Selective breeding · Grade 11
In Grade 11 you get fluent with: selective breeding and the problems of inbreeding.
In selective breeding, people choose the plants or animals with the characteristic they want, breed them, choose the best of the offspring and breed those, over many generations. That is how we have crops that resist disease, cows that give more milk and gentle dogs. Breeding close relatives over and over reduces variation, which can bring inherited defects and leave them all open to a new disease.
Key facts
- Selective breeding: choose parents with the characteristic you want, breed them, choose the best offspring and breed them, and repeat over many generations. Examples are disease-resistant crops, cows that give more milk and gentle dogs.
- Selective breeding can lead to inbreeding: there is less variation, so the organisms are more likely to suffer from inherited defects and from new diseases.
Worked examples
Example 1
Put the steps of selective breeding in order.
Answer: choose parents with the characteristic you want → breed them together → choose the offspring that show the characteristic best → breed these, and repeat over many generations
Example 2
A farmer wants cows that give more milk. Describe how she could use selective breeding.
Answer: breed the cows that give the most milk with bulls from high-yield mothers, choose the best offspring and repeat over many generations
Example 3
Give two characteristics that farmers selectively breed crop plants for.
Answer: for example: a high yield and resistance to disease (also: larger fruit, or coping with drought)
Example 4
Which technique produced these: wheat with bigger grains, bred from the best plants each year?
Answer: selective breeding
Example 5
Explain why selective breeding can cause problems for animals.
Answer: it reduces variation in the gene pool, so inbreeding makes inherited disorders more likely and the whole population could be wiped out by a new disease
Example 6
What is the main problem caused by selective breeding?
Answer: inbreeding, so there is less variation and more risk from diseases and inherited defects
Example 7
After 5 generations of selective breeding, the mean milk yield of a herd rose from 4,000 liters to 5,200 liters a year. Calculate the percentage increase.
Answer: 30%