Question
State the Hardy-Weinberg principle and explain the five conditions required for a population to be in equilibrium. Why is this principle important for understanding evolution?
Solution — Step by Step
Hardy-Weinberg equilibrium says that in an ideal population, allele frequencies and genotype frequencies remain constant from generation to generation. This is the “null hypothesis” of evolution — if nothing is acting on a population, nothing changes.
For a gene with two alleles — dominant A (frequency = p) and recessive a (frequency = q):
Genotype frequencies in the next generation follow the expansion of :
Where = frequency of AA, = frequency of Aa, and = frequency of aa. The key insight: these proportions stay the same every generation, forever — as long as the five conditions hold.
| Condition | What violates it |
|---|---|
| Large population size | Genetic drift (random allele loss) |
| Random mating | Sexual selection, mate preference |
| No mutation | New alleles entering the gene pool |
| No gene flow | Migration in or out |
| No natural selection | Some genotypes survive/reproduce better |
Every one of these violations is a mechanism of evolution. Hardy-Weinberg is essentially a list of what causes evolution.
Problem: In a population, 16% of individuals show the recessive phenotype (aa). Find allele frequencies.
Since , we get
Therefore
Genotype frequencies: AA = , Aa = , aa =
Always start from the recessive phenotype — it’s the only one where genotype (aa) equals phenotype directly.
If you test a real population and find allele frequencies are changing over generations, Hardy-Weinberg tells you something is acting on the population. The principle doesn’t describe reality — it describes the baseline you’d expect with zero evolutionary pressure.
Why This Works
Think of Hardy-Weinberg as asking: “What would happen if mating were completely random and nothing else interfered?” The binomial expansion just describes the probability of randomly drawing two alleles from the gene pool. If you pick allele A with probability p twice independently, you get AA with probability . Simple probability.
The reason genotype frequencies stabilise after just one generation of random mating (this is called Hardy-Weinberg equilibrium being reached instantly) is because random mating shuffles existing alleles without creating new ones or removing any. The allele pool itself doesn’t change.
This is why population genetics treats Hardy-Weinberg as the starting point. Real populations deviate from it — and measuring that deviation tells us which evolutionary force is acting and how strongly.
Alternative Method
For numerical problems in NEET, you can work backwards from phenotype counts directly without memorising the formula structure:
If a question gives you number of individuals instead of frequencies:
- Count aa individuals → this is
- Divide by total N to get
- Take square root to get q
- Subtract from 1 to get p
- Calculate for heterozygotes
This avoids confusion between frequency and number — a very common NEET trap.
Common Mistake
Students often try to calculate p by counting dominant phenotype individuals and dividing by total. Wrong. Dominant phenotype includes both AA and Aa individuals — you can’t separate them without genetic testing. The only phenotype that directly gives you a genotype frequency is the recessive (aa). Always start with , never with .
NEET frequently asks you to find the frequency of carriers (heterozygotes Aa), not just allele frequencies. After finding p and q, don’t forget: frequency of carriers = , not just pq. Missing the factor of 2 costs you the mark every time. This type appeared in NEET 2022 and NEET 2019.