Lec 13-1 Flashcards
Population genetics
Study of genetics at a population level
Ancestor Finch population
Had variation in beak size and shape
Certain finches with some alleles thrived under changing conditions
Much of trait variation is derived from
Variation at the molecular/genetic level
Thus is inheritable
Population genetics definition
Branch of genetics that studies the genetic makeup of groups and how group’s genetic composition changes over time
Mendelian population
A group of interbreeding sexually reproducing individuals that have a common set of alleles, a GENE POOL
Example of Mendelian Population
Wolves of Isle Royale
In 1949 single breeding pair of wolves
crossed frozen ice and populated Isle
Royale
1959 - 20 wolves
1980 – 50 wolves
Cut off from mainland populations
interbreeding
This is an example of a Mendelian
population with a common gene pool
Sometimes a Mendelian population is
Too large to practically measure all the alleles/genotypes
In these cases, a representative sample of the population is considered and then the results are extrapolated to the entire gene pool
In order to understand how genetic variation changes in a
population over time,
Need to mathematically describe the gene pool
Gene pools can be described by the types and frequencies of
Genotypes in the population
Alleles in the population
Sum of all genotypic frequencies always equals
1
Gene pools can also be described by
Allelic frequencies
Different genotypes are just
Rearrangements of a set of alleles
There is always less
Alleles than genotypes
Allelic frequencies can be calculated in either of 2 ways:
- counting numbers of alleles
- the frequencies of genotypes
Calculating genotypic frequencies
f(AA)= Number of AA individuals/ N
f=frequencies
Calculating Allelic Frequencies by Counting Alleles
General equation
Frequency of an allele= number of copies of the allele/ number of copies of all alleles at the locus
AA=two alleles for the letter A
For locus with two alleles (A and a)
Equation
p= f(A)= 2nAA + nAa/2N
q=f(a)=2naa + nAa/2N
also p+q=1
N=population size
n= number with a certain genotype
For locus with two alleles (A and a)
Equation
only works for
2 alleles and autosomal
Calculating allelic frequencies for more than 2 alleles
From genotype numbers
slide 24
Calculating allelic frequencies for more than 2 alleles
From genotype frequencies
slide 25
How does segregation of alleles in gamete formation and then the combining of alleles during fertilization alter the genetic structure of a popualation
Hardy-Weinberg Law
G.H. Hardy and Wilhelm Weinberg
Made the Hardy-Weinberg Law
Hardy Weinberg Law
Mathematical model evaluating the effect of reproduction on genotypic and allelic frequencies of a population
Among the most important principles in population genetics
Assumptions of Hardy Weinberg
- Population is large
- Random mating
- No mutations, migration, or natural selection
Predictions of Hardy Weinberg
- Allelic frequencies of a population do not change
- Genotypic frequencies stabilize (will not change) after one
generation
Genotypic frequencies will be
Frequency of AA = p2
Frequency of Aa = 2pq
Frequency of aa = q2
Where p = f(A) and q = f(a)
In other words,
The Hardy-Weinberg Law says that if the assumptions are met,
Reproduction does not itself alter allelic or genotypic
frequencies
Hardy Weinberg States that after a generation of random mating
genotypic frequencies should stabilize and achieve
the Hardy-Weinberg proportions
When genotypes are in the expected proportions of p2 (AA),
q2(aa) , and 2pq (Aa),
the population is said to be in Hardy Weinberg equilibrium
