Why is the sickle cell allele common in regions where malaria is widespread?

Heterozygotes survive malaria better than people without the allele, so they are more likely to reproduce. The allele is passed on and stays common in the population.

What is aneuploidy? Give an example.

Aneuploidy is having one or more chromosomes more or fewer than normal. Down syndrome results from three copies of chromosome 21, giving 47 chromosomes.

What is DNA sequencing, and how does it help genetic screening?

DNA sequencing determines the exact order of bases in a length of DNA. Mutations can be identified, and probes can be designed to detect them.

CCEA A-Level Life and Health Sciences 0008

Experimental Techniques

Topic overview Standard solutions, titrations and qualitative tests Rates of reaction and organic preparations Physics measurements: oscillations, resistivity, lenses and critical angle Biological tests, colorimetry, chromatography and osmosis

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Genetics, Stem Cell Research and CloningA2 option

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Genetics, Stem Cell Research and Cloning

Genetic disease, screening and counselling

3 things to know
A2 5

Overview

Sickle cell anaemia and malaria, aneuploidy, DNA sequencing, probes and PCR in screening, genetic counselling, oncogenes and tumour testing, carrier risk calculations and evaluating genetic testing.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

How can an allele be useful in one context but harmful in another? Use sickle cell anaemia as the example.

Key term

What mutation causes sickle cell anaemia?

Key term

How does haemoglobin S lead to the symptoms of sickle cell anaemia?

Questions from this subtopic

Each one with the model answer underneath.

1
How can an allele be useful in one context but harmful in another? Use sickle cell anaemia as the example.
Model answer
People heterozygous for the sickle cell allele are partly protected against malaria. People homozygous for the allele have sickle cell anaemia.
2
What mutation causes sickle cell anaemia?
Model answer
A single base substitution in the gene for the beta chain of haemoglobin changes one triplet. The amino acid valine replaces glutamic acid, producing haemoglobin S.
3
How does haemoglobin S lead to the symptoms of sickle cell anaemia?
Model answer
Haemoglobin S forms long fibres when oxygen levels are low, distorting red blood cells into a sickle shape. Sickled cells block small vessels, causing pain, and break down early, causing anaemia.
4
Why is the sickle cell allele common in regions where malaria is widespread?
Model answer
Heterozygotes survive malaria better than people without the allele, so they are more likely to reproduce. The allele is passed on and stays common in the population.
5
What is aneuploidy? Give an example.
Model answer
Aneuploidy is having one or more chromosomes more or fewer than normal. Down syndrome results from three copies of chromosome 21, giving 47 chromosomes.
6
What is DNA sequencing, and how does it help genetic screening?
Model answer
DNA sequencing determines the exact order of bases in a length of DNA. Mutations can be identified, and probes can be designed to detect them.

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