Gametes join at fertilisation to restore the normal number of chromosomes.

The idea

Fertilisation is the fusion of two gametes, usually one from each parent. Because each gamete is haploid and carries a single set of chromosomes, their fusion produces a cell with two sets of chromosomes. This restores the normal diploid number for the species. In humans, a sperm cell with 23 chromosomes fuses with an egg cell with 23 chromosomes, producing a zygote with 46 chromosomes. The zygote therefore contains genetic information from both parents, which is why offspring inherit features from each parent. The restoration of the diploid number is essential because it allows the new cell to grow and divide by mitosis while maintaining the correct chromosome number in the organism.

What earns the marks

• Fertilisation involves the fusion of two gametes, typically one from each parent.
• Each gamete is haploid and contributes a single set of chromosomes.
• Fusion of the gametes produces a cell with two sets of chromosomes, restoring the diploid number.
• In humans, the fusion of a 23-chromosome sperm and a 23-chromosome egg produces a zygote with 46 chromosomes.
• The zygote contains genetic information from both parents and can divide by mitosis to form the new organism.

Common mistakes and how to put them right

• Thinking fertilisation doubles the chromosome number permanently.

  Put it right by stating that it restores the normal diploid number.

• Believing gametes are diploid.

  Put it right by stating that gametes are haploid and contain a single set of chromosomes.

• Confusing fertilisation with cell division.

  Put it right by describing fertilisation as the fusion of two gametes.

Exam tips

• State the chromosome numbers of the gametes and the zygote when using humans as an example.
• Use the terms haploid and diploid to show that you understand the change in chromosome number.
• Link fertilisation to the inheritance of genetic information from both parents.