By the end of this lesson
- Describe how selective breeding is used in animals.
- Give examples of desired characteristics in farm and domestic animals.
- Evaluate welfare and genetic issues linked to selective breeding.
BIG QUESTION
Why do modern dairy cows produce so much more milk than their ancestors?
Generations of breeding from cows with the highest milk yield have increased the alleles linked to that trait.
Breeding animals
A breeder identifies males and females with the desired characteristic, breeds them and selects the best offspring as future parents. This is repeated over many generations.
Because animals take time to reach breeding age, progress is usually slower than in many plants. Artificial insemination lets a single selected male father many offspring.
Desired characteristics
Examples include high milk yield in cattle, fast growth and lean meat in pigs, more eggs from hens, fine wool from sheep, disease resistance and a calm temperament.
In dogs, breeders have selected for size, coat type and behaviour, producing very different breeds within one species.
Problems
Repeatedly breeding from a few related animals is inbreedingBreeding closely related individuals, which reduces genetic variation and increases the chance of inherited disorders.. It reduces genetic variation and increases the chance that offspring inherit two copies of a harmful recessive allele.
Some selected traits can cause welfare problems. For example, very high milk yield may be linked to leg and udder problems, and some dog breeds have breathing or joint difficulties.
Selective breeding in animals
Benefits often come with trade-offs.
| Animal | Desired characteristic | Possible problem |
|---|---|---|
| Dairy cattle | High milk yield. | Possible lameness or udder infection. |
| Broiler chickens | Rapid growth. | Legs may struggle to support body mass. |
| Sheep | Fine, heavy fleece. | Reduced genetic variation. |
| Dogs | Particular appearance or behaviour. | Inherited disorders from inbreeding. |
Language that earns precision
- inbreeding
- Breeding closely related individuals.
- milk yield
- The volume of milk an animal produces.
- temperament
- The typical behaviour of an animal, for example docile or aggressive.
- artificial insemination
- Placing collected sperm into a female without mating.
Common misconceptions
Claim: Animals adapt during their lives and pass on the change.
Correction: Only inherited alleles are passed on; breeders select individuals that already have the allele.
Claim: Selective breeding always improves animal health.
Correction: It may cause welfare problems if traits are selected without regard to health.
Claim: Inbreeding creates harmful alleles.
Correction: It increases the chance of two copies of an existing recessive allele coming together.
Match the answer to the command word
Link inbreeding to reduced variation and to the increased chance of inheriting two recessive harmful alleles.
Give benefits to the farmer and possible costs to animal welfare and the gene pool.
Recall, apply, then justify
Describe how selective breeding could be used to increase the milk yield of a herd of cattle.
Hint
Which cows and bulls? Then what?
Suggested marking points
- Choose cows with the highest milk yield.
- Breed them with a bull whose female relatives had a high milk yield.
- Select the female offspring with the highest milk yield to breed from.
- Repeat over many generations.
Explain why inbreeding can increase the number of animals born with inherited disorders.
Hint
Recessive alleles and related parents.
Suggested marking points
- Related parents are more likely to carry the same recessive allele.
- Offspring are more likely to be homozygous recessive.
- So the disorder is expressed more often / genetic variation is reduced.
Discuss whether selective breeding of broiler chickens for rapid growth is beneficial.
Planning hint
Producer, consumer, animal welfare, genetics.
What a strong design includes
- Faster growth means chickens reach slaughter mass sooner.
- Less feed and time per bird can lower meat prices.
- Rapid growth may cause leg or heart problems.
- Animal welfare concerns may arise.
- Reduced genetic variation may increase vulnerability to disease.
- A balanced conclusion depends on how welfare is weighed against production.