5bSelective breeding

The idea in one sentence

Selective breeding (also called artificial selection) is when humans choose which organisms breed together, so that a desired characteristic becomes more common over many generations.

Nature does not choose the parents — a farmer, breeder or plant scientist does.

The four steps you must be able to write

Whatever the organism, the same sequence earns the marks:

1. From a population showing variation, select the individuals with the desired characteristic (e.g. highest wheat yield, most milk).
2. Breed these selected individuals together (cross-pollinate the plants; mate the animals).
3. From the offspring, again select those that show the desired characteristic best.
4. Repeat over many generations until all offspring reliably show the characteristic.

That final phrase — repeated over many generations — is a mark on its own in almost every selective breeding question. Do not leave it out.

Why it works

The desired characteristic is controlled by genes. Parents with the characteristic pass those alleles to their offspring, so each generation contains a higher proportion of individuals carrying them. Selective breeding does not create new genes — it changes how common existing ones are.

Things it is easy to lose marks on

• Offspring are not identical to the parents. Fertilisation is a random process, so which gametes fuse — and therefore which alleles the offspring get — varies. That is exactly why breeders must repeat the selection every generation instead of stopping after one cross.
• Selective breeding is not genetic engineering. No genes are transferred between organisms; you are only choosing parents.
• Variation must already exist in the population. If every plant is the same height, you cannot select for tallness.

Common mistake: writing that breeders "make" or "create" a new characteristic. Say instead that they select organisms that already show it and breed from them.

Examiner tip. Candidates often lose marks because they answer a slightly different question from the one asked. Read every word of the stem: a question about plants wants pollination and seeds, a question about cattle wants mating, and a question that says 'explain' wants reasons, not just the steps.

Characteristics breeders select for in crops

The method for a plant

Suppose you want a wheat variety with a high yield:

1. Grow a large population of wheat plants — they show variation in yield.
2. Select the plants that produce the most grain.
3. Cross-pollinate them: transfer pollen from one selected plant to the stigma of another (breeders often remove the anthers first and use a brush, then cover the flower so no unwanted pollen arrives).
4. Collect and sow the seeds.
5. From the offspring, again select the highest-yielding plants and cross them.
6. Repeat over many generations until yield is consistently high.

The four words that carry the marks are variation → select → cross → repeat over generations. An answer that leaves out any one of them is usually capped.

Points worth extra marks

• Two useful features can be combined by crossing a high-yielding variety with a disease-resistant variety, then selecting offspring that show both.
• Selective breeding only works on variation that already exists in the population. The breeder chooses which parents pass their alleles on — no new genes are created, so a characteristic that no plant in the population has cannot be bred in.
• Plants are convenient to selectively breed: they produce large numbers of offspring (seeds), and many have a short life cycle, so many generations pass quickly.
• Progress is fastest at the start and then slows down, because after several generations most of the useful alleles are already present in every plant.
• A drawback worth a mark: using the same few plants as parents every generation leaves the variety with reduced genetic variation, so one new disease can destroy the whole crop.

Common mistake: not all the offspring from one cross show the characteristic, and the ratios you get rarely match the predicted ones exactly. Two reasons examiners credit are that fertilisation is random and that not all seeds germinate.

Examiner tip. Describe the process, don't just name it. Examiners report that candidates write "they breed the best plants together" and stop, which scores one mark at most. Say which characteristic you are selecting for, say that the selected plants are crossed, say that the offspring are selected from again, and say that this is repeated over many generations. If the question says explain, finish with why it matters — for example, higher yield means more food produced from the same area of land.

Characteristics breeders select for

The method for an animal

To breed cattle with a high milk yield:

1. Measure the milk yield of a herd of cows — they show variation.
2. Select the cows with the highest milk yield, and select bulls whose mothers and sisters had high yields (a bull produces no milk, so it is judged on its female relatives).
3. Mate the selected male and female — often by artificial insemination, so one outstanding bull can father very many calves.
4. Measure the milk yields of the female offspring.
5. Select the best of these offspring and breed from them.
6. Repeat over many generations until the herd reliably gives a high yield.

Why animals are harder than plants

• Fewer offspring per breeding.
• Longer life cycles and longer gestation, so each generation takes years.
• Some characteristics (milk, egg number) can only be measured in one sex.

The main disadvantage: reduced variation

Breeding closely related individuals over many generations (inbreeding) reduces genetic variation in the population. This causes problems:

• Harmful recessive alleles become more common, so inherited disease appears more often (e.g. hip problems in some dog breeds).
• With little variation, the whole population may be susceptible to the same disease, and there may be no individuals able to survive a change in the environment.

Common mistake: writing 'selective breeding causes mutations' or 'creates new species'. It does neither — it only changes how common existing alleles are.

Examiner tip. Read what the question actually asks about. Examiners note that candidates who understood the biology still scored zero because they wrote about the wrong thing — e.g. describing effects on the whole ecosystem when asked about the breeding programme itself. Underline the organism and the characteristic named in the stem before you write.

Describe vs explain

These two command words are confused more than any others, and the confusion costs whole questions.

If the question says explain, a list of steps alone will not score. If it says describe, extra reasons waste time but do not lose marks — the steps must still be there.

A reliable four-mark answer template

Select the [organisms] with the [desired characteristic]. Breed them together. Select the offspring that show the characteristic best. Repeat over many generations.

Swap in cross-pollination for plants and mating for animals. Name the actual characteristic given in the question — not just "the good ones".

Data questions

You may be given a table or graph showing, for example, mean wheat yield or mean milk yield over several generations.

• Describe the trend: yield increases / increases then levels off.
• Quote figures with units from the data, including a comparison (e.g. "rose from 4.2 to 7.6 tonnes per hectare").
• If it levels off, explain that most individuals now carry the alleles, so there is little variation left to select from.

Practical-style vocabulary

Breeding trials are experiments, and examiners report candidates being vague about these terms:

• Dependent variable — what you measure (e.g. yield in tonnes per hectare).
• Independent variable — what you change (e.g. the generation number, or the variety used).
• Control variables — kept the same for a fair test (soil type, water, mineral ions, temperature, spacing of plants; for animals, the same diet and housing).
• Reliable — repeat the trial or use a large sample of plants/animals and calculate a mean, so a single unusual individual does not distort the result. "Reliable" does not mean "accurate" and it does not mean "correct".

Examiner tip. Examiners specifically flag confusion between 'describe' and 'explain', and vagueness about 'reliable' and 'dependent variable'. Before writing, circle the command word; if it is 'explain', make sure at least one sentence begins with 'because' or 'so that'.