Intro to Population Genetics

Terms:Population: Group of interbreeding or

potentially interbreeding organisms

Population Genetics: Branch of genetics that studies the genetic makeup of groups and how the group’s genetic composition changes over time.

Microevolution: evolution at the genetic level

Terms Cont.Gene Pool: Total genetic information in a

population at one time; all the alleles in the populationEx. The genotypes of a certain population are

below:AA = 45 Aa= 35 aa=

20

In the gene pool there are 125 (A) alleles and 75 (a) alleles in this gene pool.

Population Geneticists: study how we get from the gene pool now to the gene pool in the future through microevolution

Gene PoolAllele frequency

Phenotype frequency

Genotype frequency

Allele Frequencyf(A) = (2nAA + nAa)/ 2N this is symbolized by the

letter p

f(a) = (2naa + nAa)/2N this is symbolized by the letter q

Where nAA = number of homozygous dominant individuals

nAa = number of heterozygous individualsnaa = number of homozygous recessive individualsN = number of organisms in the population Ex. Aa- One A and one a

 

Allele Frequency Calculations in a Gene Pool:

AA=50 Aa=20 aa=30

 

Calculate the genotypic and allelic frequencies from the numbers above.

Practice ProblemThe genotypic numbers of a population of

bears is below. Calculate the genotypic and allelic frequency for this gene pool:

BB – 34 Bb- 56 bb- 16

Predicting Phenotypes• You can use alleleic frequencies and

probability rules to predict future phenotypic and genotypic ratios.

• What would expect the phenotypic ratios to be of the next generation?

Allele Frequency

• Although four o’clock flowers differ phenotypically from generation to generation, the allele frequencies tend to remain the same.

The Hardy-Weinberg Genetic Law

The primary goal of population genetics is to understand the process that shapes a population’s gene pool.

First we must know what effect reproduction has on genotypic and allelic frequencies.

The Hardy-Weinberg Law allows us to model the effect of reproduction of the two frequencies.

The Hardy-Weinberg Genetic Law• The Law is actually a mathematical model

that allows us to see the effect of reproduction on genotypic and allelic frequencies.

• It makes several assumptions about the population and provides two key predictions if the assumptions are met. • Large, random mating populations • No genetic drift • No selection • No mutation • No migration

Uses for Hardy-Weinberg Equation

• Test for change in gene pool• Estimate the frequency and number of

carriers in a dominant/recessive trait• Basis for modeling Mechanisms of Gene

Pool change

Example of ProblemsIs a population in Hardy-Weinburg

equilibrium?How many carriers are found in a

population?

To find if a population is in equilibrium:Find allele frequencies (p and q)Find expected genotypic frequenciesFind expected genotypic countsRun a Chi-Square Test (Less than 3.84)

Chi-Squared Equation

O= Observed Values E=Expected Values

Sigma= “The Addition of All”

Example ProblemA population has the following genotypes

SS: 57 SC: 169 CC: 29

Is this population in Hardy-Weinberg Equilibrium?

Other exampleIn a population of 400 individuals 300

express the dominant phenotype. How many heterozygous individuals (carriers) would you expect?

Practice Problems• Two scientist working at Glacier Lake,

Colorado discovered three genotypes (RR, Rr, rr) at a locus in some weird pine tree. The observed numbers were:

RR 135Rr 44rr 11

Do the trees show Hardy-Weinberg Equilibrium at this locus?

Practice Problems• Cystic Fibrosis is a recessive disorder that is

found in approximately 1 out of every 2000 people. Assuming the population is in Hardy- Weinberg what percentage of people are carriers of the disorder.