Selective breeding is a four-step cycle
Selective breeding (artificial selection) is how humans develop a desired characteristic over many generations. Four steps: (1) identify those showing the characteristic most strongly; (2) breed those parents together; (3) select the offspring that best show it; (4) repeat over many generations. Because the trait is heritable, each round raises the frequency of its alleles until it is reliably fixed.
Know a plant and an animal example
The spec needs at least one plant and one animal example. Plants (5.10): wheat or rice bred for higher grain yield or disease resistance; crops for drought tolerance or shorter, stiffer stems (less lodging). Animals (5.11): dairy cattle for high milk yield; beef cattle for more muscle mass. For full marks, name the characteristic selected, say the strongest are chosen as parents, and that this is repeated over many generations.
Faster than nature, but cuts variation
Selective breeding fixes a valuable trait far faster than natural selection and targets one characteristic precisely. Its cost: it reduces genetic variation and narrows the gene pool, leaving the population vulnerable to new disease or environmental change; repeated inbreeding can also cause inbreeding depression (harmful recessive alleles in the homozygous form). Contrast: humans choose the parents here, whereas in natural selection the environment decides who survives.
Drawn from real examiner reports.
Humans choose, the environment does not
In selective breeding the human chooses which individuals breed — the selection is artificial, not environmental. "The strongest survive and breed", or saying the environment selects, describes natural selection instead and loses the mark. The mark-scheme answer names the human choosing parents with the desired characteristic and repeating this over many generations.
The allele is inherited, not the trait
Examiners want the allele named as what is passed on — not the "characteristic" or "trait". Humans select parents by their visible characteristic, but the offspring inherit the allele that produces it, and over generations that allele's frequency rises. Also say organisms reproduce more, rather than that their "numbers increase".
Jun 2024 P1BR Q8(a)(iii)
One cross is not enough
Selective breeding is a repeated, multi-generation process. An answer that stops at "pick the best parents and cross them" scores only partial credit — the mark scheme always rewards repeating the selection and breeding over many generations until the desired allele is fixed. Write "repeated over many generations" as a distinct final step.
Selective breeding is not gene editing
Selective breeding works only by choosing which individuals mate — no DNA is cut, copied or transferred. Describing restriction enzymes, ligase or inserting a gene is genetic modification (5c), not selective breeding. Note the 4BI1 wording: transgenic means transferring genetic material to a different species — "one organism to another" loses the mark.
Jun 2023 P1BR Q9(a)(i)
Bred traits need not aid wild survival
A selectively bred characteristic is chosen for human value (e.g. high milk yield), not for survival in the wild — it can even be a disadvantage there, so do not claim selective breeding makes organisms "better adapted to their environment". Its real trade-off is reduced genetic variation, which leaves the population exposed if a new disease appears.
Use the four-step breeding scaffold
For "describe how selective breeding could produce…" (3-4 marks), give one step per mark: identify those showing the trait most strongly, cross them, select the best offspring, repeat over many generations. Name the trait each time; do not mention enzymes or gene transfer.
Name the exact characteristic
Marks are for the trait selected, not the organism. "Wheat" or "cattle" alone scores nothing — say wheat bred for disease resistance, or cattle for high milk yield. Avoid vague words like "better"; state precisely what is selected for.
Compare: cover both processes
"Compare" or "evaluate" selective breeding versus natural selection needs both sides of each axis named — agent (humans vs environment), timescale, and genetic variation. A one-sided list with no contrast cannot reach full marks.
Selective breeding (also called artificial selection) is the process in which humans choose individuals with a desired characteristic and breed them together, then select the offspring that best show that characteristic, repeating this over many generations until the characteristic is reliably established in the population.
Two elements needed for full marks on a definition:
Deoxyribonucleic acid (DNA) carries the heritable information that determines characteristics. Selective breeding does not alter DNA directly — it changes which alleles are most common in the population by controlling which individuals reproduce.
Allele — a version of a gene. Desired characteristics are controlled by particular alleles; selective breeding gradually increases the frequency of those alleles over generations.
Define selective breeding (artificial selection). Give both essential elements.
What is selective breeding? Include both elements needed for full marks. (2 marks)
