REMEMBER: An allele is a length of DNA on a chromosome from a homolagous pair.
An allele is a different form of a gene - i.e: it codes for the same characteristic but the characteristic will be coded for differently. Example: a gene codes for eye colour. One Allele may code for blue eyes and another for brown eyes
- Similarities between organisms can be determined by similarities in DNA. Difference in DNA leads to genetic diversity.
- All members of the same species have the same genes i.e. their DNA codes for the same polypeptides. e.g. all humans have a gene which codes for eye colour.
- The TYPE of characteristic e.g. blue eyes is determined by the ALLELES of the gene (dominant or recessive). Organisms differ in alleles but NOT genes.
- The COMBINATION of alleles makes species and organisms within species individual.
- The more different alleles a species has, the greater the genetic diversity.
- The greater the genetic diversity, the more likely a species will adapt to environmental change. The species will have a wider range of alleles and so a wider range of characteristics. This increases the chance that an organism will have the characteristics needed to adapt to the new environment.
- Genetic diversity is reduced when a species has fewer alleles. This is known as a genetic 'bottleneck'. It may also be due to inbreeding
- Also known as ARTIFICIAL SELECTION.
- Involves taking samples of a species with favourable characteristics and mating them to produce offspring with these characteristics.
- Offspring which do not carry desireable characteristics are killed or spayed (neutered).
- This way, the alleles for unwanted charactertstics are phased out.
- The genetic variety is deliberately restricted to a smaller number of alleles, forming a genetic bottleneck and reduced biodiversity with each generation.
- This method is commonly used to breed high quality livestock and crop for farmers e.g. wheat has a resistance to disease through selective breeding.
The founder effect
- Occurs during primary colonization when a species first inhabits an area.
- The few individuals which inhabit the area carry only a small sample of the alleles for the whole species.
- The new population is not representative of the larger population. Therefore, biodiversity will decrease as only a few of the alleles from the species are present in the area.
- As the species on the area have fewer alleles, they are less able to adapt to change.
- Occasionally, drops in population can occur.
- Reasons may be natural (predators colonize the area) or anthropogenic (human destruction of habitats).
- The survivors will only possess a few of the alleles of the original species, so biodiversity is reduced.
- As the population re-grows, the genetic variation remains low as only a few alleles are passed down to the next generation.
- This reduction of biodiversity is known as a GENETIC BOTTLENECK.
Application and ethics
Whilst science can tell us what CAN be done, ethics tell us what OUGHT to be done.
- Research is expensive and money usually comes from an outside source. This outside source may bias the results e.g. the results would differ if the money came from a Farmer's Union, DEFRA, Greenpeace, etc.
- Scientists are peer-reviewed i.e. their work is monitored by other scienctists. Different scientists have different morals and ethics on breeding animals and the method used to do so.
When arguing for/against selective breeding, you may want to consider these points:
- Is selective breeding interfering with nature?
- What genes should be selected and who should decide this?
- How do we balance increased yield with animal welfare?
- By phasing out some alleles in species, could we be losing alleles which benefit the species and even consumers?
- Should we stop traditional methods of selective breeding?
Case study: Cattle Breeding
The breeding of cattle involves artificial selection and has done for hundreds of years. Consumers are demanding more from their meat at lower prices, so farmers are under pressure to produce quality livestock as cheaply as possible to compete. What has prompted this change?
One way of increasing the rate of selective breeding is to use ARTIFICIAL INSEMINATION. This is where semen from a desireable male is collected and inserted into the desired female vagina artificially. A single male can inseminate many females. This way, the usual process of courtship and natural selection is avoided so the breeding process is sped up.