How does natural selection change allele frequencies within a population?

Individuals whose alleles give an advantage in the current environment survive and reproduce more successfully, so they pass those alleles to more offspring and the frequency of the advantageous allele rises over the generations.

What is meant by a selection pressure?

A selection pressure is an environmental factor, such as a disease, a predator or a shortage of food, that makes the survival and reproduction of some phenotypes more likely than others.

Define the term gene pool.

The gene pool is all the alleles of all the genes present in all the individuals of a population at a given time.

Describe the phenotype of a person with the genotype Hbᴬ Hbᴬ living where malaria is endemic.

They make only normal haemoglobin and have no anaemia, but they have no protection against the malarial parasite and are at high risk of severe, potentially fatal malaria.

Describe the phenotype of a person with the genotype Hbᴬ Hbˢ.

They have sickle cell trait: about half of their haemoglobin is haemoglobin S, so they are essentially healthy and sickle only under extreme oxygen shortage, and they are substantially protected against severe malaria.

Describe the phenotype of a person with the genotype Hbˢ Hbˢ.

They have sickle cell anaemia, with severe haemolytic anaemia and painful crises caused by sickled cells blocking vessels, and without medical treatment their survival is greatly reduced.

OCR A-Level Biology B H422

The Developing Cell: Cell Division and Cell Differentiation

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Patterns of Inheritance

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Population Genetics and Epigenetics

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Population Genetics and Epigenetics

Natural selection, sickle cell and malaria

3 things to know
Components 01, 02 and 03

Overview

Recall questions on natural selection, selection pressures and the gene pool, the phenotypes of Hbᴬ Hbᴬ, Hbᴬ Hbˢ and Hbˢ Hbˢ where malaria is endemic, heterozygote advantage, and how replacing glutamic acid with valine makes haemoglobin S polymerise and sickle cells.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

How does the sickle cell trait protect against malaria?

Key term

Why does the sickle cell allele reach a high frequency where malaria is common but stay rare elsewhere?

Evaluation

What is meant by heterozygote advantage?

Questions from this subtopic

Each one with the model answer underneath.

1
How does natural selection change allele frequencies within a population?
Model answer
Individuals whose alleles give an advantage in the current environment survive and reproduce more successfully, so they pass those alleles to more offspring and the frequency of the advantageous allele rises over the generations.
2
What is meant by a selection pressure?
Model answer
A selection pressure is an environmental factor, such as a disease, a predator or a shortage of food, that makes the survival and reproduction of some phenotypes more likely than others.
3
Define the term gene pool.
Model answer
The gene pool is all the alleles of all the genes present in all the individuals of a population at a given time.
4
Describe the phenotype of a person with the genotype Hbᴬ Hbᴬ living where malaria is endemic.
Model answer
They make only normal haemoglobin and have no anaemia, but they have no protection against the malarial parasite and are at high risk of severe, potentially fatal malaria.
5
Describe the phenotype of a person with the genotype Hbᴬ Hbˢ.
Model answer
They have sickle cell trait: about half of their haemoglobin is haemoglobin S, so they are essentially healthy and sickle only under extreme oxygen shortage, and they are substantially protected against severe malaria.
6
Describe the phenotype of a person with the genotype Hbˢ Hbˢ.
Model answer
They have sickle cell anaemia, with severe haemolytic anaemia and painful crises caused by sickled cells blocking vessels, and without medical treatment their survival is greatly reduced.

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The Developing Cell: Cell Division and Cell Differentiation

The Developing Individual: Meiosis, Growth and Development

The Development of Species: Evolution and Classification

Cells and Microscopy

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