Selective breedingSpec 4.8
Selective breeding is when humans choose which organisms to breed so that the desired characteristics are passed on to the next generation. It has been used for thousands of years.
- Decide which characteristic is wanted, for example high milk yield or disease resistance.
- Choose parents that show the characteristic most strongly and breed them together.
- From the offspring, select those with the characteristic and breed them together.
- Repeat over many generations until the desired characteristic is strongly shown.
| Organism type | Characteristics selected for |
|---|---|
| Food plants (crops) | Disease resistance, larger fruit or grains, better taste, higher yield |
| Domesticated animals | More milk, more meat, more eggs, a gentle nature, wool quality |
Impact of selective breeding
- Food plants and animals give higher yields, so more food can be produced from the same land.
- Crops can be bred to resist disease, so less is lost.
- Breeding from only a few individuals reduces genetic variation in the population. If a new disease or a change in the environment appears, all individuals may be affected, and the population could die out.
- Closely related individuals are bred together (inbreeding), which can make harmful inherited conditions more common in domesticated animals.
Probability in a selective breeding cross
In a plant, disease resistance is controlled by a dominant allele R, and susceptibility by the recessive allele r. Two resistant plants with the genotype Rr are crossed. What is the probability that an offspring is susceptible to the disease?
- Gametes from each parent: R or r
- Possible offspring: RR, Rr, Rr, rr
- Only rr is susceptible: 1 out of 4
Answer: The probability is 1/4 (25%). The ratio of resistant to susceptible offspring is 3 : 1.