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BI 1: Population Genetics

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Kimberly Christian

on 19 September 2017

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Transcript of BI 1: Population Genetics

Population Genetics
A Brief Review of Genetics
Dominant Phenotype
Recessive Phenotype
Complete Dominance - p
attern of inheritance of some traits in which two alleles exist in the population: dominant (A) & recessive (a), and the expression of these alleles in heterozygous (Aa) individuals is the same as homozygous dominant (AA)
a region of DNA that codes for the expression of a specific character (ex: hair color)
variations of a specific gene that result in different traits
(ex: blonde, brown, red)
description of the two alleles an individual carries for a specific gene
description of the physical expression of an individual's genetics
individual with two of the same allele (AA or aa)
individual with two different alleles (Aa)
Using Math to Measure Evolution
study of the changes in allele and genotype frequencies in a population
The Hardy-Weinberg Principle
An example problem:
Study the population of pigs below.
Given that pink is dominant to black, what percentage of pigs shows the dominant phenotype? What percentage expresses the recessive phenotype?
With that information, calculate the following:
a. frequency of dominant allele? (p)
b. frequency of recessive allele? (q)
c. frequency of homozygous dominant genotype? (p )
d. frequency of heterozygous genotype? (2pq)
e. frequency of homozygous recessive phenotype? (q )
states that frequencies of alleles and genotypes in a population will remain constant from generation to generation (evolution will not happen), provided that all five H-W conditions are met
If those five conditions are met...the population is NOT evolving and is in Hardy-Weinberg Equilibrium.
In reality - this is impossible!
The Five Requirements for a Hardy-Weinberg Population:
1. No mutations (DNA of individuals does not change.)
2. Random mating (Individuals show no preferences for
3. No natural selection (No individuals have adaptations
over others.)
4. Extremely large population size (Ideally, infinitely
5. No gene flow (Individuals cannot enter or leave
Remember, every individual inherits two alleles per gene!
No real population can ever achieve all five H-W requirements.
So, what is the point?
H-W serves as a control group in the study of an evolving population.

If we expect a non-evolving population to show H-W ratios, then, if we do not see ratios remain constant over time, the population is experiencing evolution.
So, the math...
H-W allows us to calculate a series of variables concerning the frequencies of alleles and genotypes in a population, given we know one of the variables.
Hardy-Weinberg Variables
p=frequency of dominant allele
q=frequency of recessive allele

p =frequency of homozygous dominant
2pq=frequency of heterozygous
q =frequency of homozygous recessive
p + q = 1
p + 2pq + q = 1
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