Sickle cell anaemia is a disease caused by a mutation in the HBB gene. The presence of the mutant HBB allele in the genome of a person can be detected using a variety of techniques. DNA can be extracted and purified from cell samples taken from a person. The polymerase chain reaction (PCR) can be used to amplify the region of DNA containing the mutant HBB allele, if it is present in the genome of the person. This PCR process amplifies only the mutant HBB allele and not other DNA sequences. Outline the steps involved in PCR and explain how PCR can be carried out so that only the mutant HBB allele is amplified.

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3(b)Pcr Electrophoresis Dna ProfilingMedium2 marks

Restriction enzymes and electrophoresis can be used to determine the alleles of the HBB gene that make up the genotype of a person. The restriction enzymes:

produce one fragment when only the mutant HBB allele is present
produce two smaller fragments when only the normal HBB allele is present.

Figure 3.1 shows the results of electrophoresis of DNA samples from four people, 1 to 4. Only DNA fragments from the HBB gene locus are shown.

Figure 3.1, a gel electrophoresis diagram with four loading wells labelled 1 to 4 above the gel; lane 1 shows one band, at the higher (less migrated) position; lane 2 shows three bands, one at that higher position and two more, closely spaced, further down the gel; lane 3 shows two bands, only the two lower, closely spaced bands; lane 4 shows three bands in the same pattern as lane 2.

State which people, 1–4, show a genotype that is homozygous for the normal HBB allele, a genotype that is homozygous for the mutant HBB allele, and a genotype that is heterozygous for the HBB gene.

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3(c)Genetic Disorders ScreeningHard4 marks

Microarray analysis can be used to detect the mutant HBB allele in the genome of a person. Outline how microarray analysis is used to detect the presence of the mutant HBB allele in the genome of a person.

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