1.3 Comprehensive Guide to Genetics

Introduction to Genetics

• The study of genes, genetic variation, and heredity.
• Explores how traits are passed from parents to offspring and vary within populations.

Mendel's Laws of Inheritance

1. Law of Dominance: In contrasting traits, only the dominant trait appears in F1.
2. Law of Segregation: Alleles for a trait separate during gamete formation, so each gamete gets only one allele.
3. Law of Independent Assortment: Alleles for different traits sort independently of each other.

Genetic Crosses

• Monohybrid Cross: Involves one trait.
  • F2 Phenotypic Ratio: 3:1 (e.g., Tall:Dwarf)
  • F2 Genotypic Ratio: 1:2:1 (e.g., TT:Tt:tt)
• Dihybrid Cross: Involves two traits.
  • F2 Phenotypic Ratio: 9:3:3:1 (e.g., Round Yellow:Round Green:Wrinkled Yellow:Wrinkled Green)

Key Genetic Terminology

• Gene: Basic unit of heredity (DNA sequence).
• Allele: Alternative form of a gene.
• Homozygous: Two identical alleles (TT, tt).
• Heterozygous: Two different alleles (Tt).
• Dominant: Allele expressed phenotypically.
• Recessive: Allele masked by dominant allele.
• Phenotype: Observable characteristics.
• Genotype: Genetic makeup.

Sex Determination in Humans

• Humans have 23 pairs of chromosomes.
• Autosomes: 22 pairs (same in males and females).
• Sex Chromosomes: 1 pair (determines sex).
  • Females: XX
  • Males: XY
• Sex of child determined by the sperm (X or Y).

Sex-Linked Inheritance of Diseases

• Traits inherited through X or Y chromosomes (mostly X-linked).
• Males are more affected by X-linked recessive disorders due to having only one X chromosome.
• X-linked Diseases:
  1. Haemophilia: Blood does not clot properly (X-linked recessive).
  2. Colour Blindness: Inability to see certain colors (X-linked recessive).