(b) Selective breeding
- Syllabus
- 2024
- Topic
- —
- Level
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Learning objectives
Develop a plant variety by selective breeding
Selective breeding uses existing inherited variation: humans choose plant parents with desired phenotypes and repeatedly breed the best offspring so the associated alleles become more common.
| Stage | Plant-breeding decision |
|---|---|
| 1 define the goal | choose measurable characteristics such as high grain yield, disease resistance, short strong stems or a desired flower colour and scent |
| 2 choose parents | identify plants that show the desired characteristic or complementary characteristics |
| 3 control pollination | transfer pollen between selected parents and prevent unwanted pollen reaching the stigma |
| 4 grow many seeds | raise offspring under comparable conditions so inherited differences can be judged |
| 5 select offspring | measure the target phenotype and keep the strongest combination |
| 6 repeat | cross or self-pollinate selected plants over many generations until the characteristic is reliably inherited |
To combine red flowers with scent, cross a red unscented variety with a white scented variety, select offspring that are both red and scented, then breed those selected offspring repeatedly.
Selective breeding does not create a chosen allele on demand; it changes the frequency and combinations of alleles already present through differential reproduction. Environment must be controlled when comparing phenotypes.
Develop animal traits by selective breeding
Animal selective breeding chooses parents with useful inherited traits, measures those traits in their offspring and repeats the selection across generations.
| Stage | Animal-breeding decision |
|---|---|
| 1 define the goal | choose a measurable trait such as rapid growth, high milk yield, disease resistance or efficient feed conversion |
| 2 identify parents | use phenotype, health, pedigree and offspring records to choose animals likely to carry useful alleles |
| 3 mate selected animals | use controlled mating or artificial insemination while avoiding animals with harmful traits |
| 4 raise comparable offspring | keep nutrition and other environmental factors similar |
| 5 measure and select | breed from offspring with the strongest inherited performance |
| 6 repeat | continue over many generations so useful alleles increase in the population |
A bull does not produce milk, so compare milk yield across many daughters produced with comparable cows. Select the bull whose daughters have the highest mean yield, not the bull with the largest body.
An individual phenotype reflects genes and environment, so records from many offspring are stronger than one observation. Repeated use of closely related animals can reduce genetic diversity and increase the chance that harmful recessive alleles meet.