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.
Tissue cultureSpec 4.9BTriple only
Tissue culture is growing new cells or tissues from small pieces of tissue, using sterile conditions and a growth medium.
- Take a small piece of tissue or a few cells from the parent plant.
- Use aseptic technique so that bacteria and fungi do not contaminate the culture.
- Place the tissue on a sterile nutrient medium (such as agar) containing nutrients and growth hormones.
- The cells divide, forming a mass of cells that grow into small plants, which are genetically identical to the parent.
Advantages in plant breeding programmes
- Many identical plants can be produced quickly, from a very small amount of tissue.
- Plants with desirable characteristics, such as those produced by selective breeding or genetic engineering, can be copied exactly.
- Plants can be produced all year round, in a small space.
- Rare plants can be reproduced to help save them.
Advantages in medical research
- Large numbers of identical cells can be grown for research, so results are reliable.
- New drugs can be tested on cultured cells, reducing the need to test on animals or people.
- Cells can be used to study how diseases develop.
Quick check
What is selective breeding?
Show answer
Choosing parents with desired characteristics to breed together, and repeating this over many generations.
Name two characteristics selected for in food plants.
Show answer
Any two of: disease resistance, larger fruit or grains, better taste, higher yield.
Why can selective breeding be a risk to a population?
Show answer
It reduces genetic variation, so a new disease or change could affect all individuals.