Selective Breeding
Use of Biological Resources · 6 question types
What Selective Breeding Is
(also called ) is the process by which humans deliberately organisms with desirable characteristics together, over many generations, to produce offspring with those characteristics consistently expressed
Humans have been selectively breeding plants and animals for at least 10 000 years, ever since the first farmers chose which crops to plant from this year's harvest and which animals to keep for next year's breeding. The result is the modern range of crops and farm animals: large, productive, fast-growing, disease-resistant, and almost completely different from their wild ancestors.
The four-step process
The basic procedure is the same for any species:
- Identify the desirable characteristic you want. This might be high milk yield, large fruit, disease resistance, fast growth, or a specific colour.
- Choose parents from the population that show the characteristic most strongly. Breed them together.
- Select the offspring that also show the characteristic. The rest are discarded (not allowed to reproduce).
- Repeat for many generations. Each generation, only the best examples are kept and bred together. The characteristic becomes more and more pronounced in the population.
After enough generations (often tens to hundreds), a new variety has been produced in which every offspring reliably shows the chosen characteristic. This is called a breed in animals, or a or in plants.
Exam note: an answer that only mentions one round of breeding (choose two parents → breed them together → done) is incomplete. The process must be repeated over many generations for offspring to reliably show the desired trait.
Describing how to breed for a characteristic
Question: Describe how selective breeding could produce a characteristic such as higher wheat yield, more milk, taller cattle or brightly coloured flowers (2–4 marks).
Asked in 7 of the 23 papers. Breed together the parents showing the characteristic most strongly; select the offspring that show it and breed from those; repeat over many generations. In 3- and 4-mark versions each step is a mark; in 2-mark versions the offspring step and the repeat usually share one, so include the parent step; one scheme ignores a "repeat" that is not about the offspring or next generations.
Name the characteristic given: "desired characteristics" usually scores but one scheme ignores it alone, and "large fish" is ignored when fast growth is wanted. For a trait only one sex shows, such as milk yield, pick bulls by their mothers', sisters' or daughters' yield, saying bulls or males: "cattle" is ignored. To remove a recessive allele, breed only from unaffected animals (a test cross shows which are homozygous dominant, not carriers) and their unaffected offspring, over generations.