomal Mutation
Change in chromosome number or structure.
Examples:
Down Syndrome
Klinefelter Syndrome
Turner Syndrome
Genetic Disorders
Sickle Cell Anaemia
Cause:
Mutation in β-globin gene.
Inheritance:
Autosomal recessive.
Feature:
Sickle-shaped RBCs.
Haemophilia
Cause:
Defective blood clotting.
Inheritance:
X-linked recessive disorder.
Mostly affects males.
Colour Blindness
Inheritance:
X-linked recessive.
Affected individuals cannot distinguish certain colours.
Chromosomal Disorders
Down Syndrome
Cause:
Trisomy of chromosome 21.
Chromosome Number:
47
Characteristics:
Intellectual disability
Short stature
Broad face
Klinefelter Syndrome
Chromosome Constitution:
44 + XXY
Characteristics:
Male individual
Sterility
Poor development of secondary sexual characters
Turner Syndrome
Chromosome Constitution:
44 + XO
Characteristics:
Female individual
Underdeveloped ovaries
Sterility
Pedigree Analysis
Pedigree analysis is the study of inheritance patterns in families through generations.
Uses
Identifying carriers
Predicting genetic disorders
Genetic counseling
Key Terms for NEET
| Term | Meaning |
|---|---|
Gene | Unit of heredity |
Allele | Alternative form of a gene |
Genotype | Genetic constitution |
Phenotype | Observable character |
Homozygous | Similar alleles |
Heterozygous | Different alleles |
Dominant | Expressed trait |
Recessive | Masked trait |
Mutation | Sudden genetic change |
Frequently Asked NEET Facts
Father of Genetics – Gregor Mendel
Father of Experimental Genetics – Thomas Hunt Morgan
Chromosomal Theory – Sutton and Boveri
Law of Dominance – Mendel
Law of Segregation – Purity of Gametes
Dihybrid Ratio – 9:3:3:1
Monohybrid Ratio – 3:1
Human Male Chromosomes – 44 + XY
Human Female Chromosomes – 44 + XX
Quick Revision Table
| Topic | Key Point |
|---|---|
Monohybrid Cross | 3:1 Ratio |
Dihybrid Cross | 9:3:3:1 Ratio |
Dominance | One allele masks another |
Segregation | Alleles separate during gamete formation |
Independent Assortment | Genes assort independently |
Linkage | Genes inherited together |
Recombination | New gene combinations |
Mutation | Source of variation |
Conclusion
The Principles of Inheritance and Variation chapter forms the foundation of Genetics. Mendel's laws, chromosomal theory, linkage, sex determination, and genetic disorders are among the most important topics for NEET. A clear understanding of inheritance patterns and ratios can help students solve genetics-based questions quickly and accurately in the examination.
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