🧬Biocode Explorer - Hardy-Weinberg Equilibrium

Hardy-Weinberg Principle

Basic Genetic Concepts for Population Studies

Before diving into population genetics and the Hardy-Weinberg principle, it’s important to understand some basic genetic terms and how they are connected.

Gene: The word “gene” is one you may have heard or used many times. When it comes to defining it, we might think of it in a complex way. But in simple words, a "gene is a segment of DNA that carries instructions to produce a protein or determine a specific trait", such as hair color or blood type. Genes are the fundamental units of heredity.

Mutation: This is a very interesting process and an important source of variation. It is defined as a "permanent change in the DNA sequence of a gene or chromosome". Such changes are called mutations, and they can create new versions of a gene (alleles). Mutations are the ultimate source of genetic diversity within a population and play a key role in evolution.

Allele: An allele is a version of a gene. For example, the gene for eye color may have a “brown” allele and a “blue” allele. Different alleles can produce different traits in individuals.

Genotype: The genotype is the combination of alleles an individual carries for a gene. For example:

BB = two brown alleles form brown eyes
Bb = one brown and one blue allele form brown eyes (because brown is dominant)
bb = two blue alleles form blue eyes

Understanding Genetic Variation from Individuals to Populations

Understanding how genes, mutations, and alleles combine to form an individual’s genotype sets the stage for a bigger question: What happens to these genetic variations when we look at an entire population rather than a single person?

This is where population genetics comes in. Instead of focusing on one individual or family, population genetics examines how alleles and genotypes are distributed across a group of interbreeding individuals and how these distributions change over time. Studying these patterns helps scientists understand processes such as evolution, the spread of genetic diseases, and the maintenance of diversity in populations.

To analyze these patterns, scientists often use a fundamental model called the Hardy-Weinberg principle (HWE), which provides a baseline for predicting how allele and genotype frequencies behave under ideal conditions. In the next section, we will explore this principle in detail.

The Hardy-Weinberg Principle

The Hardy-Weinberg principle is like a starting rule in population genetics. It says: “If nothing in the environment changes, if no new mutations appear, if mating is random, and if the population is very large (Hardy-Weinberg assumptions), then the genetic makeup of that population will stay the same and remain in balance.

In simple words, allele frequencies won’t change; they’ll remain frozen in time, and the population will be in equilibrium.

Now comes the exciting part, how can we determine whether the allele frequencies in our population are in equilibrium or not? The answer lies in a simple yet powerful equation: the Hardy-Weinberg equation.

p² + 2pq + q² = 1

where:

p² = the fraction of people with two dominant alleles (like BB) [Homozygous Wil or Homozygous Dominant]
2pq = the fraction with one dominant and one recessive allele (like Bb) [Heterozygous]
q² = the fraction with two recessive alleles (like bb) [Homozygous Recessive or Homozygous Mutated]

Here, p is the frequency of the dominant allele, and q is the frequency of the recessive allele.

Since we’re only dealing with two alleles (p and q), they always add up to 1:

p+q=1

Why Is This Useful?

The Hardy-Weinberg principle acts as a “null model”, a kind of “control experiment” for populations.

If a population fits the Hardy-Weinberg equation, it means nothing is changing and the population is stable.
But if it doesn’t fit, then something interesting is going on! It could be natural selection, mutations, genetic drift, migration, or non-random mating.

So, Hardy-Weinberg is like a detective’s tool: if the data doesn’t fit, it tells us evolution might be at work.

Example: Hardy-Weinberg Equilibrium with Eye Color 👀

Imagine a small village where everyone’s eye color is decided by just one gene with two versions (alleles):

B = Brown eyes (dominant)
b = Blue eyes (recessive)

👩‍👩‍👧 Population of 100 villagers:

40 people have BB (brown eyes)
50 people have Bb (still brown eyes but carrying the blue-eye gene)
10 people have bb (blue eyes)

Curious Question:

If we look at the children and grandchildren of this village, will the number of brown-eyed and blue-eyed people stay the same… or change?

To answer this, we need to predict the allele frequencies in the next generation.

Thus, it can be said that the Hardy–Weinberg principle acts as a prediction tool sometimes even called the “null model of population genetics” because it tells us what should happen to allele and genotype frequencies if nothing disturbs the population.

Situation 1

According to the HWE assumptions, if the ratios should stay the same as long as nothing “disturbs” the population.

That means: