Sexual and asexual reproductionIB MYP Sciences: Revision notes

In asexual reproduction, one parent produces offspring. There are no gametes and no fertilisation. The offspring are genetically identical to the parent, so they are clones.

Examples:

• Binary fission: a bacterium copies its DNA, grows and divides into two.
• Budding: a small bud grows on the parent (for example in yeast) and breaks off.
• Runners: strawberry plants send out stems along the ground and new plants grow from them.
• Cloning: making identical organisms, for example from plant cuttings or in the laboratory.

Advantages:

• Only one parent is needed, so there is no need to find a mate.
• It is fast, so populations grow quickly.
• Offspring keep the useful features of a successful parent.

Disadvantages:

• There is no variation, so all offspring are equally affected by a disease or change in the environment.
• A harmful change can wipe out the whole population.

In sexual reproduction, two parents each make special sex cells called gametes. In animals these are sperm and egg cells. In fertilisation, a male gamete fuses with a female gamete to make a fertilised egg (a zygote), which divides and grows into a new organism.

The offspring receive a mixture of genes from both parents, so they show variation: they are not identical to either parent or to each other.

Advantages:

• Variation means some offspring are likely to survive a disease or a change in the environment.
• Variation allows species to adapt over time.

Disadvantages:

• Two parents are needed, so finding a mate takes time and energy.
• It is slower than asexual reproduction, and fewer offspring may be produced.

Meiosis is the type of cell division that makes gametes. Each gamete has half the usual number of chromosomes, so it is haploid. When two gametes fuse, the full number is restored.

For example, a human gamete has 23 chromosomes. A fertilised egg has 46. You do not need the stages of meiosis, only that it makes haploid gametes.