Selective breeding reduces genetic variation
Repeatedly breeding from only a few individuals reduces genetic variation and shrinks the gene pool (the number of different alleles). Two consequences: (1) inbreeding — breeding closely related organisms makes two harmful recessive alleles more likely to meet, so genetic defects appear more often; (2) reduced adaptability — the organisms are genetically similar, so a new disease or environmental change can hit the whole population, few having alleles to survive.
Selective breeding: humans pick the parents, repeated for generations
Selective breeding (artificial selection): humans choose the organisms that reproduce, breeding those with a desired characteristic so it becomes more common each generation. Two mark-scheme elements: (1) humans select the parents showing the characteristic; (2) repeated over many generations. What is really selected are the alleles: the chosen parents pass them on, so they become more common in the gene pool. A single cross is not selective breeding.
Desired characteristics bred into plants and animals
Statements 5.10 (plants) and 5.11 (animals) expect named examples. Plants: wheat bred for higher grain yield, disease resistance, or shorter stronger stems to resist lodging; fruit for larger fruit. Animals: dairy cattle for higher milk yield, chickens for more meat or eggs, sheep for wool, dogs for temperament. The mechanism matches natural selection — advantageous alleles become more common — but the selection pressure is applied by humans, not the environment.
Drawn from real examiner reports.
Allele is inherited, not the characteristic
A recurring precision error: candidates write that a characteristic or trait is passed on. The mark-scheme wording is that the allele — the version of the gene — is inherited. The selected parents pass on the alleles for the desired characteristic, so those alleles become more common each generation. Name the allele, not the characteristic.
Selective breeding is not natural selection
In selective breeding humans decide which organisms reproduce (artificial selection). In natural selection the environment selects — the best-adapted survive and reproduce. Both change allele frequencies over generations, but the source of the selection pressure differs. Do not describe the environment choosing in a selective-breeding answer.
Selective breeding vs genetic modification
Selective breeding uses normal reproduction to combine whole genomes over many generations; it never inserts or removes a gene from the DNA. Genetic modification puts a specific gene directly into the DNA in one step and can cross species. Writing that the breeder changes or adds a gene describes GM, not selective breeding — the wrong process for the context.
Breeding the best animals is too vague
The answer breeding the best animals or picking good plants scores poorly because it misses the two mark-scheme elements: that humans do the choosing (this is what makes it artificial selection) and that the process is repeated over many generations. Avoid vague words like strongest or best — name the desired characteristic and give a direction of change.
Genetically similar, not identical
When explaining reduced variation, the word identical alone is not enough. State the loss of variation with reference to alleles or genes: the organisms are genetically similar with fewer different alleles. Examiners credit genetically similar / reduced genetic variation and reject the loose word identical on its own.
Use the 5-step selection chain
For "describe how a farmer could use selective breeding" (3-4 marks), answer as a chain: select the individuals with the desired characteristic; breed them; from the offspring select the best; breed those; repeat over many generations. One marking point per step.
Combine two traits: cross then select both
To combine two desired traits (e.g. red flowers and scent), cross the two varieties, select the offspring showing both traits, breed them together, and repeat over several generations. Do not switch to genetic modification — the context wants selective breeding.
Name the characteristic and its direction
Always name the desired characteristic (e.g. highest milk yield) and give a direction of change rather than a vague make it better. Where the question asks you to select the alleles, say the alleles for the characteristic become more common each generation.
For evaluate parts, add the drawback
When a question asks for a problem or risk of selective breeding, add the consequence chain: reduced genetic variation and a smaller gene pool lead to inbreeding, harmful recessive alleles combining, and a population vulnerable to a new disease.
Selective breeding (artificial selection) — the process by which humans choose (select) the organisms with a desired characteristic and breed them together, repeated over many generations so the desired characteristic becomes more common in the population. (Two elements: humans choose the parents; repeated over generations.)
Selective breeding uses normal reproduction — the breeder does not change the DNA directly. It works because offspring inherit alleles from their parents, so choosing parents with the desired alleles makes those alleles more common each generation.
Define selective breeding (artificial selection).
What is meant by selective breeding? (2 marks)
