Population GeneticsLearning Objectives

1. Define a population, a species, microevolution and population genetics.

2. What is the population gene pool and what is a fixed allele?3. Learn how to calculate the allelic frequencies in a population

from the genotypic frequencies (same as lab exercise).4. What is the Hardy-Weinberg theorem and what is its formula or

equation?5. How is the equation for the Hardy-Weinberg equilibrium used to

calculate allelic and genotypic frequencies?6. Using the Hardy-Weinberg equation, calculate the frequency of

carriers of a particular disease within a population when given the numbers of affected individuals with a recessive disease.

7. What are the population conditions under which the Hardy-Weinberg equilibrium applies, and how are they related to the evolutionary process?

8. Define genetic drift and its two causes?9. What are the four factors of microevolution?10. How do new alleles originate?

A population is a localized group of individuals that belong to the same species.

A species is a group of populations whose individuals have the potential to interbreed and produce fertile offspring in nature.

Population genetics

Study of the extensive genetic variation within populations

Recognizes the importance of quantitative characters

The population’s gene pool consists of all alleles at all gene loci in all individuals of a population at any one time

Microevolution:

• Defined as a change in the allele frequencies in the gene pool of a population from generation to generation

• Populations not individuals are the units of evolution

- If all members of a population are homozygous for the same allele, that allele is said to be fixed

Calculating the allelic frequencies from the genotypic frequencies

What is the allelic frequency (of R and r) in this population?

Do not confuse allelic frequency with genotypic frequency -

Genotypic frequency:

RR= 320/500 = 0.64

Rr = 160/500= 0.32

rr = 20/500 = 0.04

What is the allelic frequency in a population of 500 flowers?

How many total alleles are there?

500 X 2 = 1000

Frequency of R allele in population

RR + Rr = 320 X 2 + 160= 640+160= 800

800/1000 = 0.8 =80%

Frequency of r allele = 1- 0.8 = 0.2 =20%

or

rr +Rr = 20 X 2+ 160= 200

200/1000 = 0.2

- The gene pool of a non-evolving population remains constant over the generations

- The shuffling of alleles that accompanies sexual reproduction does not alter the genetic makeup of the population

- Meiosis and random fertilization do not change the allele and genotype frequencies between generations

The frequencies of alleles and genotypes in a population’s gene pool will remain constant over generations unless acted upon by factors other than Mendelian segregation and recombination of alleles

The Hardy-Weinberg theorem describes the gene pool of a nonevolving population

Theorem assumes that fertilization is completely random and all male-female mating combinations are equally likely.

Because each gamete has only one allele for flower color, we expect that a gamete drawn from the gene pool at random has a 0.8 chance of bearing an R allele and a 0.2 chance of bearing an r allele.

Population geneticists use p to represent the frequency of one allele and q to represent the frequency of the other allele.The combined frequencies must add to 100%;

therefore p + q = 1.

If p + q = 1, then p = 1 - q and q = 1 - p.

It is possible to calculate the genotypic frequencies of RR, Rr, rr in next generation based on allelic frequency of p = 0.8 and q =0.2

Let’s watch the following video segment and see how…

Using the allelic frequencies and rule of multiplication, the probability for genotypic frequencies is:

RR = (0.8 x 0.8) = 0.64

Rr or rR = (0.8 x 0.2) + (0.2 x 0.8) = 0.16 + 0.16

= 0.32

rr = (0.2 x 0.2) = 0.04

Given R = p and r = q

The genotype frequencies should add to 1:

p2 + 2pq + q2 = 10.64 + 0.32 + 0.04 = 1.

In the wildflower example, p is the frequency of red alleles (R) and q of white alleles (r).– The probability of generating an homozygous

dominant offspring is p2 (an application of the rule of multiplication).In this example, p = 0.8 and p2 = 0.64.

– The probability of generating an homozygous recessive offspring is q2.In this example, q = 0.2 and q2 = 0.04.

– The probability of generating heterozygous offspring is 2pq.In this example, 2(0.8 x 0.2) = 0.32.

This general formula is the Hardy-Weinberg equation and it is used to calculate:

- frequencies of alleles in a gene pool if we know the frequency of genotypes

or

- the frequency of genotypes if we know the frequencies of alleles

Estimate the percentage of the human population that carries the allele for the inherited disease, phenylketonuria (PKU) - in other words, what is the genotypic frequency of heterozygote carriers?

First, we need some background info:

– About 1 in 10,000 babies born in the United States is born with PKU, which results in stunted mental development and other problems if left untreated.

– The disease is caused by a recessive allele.

http://www.ygyh.org/pku/whatisit.htm

- The frequency of homozygous recessive individuals = q2 = 1 in 10,000 or 0.0001.

- The frequency of the recessive allele (q) is the square root of 0.0001 = 0.01.

- The frequency of the dominant allele (p) is

p = 1 - q or 1 - 0.01 = 0.99.Is this what we’re looking for? No, we are looking for the

percentage of carriers (a.k.a. heterozgotes):

The frequency of carriers (heterozygous individuals) is 2pq = 2 (0.99 x 0.01) = 0.0198 or about 2%.

• About 2% of the U.S. population carries the PKU allele.

Populations at Hardy-Weinberg equilibrium must satisfy five conditions.(1) Very large population size. In small populations,

chance fluctuations in the gene pool, genetic drift, can cause genotype frequencies to change over time.

(2) No migrations. Gene flow, the transfer of alleles due to the movement of individuals or gametes into or out of our target population can change the proportions of alleles.

(3) No net mutations. If one allele can mutate into another, the gene pool will be altered.

(4) Random mating. If individuals pick mates with certain genotypes, then the mixing of gametes will not be random and the Hardy-Weinberg equilibrium does not occur.

(5) No natural selection. If there is differential survival or mating success among genotypes, then the frequencies of alleles in the next variation will deviate from the frequencies predicted by the Hardy-Weinberg equation.

Evolution usually results when any of these five conditions are not met - when a population experiences deviations from the stability predicted by the Hardy-Weinberg theory.

Genetic Driftchanges allelic frequencies in populations

1. The bottleneck effect

2. The founder effect -

Isolation event from a larger poulation (e.g. colonization)

2 causes of Genetic Drift:

Microevolution is the generation-to-generation change in a population’s frequencies of alleles.

Caused by four factors:

1. genetic drift – due to sampling/ bottleneck and founder effects

2. natural selection- accumulates and maintains favorable genotypes in a population

3. gene flow- genetic exchange due to migration of fertile individuals or gametes between populations

4. Mutation- transmitted in gametes can immediately change the gene pool of a population

New alleles originate only by mutation– rare and random.– mutations in somatic cells are lost when the

individual dies.– Only mutations in cell lines that produce

gametes can be passed along to offspring.

Diversity within a population

Humans have relatively little genetic variation

- Gene diversity- average # of heterozygous loci

about 14% in humans

- Nucleotide diversity- difference in nucleotide sequences is only 0.1%.

Any two people have the same nucleotides at 999 out of every 1,000 nucleotide of their DNA.

Macro-evolution reflects the changes within a species that take place over a long period of time as a result of natural selection