4.8 - Selective breeding
Explain how choosing parents changes inherited characteristics over generations. Apply selective breeding to food plants and domesticated animals, balancing production gains against genetic diversity and welfare.
4.8 — How selective breeding works
Individuals in a population vary. Some of that variation is inherited because the
individuals carry different alleles. Selective breeding can act on this inherited
variation: a breeder decides which organisms reproduce according to a trait that people
want.
Selective breeding
Selective breeding, also called artificial selection, is the process in which humans choose
organisms with desired inherited characteristics as parents of the next generation.
The process has a repeated cycle:
- Choose the desired inherited characteristic. For example, a breeder may want
disease resistance in wheat or high milk yield in cattle. - Identify parents that show the characteristic strongly. The organisms should be
compared under similar conditions because environment as well as genes can affect
phenotype. - Breed the selected parents. Plants are crossed by transferring pollen; animals mate
naturally or their gametes are brought together using a breeding technique. In either
case, this is sexual reproduction. - Assess the offspring and select the best. Offspring inherit alleles from both
parents, so they are not all genetically identical. - Breed selected offspring and repeat for many generations. Alleles contributing to
the desired characteristic are passed on by the chosen parents and can become more
common in the population.
Worked example: disease-resistant wheat. A breeder grows genetically varied wheat
plants where they can be compared fairly and identifies plants that suffer the least fungal
disease. These plants are crossed. Seeds are grown, the most resistant offspring are chosen
as parents, and the cycle is repeated. After many generations, inherited resistance is more
common because resistant plants have repeatedly contributed more alleles to the next
generation.
The fungus does not make individual wheat plants develop useful alleles because they need
them. The variation exists before selection, and the breeder changes which plants
reproduce. Unlike genetic engineering, selective breeding does not directly insert, remove,
or edit DNA.
4.8 — Impacts on plants and animals
The impact of selective breeding depends on the characteristic chosen and on how the
breeding population is managed.
| Organisms | Examples of a selected inherited characteristic | Possible impact |
|---|---|---|
| Food plants | higher yield; larger or better-quality edible parts; resistance to a disease, pest, or drought | more usable food can be harvested, or less crop may be lost under the relevant conditions |
| Domesticated animals | higher milk, meat, or egg production; disease resistance; fertility; longevity; calm behaviour | animals may provide more food or be healthier and easier to manage |
These are population changes, not instant changes to one organism. A cow does not inherit a
higher-milk-yield allele because it is milked often; breeders must choose parents whose high
yield has an inherited component and then select among their offspring.
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