Population Genetics 1
What is a Population?
- Population can be defined as a group of interbreeding individuals of the same species
sharing a common geographical area.
- Gene pool is the sum of the alleles in a population at a particular time.
- Evolution is the change of allele frequencies over time in a population.
Important Things To Consider
- An assumption usually made is that there is random mating among the individuals
within a particular population.
- However, remember that in many cases, there is no random mating among individuals
living in the same geographical area, due to
o Ethnic/Religious Isolation
o Linguistic/Political Isolation
o Other forms of isolation
- In those cases, there is population subdivision, or in other words, population structure.
- We won’t review the consequences of population structure in this course, but you should
know that this happens quite often in human populations.
Allele Frequency
- What is the allele frequency of the yellow (Y) allele in this population?
- Four Y alleles our of a total of ten =
o 4/10 = 0.4 or 40%
- Since there are only two alleles the frequency of the blue (B) allele is 100% - 40% = 60%
Alleles are not Loose in the Population but are Paired Together in Individuals
- These pairs are called genotypes
- Homozygotes have two copies of the same allele
- Heterozygotes have two different alleles
Which Scenario is More Likely?
- Inheritance works in randomly, where alleles combine randomly and the frequency will
depend on how frequently the allele is present in the population
- In the same way that the alleles are segregating independently in the formation of
gametes, they are found independently assorted among individuals of a population.
Hardy-Weinberg
- This random assortment of alleles to form individuals in the population is key to
understand the Hardy-Weinberg principle, or Hardy-Weinberg equilibrium.
- In fact, the Hardy-Weinberg principle can be considered an extension of the Punnet
square.
- Let’s see why….
The Punnet Square
- In the typical Punnet square, the genotype outcome of a cross is dependent on the
possible combinations of gametes transmitted by each parent.
- In this case, the probabilities of passing each allele are 0.5 (50%) if the parents are
heterozygotes and 1 (100%) when they are homozygotes. Thus, the genotype
probabilities of the resulting progeny are….
o BB:25% BY: 50% YY:25% o BB:50% BY:50%
For Individuals to Populations…
- In a population, when randomly selecting alleles from the gene pool, the probability of
selecting a particular allele is equal to the frequency of that allele
- Standard notation for allele frequencies:
o p = freq(B); 0.6 = 60% in this population
o q = freq(Y); 0.4 = 40% in this population
HWE: Generalizing the Punnett Square for Population Genetics
- In a population, the probability of individuals receiving each allele is equal to the allele
frequency in the population (in this case p and q). p and q can vary between 0 and 1.
- Note that the probability is not restricted to 0.5 or 1 as is the case when two persons mate.
o pp + pq +qp + qq = 1
Hardy-Weinberg Equilibrium
- pp + pq +qp + qq = 1
- pp + pq + pq + qq = 1
- p2 + 2pq + q2 = 1
- p + 2pq + q = 1
- p = frequency (A1)
- 1 – p = q = frequency (A2)
- p + 2pq + q = 1
The Importance of HWE
- The HWE is very important because it states that under certain circumstances
o There is a simple theoretical relationship between allele frequencies and
genotype frequencies in a given population.
o Allele frequencies will not change from generation to generation in a given
population.
- Thus, knowing genotype frequencies, we can easily estimate the allele frequencies in
the population, and those allele frequencies can be used to predict the expected
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