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 MathematicianWilhelm Weinberg – German Doctor

Population Genetics For every phenotype how many alleles do you have??? 2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 p 2 + 2pq + q 2 = 1 p 2 = frequency of the homozygous dominant genotype 2pq = frequency of the heterozygous genotype q 2 = 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 RRRrRrrr Prediction36%48%16% 1 st gen. 2 nd gen 3 rd 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…