5. Meiosis, Sex Cells & Variation

If each parent passed on all of their chromosomes, offspring would have double the normal number. The body avoids this with a special kind of cell division called meiosis, which makes sex cells (eggs and sperm).

Compare the two kinds of division:

• Mitosis makes two genetically identical body cells with the full chromosome number — for growth and repair.
• Meiosis makes four sex cells, each with half the chromosome number. When a sperm and egg join at fertilization, the full number is restored.

Meiosis is also the main source of variation, which is why siblings (other than identical twins) look different. The chromosome pairs are shuffled into different combinations, sections can swap during crossing over, and any sperm can combine with any egg. This variation is the raw material that natural selection acts on.

⚠️ Common misconception: meiosis and mitosis are not interchangeable. Mitosis = identical body cells; meiosis = varied sex cells with half the chromosomes.

💡 Tip: mitosis = copy (2 identical); meiosis = sex cells (4 different, half the chromosomes).