Principles of Inheritance and Variation

Introduction

• Genesis of inheritance patterns from Mendel and subsequent developments.
• Focus on genetic material's structure and the conversion basis of genotype to phenotype.
• Contributions from significant scientists like Watson, Crick, and others to molecular biology and evolution.

Mendelian Genetics

4.1 Mendel's Laws of Inheritance

• Mendel's Experiments: Conducted for 7 years on pea plants.
• Introduced statistical methods to biology.
• Studied characters manifested as two opposing traits (e.g., tall/dwarf, yellow/green seeds).
• Established the concept of genes (alleles as contrasting traits).

4.2 Inheritance of One Gene

• Hybridization Experiments: Tall and dwarf plants crossing resulting in F1 generation.
• F1 generation always resembled one parent; not a blend.
• F2 generation showed a 3:1 ratio of dominant to recessive traits.
• Concept of alleles being dominant or recessive.

Laws of Inheritance

• Law of Dominance: Dominant traits mask recessive traits in heterozygous conditions.
• Law of Segregation: Alleles segregate independently during gamete formation.

Incomplete Dominance

• Phenotypes are intermediate between parents (e.g., flower color in Snapdragon).

Co-dominance

• Both alleles express simultaneously (e.g., ABO blood groups in humans).

4.3 Inheritance of Two Genes

• Dihybrid Crosses: Studied two traits (e.g., seed color and shape).
• Established the 9:3:3:1 phenotypic ratio.
• Law of Independent Assortment: Traits segregate independently of other traits.

Chromosomal Basis of Inheritance

Chromosomal Theory

• Rediscovery of Mendel's work and link to chromosome behavior.
• Chromosomes as carriers of genes.
• Work of Sutton and Boveri linking chromosomes and Mendelian genetics.

Linkage and Recombination

• Morgan's Experiments: Drosophila studies showed linkage and recombination.
• Genes on the same chromosome do not always assort independently.

Advanced Topics

Polygenic Inheritance

• Traits controlled by multiple genes (e.g., human height, skin color).

Pleiotropy

• Single genes affecting multiple phenotypic traits (e.g., Phenylketonuria).

Sex Determination

• Mechanism of sex determination in various organisms.
• Human Sex Determination: XY mechanism.
• Honey Bee: Haplodiploid sex-determination.

Mutation

• Alteration in DNA sequence leading to genetic variation.

Genetic Disorders

Mendelian Disorders

• Caused by single gene mutations (e.g., Haemophilia, Sickle-cell Anemia).

Chromosomal Disorders

• Results from chromosomal abnormalities (e.g., Down's Syndrome, Klinefelter's Syndrome).

Summary

• Review of Mendel's laws and their linkage to chromosomal behavior and genetic variation.

Exercises

• Various questions covering Mendelian inheritance, genetic crosses, and genetic disorders.