AQA GCSE Biology: Sexual and Asexual Reproduction
A clear GCSE Biology revision guide to sexual and asexual reproduction: how each works, why sexual reproduction produces variation, and the advantages of each.
Organisms reproduce in two ways: sexually, mixing genes from two parents, or asexually, making identical copies. This guide explains how each works, why only sexual reproduction produces variation, and the advantages of each. The key contrast is variation versus identical copies.
Sexual reproduction
Sexual reproduction involves the fusion of male and female gametes at fertilisation. Gametes are sex cells made by meiosis. In animals the gametes are sperm and egg cells; in plants they are pollen and egg cells.
Because genetic information from two parents is combined, and because gamete formation by meiosis shuffles the genes, the offspring show genetic variation, differing from each other and from their parents.
There are two separate sources of that variation, and a good answer names both. Meiosis itself produces four gametes that are all genetically different, so no two sperm cells from one father carry the same combination. Then fertilisation is a matter of chance: which sperm reaches which egg decides which two of those combinations are put together. Two children of the same parents are therefore different from each other, which would be impossible if either step were missing.
Asexual reproduction
Asexual reproduction involves only one parent. There is no fusion of gametes and no mixing of genetic information. Cell division happens by mitosis only, so all the offspring are genetically identical to the parent, they are clones.
Mitosis is the reason. It copies the DNA exactly and divides it into two identical cells, so nothing is shuffled and nothing is combined. Bacteria dividing in two, a strawberry plant sending out a runner and a daffodil bulb splitting are all the same process at different scales.
The advantages of each
(This section is Triple award only.)
| Sexual | Asexual | |
|---|---|---|
| Parents needed | two | one |
| Cell division | meiosis, then fertilisation | mitosis only |
| Offspring | genetically different | genetically identical clones |
| Speed | slower, a mate must be found | fast |
| If the environment changes | some offspring may survive | all are affected the same way |
Sexual reproduction has one big advantage: it produces genetic variation, so if the environment changes, some individuals may have characteristics that help them survive, through natural selection. Humans can also use this variation to speed up change through selective breeding.
Asexual reproduction has different advantages: only one parent is needed, so there is no need to find a mate, and it is fast, producing many identical offspring quickly when conditions are good, which is useful for colonising an area.
The trade-off is worth stating plainly. Identical offspring are a risk, because a new disease or a change in conditions that kills one of them can kill every one of them. Variation is insurance, paid for with the time and energy of finding a mate.
Many organisms use both methods, and the pattern is usually the same: asexual when conditions are good, sexual when they are not. The malarial protist reproduces asexually in humans and sexually in the mosquito; many fungi release spores asexually and reproduce sexually to create variation; and flowering plants reproduce sexually by seeds and asexually by runners (as in strawberries) or bulb division (as in daffodils).
How this fits together
Sexual reproduction depends on gametes made by meiosis, the special cell division that halves the chromosome number, which is the subject of the meiosis guide.