Random Fertilisation & Genetic Variation

Before we can understand random fertilisation, we need to be sure what fertilisation means.

Fertilisation is the fusion (joining together) of a male gamete and a female gamete to form a zygote:

• The male gamete is a sperm.
• The female gamete is an egg cell (ovum).
• When they meet, the sperm nucleus fuses with the egg nucleus, making a single new cell called a zygote.

Gametes are special sex cells. They have only half the normal number of chromosomes — they are haploid. When the two gametes fuse, the zygote gets a full set of chromosomes again (it is diploid): half from the father, half from the mother.

This is why a child has a mixture of features from both parents — half of its genetic information comes from each gamete.

Exam tip. Be precise: fertilisation is the fusion of a sperm nucleus and an egg nucleus. Saying "the sperm and egg join" is acceptable, but naming the nuclei fusing is the safest wording.

Random fertilisation does not work alone — it builds on variation that is already there in the gametes. That earlier variation comes from meiosis.

Meiosis is the special type of cell division that makes gametes (sperm and eggs). A crucial feature of meiosis is that the gametes it produces are genetically different from one another:

• A person has two copies (alleles) of every gene — one on each chromosome of a pair.
• During meiosis these pairs are shuffled and separated in different ways.
• So every sperm (or egg) ends up with its own unique combination of alleles.

Think of a man producing sperm. No two sperm carry exactly the same mixture of alleles — each is a slightly different "shuffle" of his genetic information. The same is true for a woman's eggs.

So even before fertilisation happens, there is already a huge pool of genetically different gametes to choose from. This is the raw material that random fertilisation then mixes together.

Exam tip. A good answer links the two ideas: meiosis makes the gametes genetically different, then random fertilisation randomly combines them. Mentioning both stages is what scores top marks on a "explain why offspring vary" question.

Here is the key idea of this whole subtopic: fertilisation is random.

When fertilisation happens, any one of the many sperm could fertilise the egg. There is no rule deciding which sperm gets there first — it is down to chance.

Because of this:

1. Each sperm carries a different combination of alleles (thanks to meiosis).
2. Each egg also carries a different combination of alleles.
3. Which sperm fuses with which egg is random — any sperm can fertilise any egg.
4. So the combination of alleles in the zygote is random too.

The zygote's genes are therefore a chance mixture: a random combination of the father's gametes and the mother's gametes. A different sperm reaching the egg would have produced a genetically different child.

Here is the idea as a simple diagram:

Exam tip. The phrase examiners want is "any sperm can fertilise any egg, so the combination of alleles in the zygote is random". Use the word random (or "by chance") and the word alleles.

Put the two ideas together and you can see why offspring vary genetically:

• Meiosis makes gametes that each have a different combination of alleles.
• Random fertilisation then randomly combines one sperm with one egg.

Because both the gametes themselves and the way they pair up are varied, the zygote ends up with a new, unique combination of alleles. This means the offspring is genetically different:

• from its brothers and sisters (each came from a different sperm-and-egg combination), and
• from both parents (it is a mixture, not a copy, of their alleles).

This is why a family of children with the same two parents are all slightly different — different eye colour, height, blood group, and so on. Each child was the result of a different random fertilisation event.

It is important to be clear that this variation is genetic — it comes from the genes/alleles the offspring inherit. (Variation can also be caused by the environment, e.g. diet affecting weight, but that is a different cause and not what random fertilisation produces.)

Big idea: different gametes (from meiosis) + random fertilisation (chance pairing) = genetic variation in the offspring.

Exam tip. If a question asks why brothers and sisters are not genetically identical, the answer is that each is produced by a different combination of gametes brought together by random fertilisation.

Why does it matter that offspring are all slightly different? Because variation is useful for a species.

If every member of a species were genetically identical, a single disease or a sudden change in the environment could wipe them all out. But when there is genetic variation:

• some individuals may, by chance, have alleles that help them survive a change (e.g. resist a disease or cope with a colder climate);
• those individuals are more likely to survive and reproduce, passing their useful alleles on.

So the variation produced by meiosis and random fertilisation gives a species a better chance of surviving in a changing world. Over many generations this is the basis of natural selection and evolution.

This is one of the big advantages of sexual reproduction over asexual reproduction: sexual reproduction (with two gametes and random fertilisation) creates lots of genetic variety, whereas asexual reproduction makes genetically identical copies (clones).

Exam tip. Keep your focus on spec 3.31: the main marks are for explaining that random fertilisation produces genetic variation. The survival/evolution point is a useful "why it matters" extension but should not replace the core explanation.