If a person has two alleles of the sickle cell anaemia gene, the person

Analyze the question

The question asks what it means when a person has "two alleles of the sickle cell gene." Here, "two alleles of the sickle cell gene" is understood to mean two copies of the sickle cell allele (HbS HbS). This is the usual reading in this WASSCE question.

Concept

Every person has two alleles for each gene, one from each parent. For the haemoglobin gene:

• HbA HbA: normal, no sickle cell disease.
• HbA HbS: heterozygous, the sickle cell trait (a carrier). The person is usually healthy and has some protection against malaria.
• HbS HbS: homozygous recessive, which gives sickle cell anaemia.

Step-by-step solution

1. Having two sickle cell alleles means the genotype is HbS HbS (homozygous).
2. A carrier (option A) has only one sickle cell allele, paired with a normal one, so A is wrong.
3. Option B is wrong. A person with HbS HbS is not immune, and will certainly pass an HbS allele to their children.
4. Option C: a person with HbS HbS has the disease, so this is correct.
5. Option D is wrong. Sickle cell is common among people of African ancestry, and ancestry does not determine whether someone has the disease.

Answer: C, the person has the disease.

Tip: Count the sickle alleles.

• 0 sickle alleles: normal (AA)
• 1 sickle allele: carrier (AS)
• 2 sickle alleles: disease (SS)

Two carriers (AS × AS) have a 25% chance of a child with the disease (SS) in each pregnancy. This is a favourite WASSCE and JAMB genetics question.