The Evolution of Populations

Population GeneticsThe study of how populations change genetically over time

Studying the genetics of a population provides a foundation

for evolution Microevolution - the change in the genetic makeup of a population from generation to generation

Grass growing in soil full of heavy metals (from a mine)

Same species of grass, cannot grow on metal rich soil

Populations Localized group of the

same species that can interbreed and produce fertile offspring

Liger - the result of a cross between a male lion and a female tiger - ligers are sterile

Gene poolAll the genes (all alleles of those genes) present in a population at any given time

Allele frequencyHow often a particular allele shows up in a population is the allele frequencyWhat is the allele frequency for “B’ in this population?There are 7 “B” out of 14 total alleles, so 50%

p = “A” and q = “a”The sum of p & q = 100% of the alleles

p q

The Hardy-Weinberg TheoremIf allele frequencies stay the same from one generation to the next, then no evolution is occurring.

Hardy Weinberg EquationA = dominant allele a = recessive allelep + q = 1

p = frequency of A alleleq = frequency of a allele

p2 + 2pq + q2 = 1Where

p2 = frequency of AA genotype2pq = frequency of Aa genotypeq2 = frequency of aa genotype

H-W equilibrium:1. The population is very large

2. Mating is random3. no mutations

4. no migration (gene flow)5. no selection

If one of these conditions is broken, an evolutionary force is acting to change allele frequencies, and the population may not be in H-W equilibrium.

Recessive allele – q = ___

Dominant allele – p = ___

Recessive phenotype – q2 = ___

Homozygous dominant – p2 = ___

Heterozygous – 2pq = ____

p2 + 2pq + q2 = 1p + q = 1

Quick Check Using the Hardy Weinberg

equation: What symbol is used to

represent the dominant allele? What symbol is used to

represent the recessive allele?

p

q

Quick Check Using the Hardy Weinberg

equation: What does p2 represent in a

population? What does q2 represent? What does 2pq represent?

The proportion of homozygous

dominant individuals

The proportion of homozygous

recessive individuals

The proportion of heterozygous

individuals

Practice Problem p + q = 1 where p = “A” and q = “a”

p2 + 2pq + q2 = 1 where p2 = AA, 2pq = Aa, q2 = aa Directions: For each problem: first determine what value(s) you are given and fill those in the table next fill in the rest of the values third determine what value(s) are asked for and put that answer in the answer box.

Practice ProblemA group of 100 people has 22 that cannot taste PTC paper. Tasting is dominant (TT or Tt) and not tasting is recessive (t)What percent of the alleles in the gene pool are “T” and what percent are “t”?

p = ______q = ______p2 = ______2pq = ______q2 = ______

Practice ProblemA group of 100 pugs has 4 with a lethal recessive disease (aa). How many pugs are carriers for the disease (Aa)? p = ______

q = ______p2 = ______2pq = ______q2 = ______

More problems1. A population of hamsters has a gene

consisting of 90% M alleles (black) and 10% m alleles (gray). Mating is random.

Determine the proportion of offspring that will be black and the proportion of offspring that will be gray.

More problems2.You are working with pea plants and you

found 36 plants out of 400 that were dwarf. Calculate the frequency of the tall gene

Determine the number of heterozygous pea plants

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

What is the frequency of the gene for tasting PTC?

Determine the number of heterozygous students in this population.

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

Calculate the percentage of the herd that are carriers of the gene.

Determine the frequency of the dominant gene in this case.

5.Albinism is recessive to normal pigmentation in humans. The frequency of the albino allele was 10% in a population.

Determine the proportion of people you would expect to be albino.

6. It is known that 64% of a large population exhibits the recessive phenotype of a characteristic controlled by 2 alleles (one is dominant over the other).

Determine the following: The frequency of the recessive allele The percentage that are heterozygous for this trait The percentage that exhibit the dominant trait The percentage that are homozygous for the

dominant trait The percentage that have one or more recessive

alleles

7. Assume you placed 50 pure bred black guinea pigs (dominant) with 50 albino guinea pigs (recessive) and allowed the population to attain genetic equilibrium (over several generations).

Frequency of dominant allele – 50% Frequency of recessive allele – 50% Determine the proportion of the population

that becomes white.

Extra Credit Option Choose a simple

dominant/recessive trait in humans: Tongue rolling Free vs attached

earlobes Widow’s peak

Survey at least 50 people

Determine p, q, p2, 2pq, and q2 for your population

Name Dominant phenotype

Recessive phenotype