Selective breedingIB MYP Biology: Revision notes

Selective breeding (artificial selection) is when humans choose which organisms breed so that useful characteristics are passed on. It has been used for thousands of years to produce crops and farm animals with features people want.

It works because characteristics are controlled by alleles (versions of genes) that are inherited from parents.

Selective breeding follows the same steps every time:

1. Decide which characteristic you want (for example high milk yield).
2. Choose parents that show it best.
3. Breed them together.
4. From the offspring, select those that show the characteristic most strongly.
5. Repeat for many generations.

After each generation, more of the population carries the alleles for the characteristic.

Livestock:

• Dairy cattle bred for high milk yield.
• Cattle and chickens bred for more meat.
• Sheep bred for better wool.

Crops:

• Wheat and rice bred for disease resistance.
• Fruit bred for larger size or better taste.
• Crops bred for higher yield or drought tolerance.

To combine features, breeders cross two parents, each with a useful feature, then select offspring that have both.

Selective breeding can:

• Increase yield so more food is produced from the same land.
• Produce disease-resistant crops and animals, so less is lost and less pesticide or medicine is needed.
• Improve quality, such as taste, size or speed.
• Be cheaper and more easily accepted than genetic engineering, because no genes are transferred between species.

Inbreeding (breeding close relatives) is a problem when breeders use only a few animals or plants:

• Genetic variation falls, so the population has a smaller gene pool.
• Harmful recessive alleles are more likely to be inherited from both parents, so genetic disorders become more common.
• A new disease or change in climate could affect the whole population because none have resistance.

Breeding for one feature can also cause other useful features to be lost. Selective breeding is slow, taking many generations.

In natural selection the environment decides which organisms survive and reproduce, so features that help survival become more common. In selective breeding, humans decide.

• Natural selection favours features that help survival. Selective breeding favours features that humans want, such as milk yield or speed.
• Selective breeding can change a population in fewer generations.
• In both cases only some individuals pass on their alleles, and the population changes over generations.