## Using the Hardy Weinberg equilibrium

Scientists use the Hardy-Weinberg equilibrium when they know the proportions of the different alleles in the population and want to predict the proportions of the different genotypes in the population. Here's a simple example using one locus with two alleles.

Suppose that you decide to measure the proportion of the alleles A and a in a population. Call these proportions p and q, where p is the proportion of A alleles and q is the proportion of a alleles. Because of the way proportions work (they represent portions of 100 percent), you know that p + q = 1. If 70 percent of the alleles are A, 30 percent are a, and p = 0.7 and q = 0.3.

So now you've got the proportion of A and a, and you want the proportion of AA, Aa, and aa. Here's how you do it:

Plug in the numbers you got for p and q, and you get 0.49 + 0.42 + 0.09

Translation: If 70 percent of the gametes are A, 49 percent of the offspring will be AA (0.7 x 0.7 = 0.49). If you don't find that result, you know that other forces are at play in this population.

Imagine that the a allele is a lethal recessive gene. Anyone unlucky enough to get two copies of this gene dies, which means that you won't find any individuals in the population that are aa. That result is a deviation from the Hardy-Weinberg equilibrium. In this particular example, this deviation is the result of natural selection selecting against people with the aa allele.