Genetic Inheritance & Genetic Crossing: DP IB Biology Revision Notes 2023

Inheritance: Gametes & Fertilisation

• Gametes: Sex cells of an organism (e.g., sperm and egg in humans)
  • Egg is larger due to food reserves for embryo nourishment
  • Sperm has many mitochondria for movement energy
• Fertilisation: Fusion of gametes to form a zygote
  • Gametes are formed during meiosis and are haploid (one copy of each chromosome)
  • Human gametes contain 23 single chromosomes
  • After fusion: zygote is diploid (two alleles of each gene)
  • Homozygous: Same alleles; Heterozygous: Different alleles

Genetic Crosses in Flowering Plants

• Gregor Mendel: Conducted breeding experiments in pea plants
  • Known as the Father of Genetics
  • Experimented on characteristics like plant height and flower color
  • Transferred pollen manually to control fertilisation
  • Discovered predictable inheritance patterns
• Mendel's Results:
  • First generation: Offspring show characteristic of one parent
  • Second generation: 3:1 ratio, characteristics of both parents
  • Identified concepts of dominant and recessive genes
  • Introduced the idea of alleles (different forms of a gene)

Monohybrid Crosses

• Monohybrid Trait: Controlled by one gene
  • Typically involves two alleles (dominant or recessive, or co-dominant)
• Punnett Grid: Predicts offspring's genotype and phenotype
  • Steps to construct a Punnett grid:
    1. Write parental phenotypes and genotypes
    2. Determine possible gamete genotypes
    3. Place parental genotypes on a grid
    4. Combine gametes to form offspring genotypes
    5. List phenotype and genotype ratios

Worked Example: Sweet Peas

• Green (G) vs Yellow (g) Pods:
  • Step 1: Parental genotypes GG and gg
  • Step 2: Possible gametes G, g
  • Step 3: Setup Punnett grid
  • Step 4: Combine gametes (all offspring Gg, green)
  • Step 5: Cross F1 generation (Gg x Gg)
  • Step 6: Results in F2 generation (3:1 phenotype ratio, 1 GG: 2 Gg: 1 gg)

Plant Reproduction Methods

• Self-Pollination: Male & female parts in the same flower/plant
• Cross-Pollination: Between different plants by artificial means
  • Allows selection of desirable traits for breeding
  • Genetic crosses help predict breeding outcomes

The information extracted provides a framework for understanding genetic inheritance and the principles of genetic crossing in a biological context, specifically through the study of Mendel's experiments with pea plants.