3.2.3 The Founder Effect, Genetic bottlenecks and selective breeding

In this lesson we looked at the following objectives:

3.2.2 Similarities and differences in DNA result in genetic diversity

  • Differences in DNA lead to genetic diversity
  • Selective breeding and its’ influence on genetic diversity
  • the founder effect
  • genetic bottlenecks

You produced some short (in some cases very short!) presentations about what each of these were. To recap:

  • both are an example of a phenomenon called genetic drift where the frequency at which different alleles occur in a population changes over time.

The founder effect

the founder effect

The founder effect is when a small number of individuals from a larger population break away from the main population and colonize a new area. The new population has a selection of the alleles of the main population but they won’t have all the alleles that the main population contains. Thus, the breakaway population contains alleles at a different frequency from the main population.

Look at the diagram above, the ratio of pink to red alleles in the main population is 2:1 whereas in the founder population it is 1:3. Another way of stating this is that the frequency of pink alleles in the main population is 2 out of 3 whereas in the founder population it is 1 out of 4.

We looked at the example of the Amish people. Here is the paper that we read in the lesson: Amish paper

And here I am talking (not too eloquently) about the founder effect:

Genetic bottlenecks

genetic bottlenecks

A genetic bottleneck occurs when some kind of event reduces the numbers of a species greatly. Many examples of this phenomenon are human caused but by no means all. Think of any endangered species alive today and you are thinking of a species that is undergoing a genetic bottleneck. Large amounts of genetic diversity are lost when the population is whittled down to only a few individuals. The diagram illustrates this. In the original bottle there are a range of different alleles but when the bottleneck occurs the remaining individuals only posses a blue and yellow allele. When numbers expand again all the individuals posses these alleles and there is very little variation compared to the original population.

Good examples of this are seen in elephant seals and cheetahs.

Selective breeding

In any selective breeding programme you are likely to see a special case of the founder effect. We discussed expamples relating to pedigree dog breeds but we could equally have chosen any kind of agricultural or domesticated species where mating between individuals is restricted.

Mating between members of the same family is called inbreeding. Lets take a herd of cows as an example of a domesticated species. If we chose to mate a cow and bull based on their physical characteristics, and we produce offspring which have desirable characteristics, so we mate the offspring with each other, then we are selectively inbreeding.

If we continue this process then we may generate a very large number of individuals all descended from our original bull and cow. The bull and the cow are the founders of this population and all their descendents are members of the same ‘family’. Hence not much genetic variation. To combat this effect farmers need to carefully manage their herds to reduce inbreeding between individuals carrying alleles for the same genetic disorders.

Here’s a link to a very detailed but excellent overview of the whole topic. It’s written for farmers and agricultural workers so offers an interesting perspective.

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