Hardy-Weinberg Principle
Godfrey Hardy
(1877-1947)
p+q=1
Wilhelm Weinberg
(1862-1937)
Allele frequencies, assuming 2 alleles, one
dominant over the other
p= frequency of dominant allele
q= frequency of recessive allele
p² + 2pq + q² = 1
Genotype frequencies
P2 = frequency of homozygous
dominants
2pq = frequency of heterozygotes
q2 = frequency of homozygous
recessives
These equations allow us to calculate allele
frequencies in a population. Of course allele
frequencies in a population can change due
to any of the following factors:
• Gene flow
• Mutation
• Natural selection
• Genetic Drift
Thus, the Hardy-Weinberg equations are
only valid for a population that is not
undergoing any of these changes i.e. not
evolving. They do inform us if a population
is in equilibrium and we can use them to tell
if a population has evolved.
For a population to be in equilibrium (not evolving), the
following factors have to be present:
• No mutation
• No natural selection
• No gene flow into or out of then population
• Large population to avoid genetic drift problems
• All mating is totally random and is always successsful
Simulation of Hardy-Weinberg Equilibrium conditions:
http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/lif
e4e_15-6-OSU.swf
If all of these conditions are met, a population is said to be in
genetic equilibrium and is not evolving. If we can detect a
change in allele frequency, however, we can conclude that a
population has evolved.
Hardy –Weinberg can also be used to demonstrate why it is that recessive alleles do
not disappear in populations in genetic equilibrium:
http://bcs.whfreeman.com/pierce1e/pages/bcsmain_body.asp?s=23000&n=00020&i=23020.01&v=chapter&o=|00010|00020|00030|
00060|&ns=0&t=&uid=0&rau=0
Sample H-W question
http://nhscience.lonestar.edu/biol/hwe/
q1d.html