What is fertilisation?

Fertilisation is the process whereby two gametes join to form a zygote.

The zygote in question receives one of each of its pairs of chromosomes from each parent (the sperm and the egg), and it completes the transfer of genetic information to the next generation.

What is the process of fertilisation?

Image made by Ben Whitten

Male and female haploid (n) sex cells fuse to produce a diploid (2n) zygote.

(Note: haploid and diploid refer to chromosome number, so in humans, gametes produced by meiosis contain 23 chromosomes which are haploid and restore the number of chromosomes to 46 (diploid) via fertilisation.) Two gametes from unrelated individuals (from the same species) need to combine (male and female) to produce a new individual of the species, and this process is called sexual reproduction. Organisms produced via sexual reproduction have a different combination of DNA in comparison to their parents, which is comparable to asexual reproduction (i.e. mitosis, binary fission) in which genetic content is identical in daughter cells. It is important to note that gametes produced by humans have 23 chromosomes (as mentioned above), which are made up of 22 autosomes (non-sex chromosomes), and 1 sex chromosome (either X or Y).

What is the role of the sex chromosome in fertilisation?

The sex chromosome is the determinant of the offspring's biological sex (male, female or other). In humans, female gametes (usually) contain 22 autosomes and an X chromosome, and male gametes contain 22 autosomes and 50% have an X chromosome, while the other 50% have a Y chromosome. In fertilisation, this means that there is a 50% chance of a sperm cell bearing a Y chromosome and fusing with an ovum to create a male zygote (XY), and a 50% chance of a sperm cell bearing an X chromosome and fusing with an ovum to create a female zygote (XX). (Note: Errors can sometimes occur which results in chromosome combinations such as XXY, X, Y, XXXY etc. which you'll learn a bit about in mutations.)

What is apoptosis? Is it bad?

Apoptosis is a form of cell death that is pre-programmed and occurs once a cell reaches a certain age. This programmed cell death is not bad, however. Apoptosis is actually a vital process in embryonic development, particularly in regard to the shaping of organs and tissues. An example is apoptosis of cells in a human embryo of the webbing in between its fingers and toes. Without apoptosis, we'd all look like frogs!