Transcript Population Genetics - Solon City Schools

Population
Genetics
Relative Frequency of an Allele



The number of times an
allele occurs in the gene
pool, given as a percentage
Relative frequency has
nothing to do with dominant
or recessive
The recessive allele can
occur more frequently
Population Genetics review

Given a population of 300 plants…


How many total height genes are there?
Given that 100 plants are short (recessive trait),
200 are tall, and 50 are homozygous tall, how
many are there of each genotype?




How many T alleles are there in the gene pool?


Homozygous recessive
Homozygous dominant
Heterozygous
What is this alleles’ frequency in the population?
How many t alleles are there in the gene pool?

What is this alleles’ frequency in the population?
Sources of Genetic Variation

What do you think are some sources of
genetic variation?
A note on sexual reproduction…



Sexual reproduction can produce many
different phenotypes
Sexual reproduction does NOT change
relative frequency of alleles in a population
Think about shuffling a deck of cards


Shuffling cards gives you different hands
It won’t change the number of kings in a deck
Population Genetics

In the early 1900s these two men discovered
how the frequency of a trait’s alleles in a
population could be described
mathematically.
G H Hardy – British Mathematician
Wilhelm Weinberg – German Doctor
Population Genetics

For every phenotype how many alleles do
you have???

2


1 from Mom and 1 from Dad
These scientists figured out an equation that
can be used to figure out the percentages of
alleles and genotypes that are in a
population.
Genetic Equilibrium:
Hardy-Weinberg Principle


Allele frequency in a population will
remain constant unless an outside
factor causes those frequencies to
change
When allele frequencies remain
constant, we call this genetic
equilibrium
Genetic Equilibrium

In order for their equation to work the
population has to be in GENETIC
EQUILIBRIUM

This means that there is no change in the gene
pool = no evolution
Genetic Equilibrium


1.) Population size is large
2.) No gene flow in the population



3.) No mutations
4.) No environmental factors causing natural
selection


No new organisms introducing more alleles
No trait is favorable over another
5.) Random mating must occur
The Hardy-Weinberg Equation




p2 + 2pq + q2 = 1
p2 = frequency of the homozygous dominant
genotype
2pq = frequency of the heterozygous
genotype
q2 = frequency of the homozygous recessive
genotype
Hardy-Weinberg




p – frequency of the dominant allele
q – frequency of the recessive allele
Because there are only 2 alleles, the
frequency of the dominant allele (p) and the
frequency of the recessive allele (q) will add
up to 1 or 100%
p+q=1
Hardy-Weinberg


In reality, no population satisfies the HardyWeinberg equilibrium completely
However, in large populations with little
migration and little natural selection, it can
approximate gene frequencies
Hardy-Weinberg Example

In a population of 100 people 28 of them were found
to have freckles and 72 were not. We learned in
class during our genetics unit that having freckles is
a recessive trait and not having them is because of
a dominant trait. If this population is in genetic
equilibrium then solve for the allelic frequencies and
the variables in the hardy-weinberg equation:
Queens full of Jacks!



Let’s Mate!
red card=dominant allele=R
black card=recessive allele=r
P2 + 2pq + q2
Prediction
1st gen.
2nd gen
3rd gen
RR
Rr
rr
36%
48%
16%
Predicted vs Actual

If this population is in equilibrium, we should
have the predicted % for our genotypes…

We have…20 rr envelopes and 30RR
envelopes
Are we in equilibrium?

What should happen?
If we are evolving…
If we are not…