population genetics. relative frequency of an allele the number of times an allele occurs in the...
TRANSCRIPT
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? Homozygous recessive Homozygous dominant Heterozygous
How many T alleles are there in the gene pool? 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
No new organisms introducing more alleles
3.) No mutations 4.) No environmental factors causing natural
selection 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 Hardy-Weinberg 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
RR Rr rr
Prediction 36% 48% 16%
1st gen.
2nd gen
3rd gen
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…