Inheritance: Inheritance

17.3 Inheritance

Key Definitions

• Inheritance: Transmission of genetic information from generation to generation.
• Allele: Alternative form of a gene.
• Phenotype: Observable characteristics.
• Genotype: Genetic makeup (alleles present).
• Homozygous: Two identical alleles (AA or aa).
• Heterozygous: Two different alleles (Aa).
• Dominant: Expressed if at least one copy present (A).
• Recessive: Expressed only if two copies present (aa).

Monohybrid Crosses

• Use Punnett squares to predict offspring ratios.
• Examples:
  • Cross of heterozygous parents (Aa × Aa) → 3:1 phenotypic ratio.
  • Cross of heterozygous × homozygous recessive (Aa × aa) → 1:1 ratio.

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Special Cases

• Codominance: Both alleles expressed equally.
  • Example: ABO blood group system.
  • Alleles: Iᴬ, Iᴮ, Iᵒ → phenotypes A, B, AB, O.
• Sex Determination:
  • Female → XX chromosomes.
  • Male → XY chromosomes.

Mutations

• Gene mutation: Random change in base sequence (e.g., sickle cell anaemia).
• Chromosome mutation: Change in chromosome number/structure (e.g., Down’s syndrome – 47 chromosomes).
• Causes: Ionising radiation, certain chemicals.
• Source of genetic variation.

Key Points

• Inheritance patterns explain why offspring resemble parents.
• Ratios often differ from expected due to small sample size.
• Variation arises from meiosis, fertilisation, mutation.

Written And Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions And 11 World Records For Educate A Change O Level And IGCSE Biology Full Scale Course