Principles of Inheritance and VariationClass 12th Biology: Chapter 5
The chapter Principles of Inheritance and Variation explores the mechanisms of heredity and the causes of genetic variations. It delves into the foundational work of Gregor Mendel, chromosomal theory, genetic disorders, and other key concepts of genetics, forming the basis for understanding biology at the molecular and organismal levels.
1. Introduction to Genetics
Genetics is the branch of biology that studies heredity and variation.
- Heredity: Transmission of genetic traits from parents to offspring.
- Variation: Differences in traits among individuals of a species.
2. Mendel's Experiments
Gregor Johann Mendel, the "Father of Genetics," conducted hybridization experiments on pea plants (Pisum sativum).
Key Features of Mendel’s Work:
- Focused on distinct traits (e.g., flower color, seed shape).
- Conducted controlled cross-pollination experiments.
- Applied statistical analysis to study inheritance patterns.
3. Mendel’s Laws of Inheritance
(a) Law of Dominance
- In a heterozygous individual, the dominant allele masks the expression of the recessive allele.
- Example: In a cross between a tall (TT) and a dwarf (tt) pea plant, the F1 generation shows all tall plants.
(b) Law of Segregation
- Each individual has two alleles for a trait, which segregate during gamete formation.
- Example: In the F2 generation of a monohybrid cross, the ratio is 3:1 (dominant:recessive).
(c) Law of Independent Assortment
- Alleles of different traits segregate independently of each other.
- Observed in dihybrid crosses with a phenotypic ratio of 9:3:3:1.
4. Chromosomal Theory of Inheritance
Proposed by Sutton and Boveri, this theory links Mendel’s principles with the behavior of chromosomes during meiosis.
Key Points:
- Genes are located on chromosomes.
- Chromosomes segregate and assort independently, mirroring Mendelian inheritance.
5. Sex Determination
Mechanism by which the sex of an organism is determined.
(a) XX-XY Mechanism: Found in humans and Drosophila.
- Males: XY (heterogametic).
- Females: XX (homogametic).
(b) Other Mechanisms:
- ZZ-ZW: Seen in birds.
- Haplo-Diploidy: Found in honeybees, where females are diploid and males are haploid.
6. Genetic Disorders
(a) Mendelian Disorders:
Caused by mutations in a single gene.
- Examples:
- Sickle Cell Anemia: Mutation in the hemoglobin gene causes sickle-shaped red blood cells.
- Phenylketonuria (PKU): Deficiency of the enzyme phenylalanine hydroxylase leads to mental retardation.
(b) Mendelian Disorders:
Caused by abnormalities in chromosome number or structure.
- Examples:
- Down Syndrome: Trisomy of chromosome 21.
- Turner Syndrome: Monosomy of the X chromosome (XO).
- Klinefelter Syndrome: Extra X chromosome in males (XXY).
7. Linkage and Recombination
(a) Linkage:
- Genes located close to each other on the same chromosome tend to be inherited together.
- Example: Eye color and wing size in Drosophila.
(b) Recombination:
- Exchange of genetic material between homologous chromosomes during meiosis.
- Results in new gene combinations, increasing genetic diversity.
8. Variations and Mutations 
(a) Types of Variations:
- Continuous Variation: Traits show a range of phenotypes (e.g., height).
- Discontinuous Variation: Traits have distinct categories (e.g., blood group).
(b) Mutations:
- Sudden heritable changes in the genetic material.
- Example: Mutation in hemoglobin causes Sickle Cell Anemia.
9. Pedigree Analysis 
A chart representing the inheritance pattern of a trait across generations.
- Useful for identifying Mendelian disorders.
- Symbols:
- Square: Male.
- Circle: Female.
- Shaded: Affected individual.
10. Human Genome Project (HGP)
An international effort to map and sequence the entire human genome.
Key Outcomes:
- Identified approximately 20,000-25,000 genes.
- Provided insights into genetic disorders and their treatments.
11. Importance of the Chapter
- Explains the fundamental principles of heredity and variation.
- Provides insights into genetic disorders, paving the way for medical advancements.
- Forms the foundation for further studies in genetics and biotechnology.
