Heterozygote advantage: sickle cell and malaria
About this app
A population over many generations: malaria strikes AA, sickle cell anaemia strikes SS, carriers survive best. The frequency of the sickle cell allele settles at q* = s/(s+t) (balanced polymorphism). With a malaria switch, slow motion for selection and mating, and a random mode with genetic drift.
With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.
Balanced polymorphism
People with one sickle cell allele (AS) are protected against severe malaria, people with two alleles (SS) develop sickle cell anaemia. In malaria regions carriers therefore have the highest fitness: AA has 1 − s, AS 1, SS 1 − t. With random mating the genotypes arise in the ratio p², 2pq and q². The frequency of allele S rises while it is rare, because it then sits almost only in favoured AS, and it falls when it is common, because more SS arise. At equilibrium q* = s/(s + t), so about 0.13 for s = 0.15 and t = 1.
What the app shows
The population appears as figures in three genotype colours, mixed or sorted by genotype. In each generation the victims of malaria and anaemia fade first, then the new generation arises. Alongside run the course of q with the equilibrium line q*, the carrier share 2pq and bars for alleles, genotypes and fitness. A random mode works with a finite population and shows genetic drift.
Things to try
Start with a rare allele and watch the curve approach q*, then with a common allele: it reaches the same line from above. Switch malaria off and see how slowly the allele becomes rarer afterwards. In random mode, shrink the population and compare several runs: the smaller the group, the more q fluctuates around the equilibrium.
Frequently asked questions
- What is the heterozygote advantage in sickle cell anaemia?
- Carriers with one normal and one sickle cell allele (AS) hardly suffer from sickle cell anaemia and are also protected against severe malaria. In malaria regions they therefore survive better than both homozygotes. This advantage of the heterozygotes keeps the sickle cell allele in the population.
- Why does the sickle cell allele not disappear?
- When the allele is rare it sits almost only in carriers, who survive best in malaria regions, so it becomes more common. When it is common, more SS homozygotes arise, who rarely have children, so it becomes rarer. In between the equilibrium q* = s/(s + t) is reached.
- What happens to the sickle cell allele without malaria?
- Without malaria the carriers lose their advantage and selection only acts against SS. The allele then becomes rarer, but only slowly, because most copies hide in carriers. With t = 1, q_n = q₀/(1 + n·q₀); starting at 0.13 the frequency halves only after about eight generations.
- What can I set in the Heterozygote Advantage app?
- You set the malaria pressure s, the disadvantage t of SS, the starting value and the time span, switch malaria off, run generations in slow motion and sort the population by genotype. In random mode you choose the population size and start new runs.
Subject: Biology (Gymnasium)
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