Question

Sickle cell disorder is a genetic disorder caused by a substitution mutation in the gene encoding haemoglobin, the protein on the blood cells responsible for carrying oxygen. This mutation causes the haemoglobin protein to fold incorrectly preventing the protein from carrying oxygen to the red blood cells. Sickle cell anaemia is inherited as a codominant trait meaning that both alleles are expressed equally and that one is not dominant over the other. The allele for normal Haemoglobin is represented by S, and the allele for abnormal haemoglobin is represented by s.
If two heterozygous individuals mate and produce offspring, what proportion of their offspring are expected to have fully sickled red blood cells?
A) 1
B) 0.75
C) 0.5
D) 0.25
E) 0

Answer 1

Hint: Sickle cell anaemia is a disease that affects the structure of the haemoglobin protein. For an individual to have this disease, it is necessary that they are homozygous for the alleles. A heterozygous individual, who has one normal allele and one mutated allele, will show no symptoms of sickle cell disorder, but he or she will be a carrier for condition and is said to have sickle cell trait.

Complete Answer:
Two heterozygous individuals mean two carriers or individuals with sickle cell trait. If the normal allele is S, then the abnormal allele is s. Therefore they are both genotypically Ss for sickle cell anaemia.
- Crossing S and s from one parent with S and s from another gives the combinations SS, Ss, sS, and ss. If total offspring are 4, one will be homozygous recessive. Therefore the proportion who have fully sickled RBCs are ¼ or 0.25. Option D is the correct answer.

	S	s
S	SS	Ss
s	Ss	ss

- Option A is incorrect. The possibility of all offspring having sickled RBCs will occur only if both parents have Sickle cell anaemia themselves.
- It is possible for 0,75 to be normal, but not for 0.75 to have the trait in any combination. Option B is incorrect. If one parent has the disorder, and the other is heterozygous, carrying the defective allele, then 0.5 will have sickled red blood cells. Option C is not the correct answer.
- With both parents being carriers for the disorder, the chance that none of the offspring will have it cannot be 0. Option E is incorrect.

Hence the correct answer is option B.

Note: Haemoglobin is a quaternary protein. It has four subunits, two of which are alpha globin and two are beta globin chains. The glutamate to valine substitution occurs in the beta globin gene, HBB.