Principles of Inheritance and Variation

Introduction

• Concepts of inheritance and variation in biology primarily stem from the work of Gregor Mendel.
• Understanding the structure of DNA and molecular biology has been crucial to comprehending inheritance and evolution.
• Evolution and genetic variation have been further explored through molecular genetics and bioinformatics.

Mendel’s Laws of Inheritance

Key Experiments

• Mendel conducted hybridization experiments on garden peas (1856-1863).
• Introduced statistical analysis and mathematical logic to biology.
• Developed basic inheritance principles later expanded by other scientists.

Mendel’s Findings

• Conducted cross-pollination with 14 true-breeding pea plant varieties.
• Proposed laws of inheritance based on contrasting traits (e.g., tall vs. dwarf).

Inheritance of One Gene

Monohybrid Cross

• Focus on the inheritance of a single trait (e.g., plant height).
• F1 Generation: All offspring resembled one parent (e.g., tall).
• F2 Generation: Traits reappeared in a 3:1 ratio, with no blending observed.

Alleles and Genotype

• Genes (factors) as units of inheritance; occur in pairs (alleles).
• Dominance and recessiveness explained using alleles (e.g., T for tall, t for dwarf).

Laws of Inheritance

• Law of Dominance: One allele can mask the presence of another.
• Law of Segregation: Alleles segregate during gamete formation.

Incomplete Dominance

• Occurs when F1 has a phenotype between two parents (e.g., Snapdragon flower color).

Co-dominance

• F1 resembles both parents (e.g., ABO blood grouping in humans).
• Multiple alleles and the additive effect of alleles on phenotype.

Inheritance of Two Genes

Dihybrid Cross

• Example: Yellow round seeds crossed with green wrinkled seeds.
• Traits segregate independently, leading to a 9:3:3:1 phenotypic ratio.

Law of Independent Assortment

• Two or more traits are inherited independently of each other.

Chromosomal Theory of Inheritance

• Rediscovery and validation of Mendel’s work (early 20th century).
• Chromosomes identified as carriers of genes, supported by microscopy.
• Sutton and Boveri linked chromosomal movement to Mendel’s laws.

Linkage and Recombination

• Morgan’s studies on Drosophila melanogaster provided insight into genetic linkage.
• Linkage affects the predicted phenotypic ratios due to genes being close on the same chromosome.

Polygenic Inheritance

• Traits like human height are controlled by multiple genes.
• Polygenic traits display a range rather than distinct categories.

Pleiotropy

• A single gene affects multiple phenotypic traits.
• Example: Phenylketonuria affects mental capacity and pigmentation.

Sex Determination

• Mechanisms differ across species (e.g., XY system in humans, ZW in birds).
• Haplodiploid system in honey bees (haploid males, diploid females).

Mutation

• Alterations in DNA sequences (e.g., due to mutagens like UV radiation).
• Cause genotypic and phenotypic variations.

Genetic Disorders

Pedigree Analysis

• Traces inheritance patterns in families, used to study traits and disorders.

Mendelian Disorders

• Disorders caused by single-gene mutations (e.g., Hemophilia, Sickle-cell anemia).
• Can be autosomal or sex-linked, dominant or recessive.

Chromosomal Disorders

• Caused by abnormalities in chromosome number or structure (e.g., Down's syndrome, Turner’s syndrome).

Summary

• Genetics explains the transmission of traits and variations through mechanisms like segregation and assortment of alleles.
• Inheritance patterns include complete/incomplete dominance and co-dominance.
• Understanding genetic disorders involves studying mutation effects and chromosomal alterations.