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10/10 Daily Catalyst

- 1. Define allele frequency?
- 2. What causes evolution?
- 3. What is effected more by changes in allele frequencies?

10/10 Daily Catalyst Answers

- 1. Define allele frequency?
- How often alleles appear in the population
- 2. What causes evolution?
- Mutations, genetic drift, gene flow, natural and artificial selection, and sexual reproduction.
- 3. What is effected more by changes in allele frequencies, large or small populations?
- Small populations

10/10 Class Business

- Evolution quiz #7 TOMORROW
- Grades are due TOMORROW
- 10/16 Dress like a boss day
- 10/17 Parent night
- Evolution exam 10/21

10/10 Agenda

- Daily Catalyst
- Class Business
- Objective
- Hardy Weinberg notes
- Practice Time
- Reading

Objective

- We will be able to use data from Hardy-Weinberg equilibrium to analyze genetic drift and effects of selection on a population.

Causes of Evolution

- 1. Genetic Drift
- 2. Gene flow
- 3. Mutations
- 4. Natural Selection
- 5. Sexual Reproduction

When there is no change in allele frequency, the population is said to be stable and non-evolving.

- This is called HARDY-WEINBERG EQUILIBRIUM.
- Named after two scientists describing a stable population.

In your opinion thus far, is Hardy-Weinberg Equilibrium highly likely? Why or why not?

Hardy-Weinberg Equilibrium

FIVE Conditions of Hardy-Weinberg Equilibrium:

The population must be large.

Population must be isolated to prevent gene flow(No immigration or emigration).

No mutations occur.

Mating is completely random.

No natural selection (Every offspring has an equal chance of survival without regard to phenotypes).

Hardy-Weinberg Equilibrium

- Condition #1 can be met. It is important to have large populations in order that the loss or addition of genes is not a factor. By contrast, small populations experience genetic drift. Additionally, if a small population moves to another area or becomes isolated, the gene pool will be different from the original gene pool. And the founder effect comes into play.
- Condition #2 can only be met if the population is isolated. If individuals immigrate or emigrate from the population, the allele frequencies change and evolution occurs.
- Condition #3 cannot ever be met since mutations always occur. Thus mutational equilibrium can never be met.

Hardy-Weinberg Equilibrium

Condition #4 can never be met. Mating is never random. Pollen from an apple tree in Ohio is more likely to pollinate a tree in Ohio than one in Washington state.

Condition #5 can never be met. There will always be variation. Variation can help organisms survive longer and/or reproduce more effectively.

Since 3 out of the 5 H-W conditions can never be met, evolution DOES occur and allele frequencies do indeed change.

How do we calculate the equilibrium?

- Allele Frequency:
- p + q = 1
- Genotype frequency:
- p2 + 2pq + q2 = 1

- q represents the recessive trait
- pq represents the…

Applying the H-W Model

Here we go with our frogs again! Let’s suppose that in a population of 100 frogs, 36 were green (CGCG), 48 were blue (CGCR) and 16 were red (CRCR) and there was total random mating.

Thus, it can be assumed that 60% of all the gametes (eggs and sperm) should carry the CG allele and 40% of the gametes should carry the CRallele.

Applying the H-W Model

A population Punnett square is shown above. It indicates that the next generation should have the following offspring distribution: 36% green (CGCG), 48% blue(CGCR), 16% red (CRCR). When the second generation gets ready to reproduce, the results will be the same as before.

Applying the H-W Model

So, the allele frequency remains at 0.40 CG and 0.60 CRthus no evolution is taking place.

Let’s suppose that there is an environmental change that makes red frogs more obvious to predators. How is the population affected and now the population consists of 36 green, 48 blue, and 6 red frogs?

Now, allele frequencies are changing and there is an advantage to being green or blue but NOT red. Evolution is indeed occurring.

Deriving the H-W Model

Examine this Punnett square again. If p represents the allele frequency of CG (dominant) and q represents the allele frequency of CR (recessive) then two equations for a population in Hardy-Weinberg equilibrium can be derived where the following genotypes are represented by:

CGCG = p2CRCR = q2CGCR = 2pq

Mathematically then p + q = 0.60 + 0.40 = 1 (1st H-W equation)

So, the Punnett square effectively crossed (p + q ) (p + q ) which gives

p2 + 2pq + q2 = 1 (2nd H-W equation)

Example #1

- If 9% of the population has blue eyes, what percent of the population is hybrid for brown eyes? Homozygous for brown eyes?
- 1. The trait for blue eyes is homozygous recessive (bb) is represented by q2.
- Q2= 9% or .09
- Why q2 and not just q?
- 2. To solve for just q, take the square root of q2.
- Square root of .09=.3

Example #1

- 3. Now q= .3
- How do we get p?
- Use the equation p + q = 1
- P + .3 = 1
- P= .7
- 4. The hybrid condition is 2pq
- Do we have the p and q? YES!
- 2(.7)(.3)= 42%
- 42% of the population is hybrid for brown eyes

Example #1

- 5. Homozygous dominate is just p2
- Do we know p? YES!
- (.7)squared = 49%

Example #2

- Determine the percent of the population that is homozygous dominant if the percent of the population that is homozygous recessive is 16%.
- What do we know?
- bb= 16%
- BB= ?

Example #3

- Determine the percent of the population that is hybrid if the allelic frequency of the recessive trait is .5?
- What do we know?
- What do we need to find?

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