Population Genetics (Outline)

• Definition of terms of population genetics: population, species, gene, pool, gene flow

• Calculation of genotypic of homozygous dominant, recessive, or heterozygous individuals , when given appropriate information

• Calculation of allelic frequencies of dominant and recessive alleles • Definition of microevolution and macroevolution. • Hardy-Weinberg Equilibrium as state on non-evolving population and its

condition http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/life4e_15-6-OSU.swf • What is the Hardy-Weinberg Equilibrium and what are its conditions. • Source of new alleles in any population • Applications of Population Genetics

Population genetics as a field Study of the extensive genetic variation

within populations that already exist Recognizes the importance of quantitative

characters

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 a nature.

Population Genetics

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

Changes allelic frequencies in populations

Evolution Microevolution changes in allelic

frequencies within the gene pool of a population from generation to generation

Macroevolution changes in allelic

frequencies over 100’s and 1000’s of generations

• 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

Allele Frequencies

Allele frequency = # of particular allele

Total # of alleles in the population

Count both chromosomes of each individual Allele frequencies affect the frequencies of the three

genotypes

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

Calculating the allelic frequencies from the genotypic frequencies

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

- 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

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 Drift changes allelic frequencies in populations

The bottleneck effect The founder effect

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.

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

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

Example Phenylketonuria (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

Example: PKU an autosomal recessive trait

Table 14.1

Phenotypic Frequencies vary between populations

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) is p = 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.

Table 14.3

The Carrier Frequency of an Autosomal Recessive (Cystic Fibrosis)