Sexual reproduction

1. Sexual reproduction takes two parents, one supplying each kind of sex cell.
2. A male gamete and a female gamete meet and their nuclei join together, an event called fertilisation.
3. In animals these gametes are the sperm and egg, while in flowering plants they are the pollen and egg cell.
4. Fertilisation produces a single cell, the zygote, which is the very first cell of the new individual.
5. The zygote then keeps dividing by mitosis to build the growing offspring.
6. Since its genes come from two different parents mixed together, the offspring shows variation and is never an exact copy of either one.

Asexual reproduction

1. Asexual reproduction takes just one parent, with no partner involved.
2. No sex cells are made and nothing fuses, so there is no fertilisation at all.
3. The parent simply grows new cells by mitosis, and these carry exactly the same genes, making the offspring genetically identical to it and to one another.
4. A set of such identical offspring from a single parent are known as clones.
5. With no genes brought in from a second parent, this kind of reproduction gives no variation.
6. The one thing that can make two clones differ genetically is a chance mutation in the DNA.

Gametes and meiosis

1. Body cells are diploid: they carry two sets of chromosomes.
2. Gametes are haploid: they carry only one set of chromosomes, half the number in a body cell.
3. Gametes are made by a type of cell division called meiosis, which halves the chromosome number.
4. The cells meiosis makes are all genetically different from each other, whereas mitosis turns out cells that are exact genetic copies.
5. At fertilisation two haploid gametes fuse, which restores the full diploid number.
6. This keeps the chromosome number the same from one generation to the next, while combining genetic information from both parents.
7. In humans, each body cell has $46$ chromosomes and each gamete has $23$, so fertilisation restores the full $46$ in the zygote.