Population Genetics (Learning Objectives)faculty.sdmiramar.edu/bhaidar/Bio 130/lectures/Population...
Transcript of Population Genetics (Learning Objectives)faculty.sdmiramar.edu/bhaidar/Bio 130/lectures/Population...
Population Genetics (Learning Objectives)
• Define the terms population, species, allelic and genotypic frequencies, gene pool, and fixed allele, genetic drift, bottle-neck effect, founder effect.
• Explain the difference between microevolution and macroevolution.• Review how genotypic and allelic frequencies are calculated.
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).
• Visit this website to learn the factors that lead to changes in genotypic and allelic frequencies between generations: 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.• What are the factors that lead to microevolution?• What is the source of new alleles within any population?
A population is a localized group of interbreeding individuals in a given geographic area
A species is a group of populations who interbreed and produce fertile offspring
Definitions
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
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
Frequency of a trait varies in different populations. Example: PKU an autosomal recessive trait
Table 14.1
Phenotype Frequencies
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
What is the allelic frequency (of R and r) in this population?
Calculating the allelic frequencies from the genotypic frequencies
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%
or rr +Rr = 20 X 2+ 160= 200200/1000 = 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 Hardy-Weinberg theorem describes the gene pool of a non-evolving population
Hardy Weinberg animationhttp://zoology.okstate.edu/zoo_lrc/biol1114/t
utorials/Flash/life4e_15-6-OSU.swf
practice questionshttp://science.nhmccd.edu/biol/hwe.html
p + q = 1
p = allele frequency of one alleleq = allele frequency of a second allele
p2 + 2pq + q2 = 1
p2 and q2 Frequencies for each homozygote
2pq Frequency for heterozygotes
All of the allele frequencies together equals 1
All of the genotype frequencies together equals 1
Hardy-Weinberg Equation
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 results when any of these five conditions are not met - when a population experiences deviations from the stability predicted by the Hardy-Weinberg theory.
Caused by four 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
Microevolution
Figure 14.3
Calculating the Carrier Frequency of an Autosomal Recessive
What is the probability that two unrelated Caucasians will have an affected child?
Probability that both are carriers =1/23 x 1/23 = 1/529
Probability that their child has CF = 1/4 Therefore, probability = 1/529 x 1/4 =
1/2,116
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.