8.3 - Gene pools

What is the definition of a population?

All the organisms of a particular species that live in the same habitat

What are some examples of selection pressures?

• Predation

• Disease

• Competition (e.g. food, habitats, mates, etc)

• Environmental conditions (e.g. temperature)

How do selection pressures change allele frequencies within a population?

• Organisms with advantageous characteristics are more likely to survive & reproduce to produce offspring

• Therefore, their favourable alleles get passed on, while unfavourable alleles die out

What is directional selection?

• Occurs when one extreme phenotype (e.g. tallest) is favoured over the other extreme (e.g. shortest)

• This happens when the environment changes in a particular way

What is stabilising selection?

• Occurs when the intermediate phenotype is selected over the extreme phenotypes, & tends to occur when the environment doesn’t change much (e.g. babies of intermediate birth weight are most likely to survive)

• Individuals closest to the mean are favoured, & any new characteristics are selected against → results in low diversity

What is disruptive selection?

• Occurs when both extremes of phenotype are favoured over the intermediate types (e.g. birds with large & small beaks feed on large & small seeds respectively & both do well, but birds with intermediate beaks have no advantage, & are selected against)

• Over time, the population becomes phenotypically divided & new species may develop

What is the definition of genetic drift?

• A change in a population’s allele frequencies that occurs due to chance, rather than selection pressures

• In other words, it is caused by ‘sampling error’ during reproduction

What is meant by a population bottleneck?

A catastrophic event (e.g. volcanic eruption, wildfire, flood, etc) dramatically reduces the size of a population, thereby decreasing the variety of alleles in the gene pool & causing large changes in allele frequencies

What is meant by the founder effect?

A small number of individuals become isolated, forming a new population with a limited gene pool, with allele frequencies not reflective of the original population

What is the Hardy-Weinberg principle?

Allows us to estimate the frequency of alleles in a population, as well as if allele frequency is changing over time

What are some assumptions made by the Hardy-Weinberg principle?

• No mutations occur to create new alleles

• No migration in or out of the population

• No selection, so alleles are all equally passed onto the next generation

• Random mating

• Large population

Explain the Hardy-Weinberg equation for calculating allele frequency

• The frequencies of each allele for a characteristic must add up to 1.0

• the equation is therefore: p + q = 1

  • where p = frequency of the dominant allele

  • where q = frequency of the recessive allele

Explain the Hardy-Weinberg equation for calculating genotype frequency

• The frequencies of each genotype for a characteristic must add up to 1.0

• The equation is therefore: p² + 2pq + q² = 1

  • where p² = frequency of homozygous dominant genotype

  • where 2pq = frequency of heterozygous genotype

  • q² = frequency of homozygous recessive genotype

What is the step-by-step method to using Hardy-Weinberg calculations?

(all calculations must be carried out using proportions, not percentages)

1. Examine the questions to determine what piece of information you have been given out the population → in most cases, this is the percentage/frequency of the homozygous recessive phenotype (q²) or the dominant phenotype (p²)

2. Find out the value of q by taking the square root of q²

3. Determine p by subtracting q from 1 (i.e. p = 1 - q)

4. Determine p² by multiplying p by itself

5. Determine 2pq by multiplying p x q x 2

6. Check your calculations are correct by adding up the vales for p² + q² + 2pq (the sum should equal 1 or 100%)

Answer this Hardy-Weinberg question

In humans, the ability to taste the chemical PTC is inherited as a dominant characteristic. 360 out of 1000 college students could not taste the chemical

Frequency of recessive phenotype (360 out of 1000)

(a) State the frequency of the gene for tasting PTC

(b) Determine the number of heterozygous students in this population

• (a) = 64%

• (b) = 480

Answer this Hardy-Weinberg question

A type of deformity appears in 4% of a large herd of cattle. Assuming the deformity was caused by a recessive gene

Data: Frequency of recessive genotype (4% deformity)

(a) Calculate the percentage of the herd that are carriers of the gene

(b) Determine the frequency of the dominant gene in this case

• (a) = 32%

• (b) = 0.8 (8%)