Science Puzzle
Genetic Thread Count
Most human body cells contain 46 chromosomes, arranged in 23 matching pairs. But egg cells and sperm cells each contain only 23 chromosomes, one from each pair.
Why do sex cells carry only half the normal number of chromosomes?
The Answer
Because fertilisation doubles the chromosome number. When a sperm fertilises an egg, the two cells fuse and their genetic material combines. If both cells carried 46 chromosomes, the resulting fertilised egg would have 92, and every generation would double the number again.
To prevent this runaway doubling, sex cells are produced by a special type of cell division called meiosis, which halves the chromosome count. Ordinary cell division (mitosis) preserves the full count; meiosis reduces it by half.
The result: each parent contributes exactly 23 chromosomes to the offspring, giving the offspring the correct 46 total, 23 from the mother and 23 from the father. The specific which chromosome from each pair is passed on is shuffled randomly during meiosis, which is one source of genetic variation between siblings.
The principle: Chromosomal inheritance and meiosis. Sex cells carry half the usual chromosome number so that fertilisation restores the correct full count rather than doubling it with each generation.