Population Genetics (Learning Objectives)

• Recognize the quantitative nature of the study of population genetics and its connection to the study of genetics and its applications.

• Define the terms population, species, allelic and genotypic frequencies, gene pool, and fixed allele, genetic drift, bottle-neck effect, founder effect.

• Learn how to calculate the genotypic and allelic frequencies in a population.Given the appropriate information about a population you should be able to calculate the genotypic and allelic frequencies of homozygous dominant, recessive, or heterozygous individuals (following the example discussed in class), and the chance or probability that two unrelated individuals in a particular population will have an affected child for an autosomal recessive condition

• Explain the difference between microevolution and macroevolution.• Visit this website to learn the factors that lead to changes in genotypic and

allelic frequencies between generations, i.e . the factors that lead to microevolution: http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/life4e_15-6-OSU.swf

• Recognize Hardy-Weinberg equilibrium of a non-evolving population and the factors necessary to satisfy this equilibrium.

• Learn about the only origin of new alleles in populations.

Population geneticsStudy of the extensive genetic variation

within populations that already exist

Recognizes the importance of quantitative characters

A population is a localized group ofindividuals that belong to the samespecies.

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

Population Genetics

Alu genotypic and phenotypic frequencies among Bio 210A

students

1 2 3 4

Fall 2013 (Monday)

1 2 7 8 3 4 5 6 16 17 15 .

12 13 10 11 14 18 19 24 22 21 20

Fall 2013 (Wednesday)

59 45 50 60 48 65 58 61 41 46 39 47

53 52 51 36 54 55 56 57 68 67 66 64

Calculating the allelic frequencies from the genotypic frequencies What is the allelic frequency (of R and r) in this population?

Allele Frequencies

Allele frequency =# of particular allele

Total # of alleles in the population

Count both chromosomes of each individualAllele frequencies affect the frequencies of the three

genotypes

Genotypic frequencyRR= 320/500 = 0.64 Rr = 160/500= 0.32rr = 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 populationRR + Rr = 320 X 2 + 160= 640+160= 800

800/1000 = 0.8 =80%Frequency of r allele = 1- 0.8 = 0.2 =20%

orrr +Rr = 20 X 2+ 160= 200200/1000 = 0.2

Population Genetics Calculations

Determine the genotypic and phenotypic frequencies in an existing population, using Hardy-Weinberg equilirium.

In the earlier calculations of allelic frequencies in flower population, as 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.

Calculating the genotypic frequencies of RR, Rr, rr in next generation based on allelic frequency of p = 0.8 and q =0.2

The genotype frequencies should add to 1:p2 + 2pq + q2 = 1

In the wildflower example p is the frequency of red alleles (R) and q of white alleles (r).– The probability of generating an RR 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 rr offspring is q2.In this example, q = 0.2 and q2 = 0.04.

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

This general formula is the Hardy-Weinberg equation 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

Applications of Population Genetics

1. Calculation of the % carriers in the population for a certain disorder

2. Calculating the chance or probability that two unrelated individuals in a particular population will have an affected child for an autosomal recessive condition

ExamplePhenylketonuria (PKU) in an autosomal recessive genetic disease that can lead to mental retardation, if unmanaged

– All babies born in the United States are screened for PKU.

– Information can be used to calculate the % carriers in the population

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

Phenotypic Frequencies vary between populations

Example: PKU an autosomal recessive trait

Calculation of % PKU carriers from screening

About 1 in 10,000 babies in US are born with PKU- 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) isp = 1 - q or 1 -

0.01 = 0.99.The frequency of carriers (heterozygous individuals) is

2pq = 2 x 0.99 x 0.01 = 0.0198 or about 2%.• About 2% of the U.S. population carries the PKU allele.

The Carrier Frequency of an Autosomal Recessive (Cystic Fibrosis)

Calculating the chance or probability that two unrelated individuals within a population

will have an affected childProbability that both are carriers =

1/23 x 1/23 = 1/529Probability that their child has CF = 1/4 Therefore, probability = 1/529 x 1/4 =

1/2,116

Definitions• Gene pool = The collection of all alleles in the

members of the population

• Population genetics = The study of the genetics of a population and how the alleles vary with time

• Gene Flow = Movement of alleles between populations when people migrate and mate

Calculation of genotypic & allelic frequencies in populations

Evolution

Microevolution small changes due to changing allelic frequencies within a population from generation to generation

Macroevolution large changes in allelic frequencies over 100’s and 1000’s of generations leading to the formation of new species

Microevolution:• A change in the allele frequencies in the

gene pool of a population from generationto generation

• Populations not individuals are the unitsof evolution

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

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

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

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 non-evolving population

Hardy Weinberg animation http://zoology.okstate.edu/zoo_lrc/biol1114/t

utorials/Flash/life4e_15-6-OSU.swf

practice questions http://nhscience.lonestar.edu/biol/hwe.html

Caused by factors:1. Non-Random mating2. Genetic drift – due to sampling/ bottleneck &

founder effects, geographic & cultural separation

3. Migration- of fertile individuals4. Mutation- in germline cells transmitted in

gamete5. Natural selection- accumulates and maintains

favorable genotypes in a population

Macroevolution

Populations at Hardy-Weinberg equilibrium must satisfy six conditions.(1) Very large population size.(2) Random mating.(3) No migrations.(4) No natural selection.(5) No genetic drift(6) No net mutations.

Evolution 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

The bottleneck effect

The founder effect

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.