HARDY-WEINBERG LAW
What is it?
Hardy-Weinberg principle/law is used to calculate the probability of genotypes and the frequencies. It can find the frequency of each genotype to then calculate the frequency of phenotype traits of a population, or vise versa.
This calculation can be confusing, and often the AP test questions will try to trick you. Here is a sample gene pool to help explain (it is grossly oversimplified, but for these purposes, we're keeping it easy to understand).
RR Rr RR rr Rr RR rr Rr
In this gene pool, we have 8 individuals. It is important to remember that p is the frequency of the dominant allele as a genotype. In this gene pool, we can count 9 Rs, meaning that the dominant allele appears in 56.25% of the gene pool. We now know that p = 0.5625 (9/16).
Next, we can find q, which is the frequency of the recessive allele as a genotype. We could do the same math process as above to find it, or we could use logic to figure out that if we know the frequency of R, we can just subtract from 1 to then find the frequency of r. 1 - 0.5625 = 0.4375 = q If your brain works better in percentages, this means that 43.75% of the alleles in the gene pool are recessive.
Now that you have p and q, you can plug those values into the equation to calculate the frequency of the homozygous dominant (RR), the frequency of homozygous recessive (rr), or the frequency of heterozygous (Rr)
Not too bad, right?
Here's where it can be tricky.
On an AP question, they may tell you that the trait of rolling your tongue is dominant, and that _____% of the population cannot roll their tongue. This is NOT q! This is q²! The information only tells us the phenotype, but since a dominant allele would override a recessive allele, we don't know how much of the population can roll their tongue yet carry a recessive allele.
Similarly, if they said that _____% of the population can roll their tongue, this is p², NOT p.
If you can catch this, it's quite simple, all you would have to do to find p or q would be to take the square root and then you can proceed with the calculation as normal, but if you miss this step, you'll fall into the trap and get it wrong! Most questions will tell you one piece of information and ask you to find another, so pay close attention to both what they give you (whether they're giving you a genotype or phenotype frequency), as well as what they're asking you to find! Some problems will take more steps than others!
Below is a simple sample problem to give you a better sense of how a solved problem looks.
The calculations are made under 5 assumptions:
- A large breeding population
- Random mating
- No change in allelic frequency due to mutation
- No immigration or emigration
- No natural selection
But these assumptions will almost certainly never be true altogether, so what's the point of the calculation? To provide a control group! By creating controlled numbers, we can then analyze real situations of population shift, relative to which of the assumptions apply or not.
If you've forgotten some of our genetic vocab, the above information probably doesn't mean much to you! Review the terms below if you need to :)
Dominant vs. Recessive - Dominant alleles are named as such because they will 'override' the effect of a recessive allele. A recessive trait can only be physically represented if no dominant gene is present.
Homozygous vs. Heterozygous - Homozygous is when the 2 alleles have the same phenotype effect, this can occur as homozygous dominant (RR) or homozygous recessive (rr). Heterozygous would be one dominant and one recessive (Rr).
Gene Pool - The gene pool refers to the collective presence of each kind of allele in a population from each individual.
Population - When we say population, we mean a group of interbreeding individuals of a species.
