Cystic fibrosis is an autosomal recessive disorder caused by mutations in the CFTR gene - HSC - SSCE Biology - Question 10 - 2021 - Paper 1

Cystic fibrosis is an autosomal recessive disorder, meaning that an individual needs to inherit two copies of the recessive allele to express the disease phenotype.

From the data provided, we can identify the allele frequencies:

• Allele A: 98.33%
• Allele a1: 1.13%
• Allele a2: 0.08%
• Allele a3: 0.07%

The common genotypes that potentially lead to cystic fibrosis would be combinations of the recessive alleles (a1, a2, and a3). Given the high frequency of the dominant allele A (98.33%), individuals with at least one A allele will not exhibit cystic fibrosis.

Thus, the most common genotype will be the homozygous dominant genotype AA, which can be determined by:

• Frequency of AA:

P(AA) = p_A^2 = (0.9833)^2 ext{ (where } p_A = 0.9833 ext{ or 98.33%)}

This results in approximately 96.67%, so the most common genotype of cystic fibrosis patients (those who exhibit the disorder) will be those with homozygous recessive genotypes (aa), where 'a' represents any of the alleles a1, a2, or a3.

However, due to the low frequency of these alleles, while cystic fibrosis patients will predominantly possess a more significant representation of a1, a2, and a3, the commonest genotype for observed cases will remain largely A, due to the predominance of allele A in the population.