Two ways of reproducing
Reproduction makes new individuals of a species. There are two types.
Key idea
Asexual reproduction: one parent, no gametes (sex cells), offspring are genetically identical to the parent (a clone).
Sexual reproduction: two parents, involves gametes and fertilisation, offspring show genetic variation.
Asexual reproduction
Only one parent is needed and no sex cells join. Examples include bacteria dividing, and plants such as the potato and strawberry producing runners or tubers.
- Advantages: fast; only one parent needed; offspring well suited if conditions stay the same.
- Disadvantages: no variation, so a whole population can be wiped out by one disease or change.
Sexual reproduction
Two gametes (a male sex cell and a female sex cell) join at fertilisation to form a zygote. Gametes have half the number of chromosomes, so when they fuse the normal number is restored. The zygote then divides many times by mitosis to grow into a new organism.
In flowering plants the male gamete is inside a pollen grain and the female gamete is an egg cell in the ovule; in humans the gametes are the sperm and the egg cell. Because the offspring receive genes from two different parents, no two individuals (except identical twins) are exactly alike.
Remember
- Sexual reproduction produces variation, which helps a species survive changes and is the raw material for evolution and selective breeding.
Worked example
A human body cell has 46 chromosomes. Each gamete (egg or sperm) has 23. At fertilisation, 23 + 23 = 46, so the zygote has the full number, with genes from both parents giving variation.