Meiosis: Why No Two Siblings Are Alike
One cell divides twice into four, each with half the chromosomes — and three separate mechanisms guarantee that no two are the same.
Builds on: 4.1 Mendel: Traits Come in Units2.2 Replication: Copying Three Billion Letters
The halving problem
You have 46 chromosomes, in 23 pairs — one of each pair from each parent. If a sperm and an egg each carried 46, your children would have 92, their children 184, and the whole thing collapses within a few generations.
So gametes must carry half: 23 chromosomes, one from each pair. Cells with the full set are diploid (2n); gametes are haploid (n). The division that halves the number is meiosis, and it is used for nothing else.
Compare it with mitosis, the ordinary division that grows you and repairs you: one cell → two identical diploid cells. Meiosis: one cell → four different haploid cells. Different purpose, different outcome.
Two divisions, four cells
The chromosomes are copied once, then the cell divides twice. That is what gets you from 2n to n.
- Division I separates the pairs — one whole chromosome of each pair to each side. This is the reduction step.
- Division II separates the copies within each chromosome, giving four haploid cells in total.
Three sources of variety
Meiosis is not just division — it is a shuffling machine, and it has three independent mechanisms.
1. Independent assortment. When the 23 pairs line up before Division I, each pair orients at random, independently of the others. Whether you pass on your mother’s chromosome 7 has nothing to do with whose chromosome 12 you pass on. That gives 2²³ = 8,388,608 possible combinations from assortment alone.
2. Crossing over. Before separating, paired chromosomes physically swap matching segments. The chromosome you pass on is therefore not your mother’s or your father’s — it is a mosaic of both, in a new combination that has never existed.
3. Random fertilisation. Any one of millions of possible sperm meets any one of millions of possible eggs.
This is why siblings resemble each other without being identical, and why identical twins — who skip the shuffle entirely by splitting from one fertilised egg — are so striking.
The link back to Mendel
Mendel’s rules are meiosis, described before anyone had seen it. His “units” separate cleanly because homologous chromosomes physically separate at Division I. They assort independently because the pairs line up independently. Mendel inferred the mechanism from ratios alone; when chromosomes were finally watched under a microscope decades later, they were doing exactly what he had predicted.
If a pair fails to separate — nondisjunction — a gamete ends up with an extra chromosome or none. An extra chromosome 21 gives Down syndrome. The risk rises with maternal age because human egg cells begin meiosis before birth and pause partway through, sometimes for decades, before finishing.
⚗️ Lab — The Meiosis Stepper
Step through one cell becoming four, and watch where the variety comes from.
- At step 2, chromosomes swap segments — the colours mix. Turn crossing over off and compare.
- At step 3, notice the pairs line up independently. That alone gives 2²³ possibilities.
- The final four gametes are all different. Mitosis would have produced two identical cells.
Work it by hand — 0 / 2
No multiple choice here: compute the value and type it. Suffixes like 2.5k or 20m are understood; answers within ±1% count.