Genetic variation in populations can be analyzed and quantified by the frequency of alleles. Two fundamental calculations are central to population genetics: allele frequencies and genotype frequencies. **Genotype frequency** in a population is the number of individuals with a given genotype divided by the total number of individuals in the population. In population genetics, the **genotype frequency** is the frequency or proportion (i.e., 0 < *f* < 1) of genotypes in a population.

Although allele and genotype frequencies are related, it is important to clearly distinguish them.

**Genotype frequency** may also be used in the future (for "genomic profiling") to predict someone's having a disease or even a birth defect. It can also be used to determine ethnic diversity.

## Numerical example

As an example, let's consider a population of 100 four-o-'clock plants (*Mirabilis jalapa*) with the following genotypes:

**AA**

**Aa**

**aa**

When calculating an allele frequency for a diploid species, remember that homozygous individuals have two copies of an allele, whereas heterozygotes have only one. In our example, each of the 42 pink-flowered heterozygotes has one copy of the **a** allele, and each of the 9 white-flowered homozygotes has two copies. Therefore, the allele frequency for **a** (the white color allele) equals

This result tells us that the allele frequency of **a** is 0.3. In other words, 30% of the alleles for this gene in the population are the **a** allele.

Compare genotype frequency: let's now calculate the genotype frequency of **aa** homozygotes (white-flowered plants).

Allele and genotype frequencies always sum to less than or equal to one (in other words, less than or equal to 100%).

The Hardy–Weinberg law describes the relationship between allele and genotype frequencies when a population is not evolving. Let's examine the Hardy–Weinberg equation using the population of four-o'clock plants that we considered above:

if the allele **A** frequency is denoted by the symbol **p** and the allele **a** frequency denoted by **q**, then **p+q=1**. For example, if **p**=0.7, then **q** must be 0.3. In other words, if the allele frequency of **A** equals 70%, the remaining 30% of the alleles must be **a**, because together they equal 100%.

For a gene that exists in two alleles, the Hardy–Weinberg equation states that **( p^{2}) + (2pq) + (q^{2}) = 1**

If we apply this equation to our flower color gene, then

If **p**=0.7 and **q**=0.3, then

^{2}= 0.49

^{2}= 0.09

This result tells us that, if the allele frequency of **A** is 70% and the allele frequency of **a** is 30%, the expected genotype frequency of **AA** is 49%, **Aa** is 42%, and **aa** is 9%.

Genotype frequencies may be represented by a De Finetti diagram.