Meiosis makes variety

In sexual reproduction, both parents give genetic material. Here is the problem: if each parent passed on its full set, the number of chromosomes would double in every generation.

Chromosomes are thread-like structures in the nucleus that carry genetic information. Humans have 23 pairs: 46 in all. If each parent gave all 46, a child would have 92, their child 184... But every human has 46.

Meiosis solves it. It halves the number: from diploid (two sets) in body cells to haploid (one set) in the gametes. Each human sperm or egg has 23 chromosomes, one from each pair. When they join: 23 + 23 = 46. (In plants, the pollen carries the male gametes, and the ovule holds the egg.)

Meiosis also shuffles. The chromosomes of each pair separate at random: which one goes into a gamete is chance. Try the bead activity: use beads for 3 pairs of characters (hair colour, hair type, eye colour). You can make 8 different combinations.

With 23 pairs, each carrying many characters, the number of combinations is enormous. That's why children differ from their parents and from their brothers and sisters.

"Why does variety matter?" When conditions change, some individuals may be better suited to survive: for example, people who can manage on less oxygen at high altitudes, or who can digest milk as adults. Over very long times, variation helps species evolve.