what is the hardy-weinberg principle?

(p² + 2pq + q² = 1)
p is the frequency of the dominant allele
q is the frequency of the recessive allele for a trait controlled by a pair of alleles (A and a).
In other words, p equals all of the alleles in individuals who are homozygous dominant (AA) and half of the alleles in people who are heterozygous (Aa) for this trait in a population.
This equation is used to discover the probable genotype frequencies in a population and to track their changes from one generation to another.

The Hardy-Weinberg principle tells us that If nothing occurs that might cause a change in allele frequencies or genotype frequencies in a population, then:

a. Allele frequencies and genotype frequencies reach genetic equilibrium in just one generation.

b. At equilibrium, allele frequencies and genotype frequencies do not change from one generation to the next.


They used a simple probability model to figure that out.  The following gives a decent description of their reasoning.

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?Now Hardy and Weinberg independently reasoned that we could model the mating of a population by considering picking alleles from that population's gene pool at random. A way to think about this is to imagine that the gene pool of a population is like a big bucket of alleles. Each time two individuals mate, it's as though you reach into that bucket, pick out one allele, which would be a gamete from one individual, then reach in again, pick out another allele, which would be a gamete from another individual, and join them. And that would be a new individual in the next generation, and you throw those two alleles in another bucket, which represents the gene pool of the next generation." (Professor Stephen Nowicki)
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Hardy-Weinberg equilibrium relies upon 5 major assumptions.  The following are those assumptions, with questions related to the random selection of alleles from a bucket (as described above) that should help you see how each one, if not met, would throw off equilibrium.

1. No Natural Selection
What if one type of allele was less slippery than the other?

2. Infinite Population Size
What if you were to select only 10 pairs of alleles?

3. No Gene Flow
What if someone were to pour part of their bucketful of alleles into your parent bucket?

4. Random Mating
What if you were to select the second allele of each pair not at random, but in order to make it match the type of allele that had just been drawn?

5. No Mutations
What if some of the alleles of one type, in either bucket, were to spontaneously change into a new, third allele?


In natural populations, it is unlikely that all 5 of them will be met. Still, not all of the allele/genotype frequencies in a population would necessarily be altered: many could be in equilibrium while others changed.

If a particular gene?s genotypes conform to p^2 + 2pq + q^2 = 1 (where p^2 is the frequency of the A1A1 genotype, 2pq is the frequency of the A1A2 genotype, and q^2 is the frequency of the A2A2 genotype), then that gene is in Hardy-Weinberg equilibrium.

A departure from Hardy-Weinberg equilibrium in a sexually reproducing population means that the population is evolving at that locus.  Evolution at this level (below the species level) is referred to as microevolution.

It's an evil robot. I went to Hardy Weinberg Middle School and the Principle was a really big and very mean guy. Some say he was a monster, but I think he was a nasty android.