Edexcel A Level (IAL) Biology -3.10 Meiosis & Variation- Study Notes- New Syllabus

Role of Meiosis in Genetic Variation

🌱 Introduction

Meiosis is a special type of cell division that occurs in reproductive cells to produce gametes (sperm or egg cells). It ensures that each gamete has half the normal number of chromosomes (haploid) and that no two gametes are genetically identical. This variation forms the foundation of evolution and diversity in living organisms.

🧬 Overview of Meiosis

• Takes place in two divisions:
  • Meiosis I → separates homologous chromosomes
  • Meiosis II → separates sister chromatids
• Produces 4 haploid cells from one diploid parent cell.
• Genetic variation arises mainly during Meiosis I through:
  • Independent assortment (Metaphase I)
  • Crossing over (Prophase I)

🌗 Independent Assortment (at Metaphase I)

• 📍 What happens:
  • In Metaphase I, homologous chromosomes line up in pairs at the equator.
  • The orientation of each pair is random – each homolog can face either pole.
  • When they separate in Anaphase I, the combination of maternal and paternal chromosomes that go into each gamete differs every time.
• Result: New combinations of chromosomes in gametes → each gamete carries a unique mix of parental DNA.
• Example: If an organism has 3 pairs of chromosomes, possible combinations = 2³ = 8 gamete types!

🌈 Crossing Over (at Prophase I)

• 📍 What happens:
  • Homologous chromosomes pair up to form bivalents.
  • Chromatids overlap at points called chiasmata.
  • Sections of chromatids are exchanged between homologous chromosomes.
• Result: Produces recombinant chromatids with new allele combinations → each gamete carries a unique set of genes.
• Remember:
  • Crossing over → mixes alleles on the same chromosome (breaks linkage).
  • Independent assortment → mixes whole chromosomes from different parents.

🧩 Combined Effect

• Both mechanisms ensure genetic diversity among gametes:
  • Independent assortment → rearranges whole chromosomes.
  • Crossing over → rearranges DNA sections within chromosomes.
• Thus, even siblings (except identical twins) have different gene combinations.

📊 Summary Table

⚡ Quick Recap 
Meiosis → produces 4 genetically different haploid gametes.
Independent assortment (Metaphase I): random arrangement of chromosomes → new chromosome combinations.
Crossing over (Prophase I): exchange of DNA segments → new allele combinations.
Both processes increase genetic variation – essential for natural selection & evolution.