Question

Individuals heterozygous for the sickle-cell allele are generally healthy but show phenotypic effects of the allele under some circumstances (see Figure 14.17). Explain in terms of gene expression.

Short answer

Heterozygous individuals possess both sickle cell and wild-type alleles. As a result, they have both regular and sickle-cell hemoglobin molecules in their blood. Such individuals are healthy under normal conditions.

However, they show symptoms of the sickle-cell disease during prolonged oxygen deficiency.

Step-by-step solution

Sickle cell disease

Sickle cell disease is a result of substitution mutation.It occurs when the valine in the beta-globin chain of the hemoglobin molecule is substituted by glutamic acid.This leads to the synthesis of an abnormal beta-chain in the hemoglobin.

The hemoglobin protein aggregates to form long polymers, thereby transforming into a sickle shape.

Homozygous and heterozygous individuals for the sickle-cell disease

A homozygous individual possesses both sickle cell alleles. As a result, all of the hemoglobin molecules are sickle-shaped. However, heterozygous individuals have both wild-type and sickle-cell allele.

These two alleles are codominant; thus, such individuals have both regular and sickle cell hemoglobin molecules.

Effect of sickle-cell trait on heterozygous individual

Individuals heterozygous for sickle-cell allele are said to have sickle-cell traits. This is becausethe presence of one sickle-cell allele affects the phenotype. However, such individuals are usually healthy as they have both regular and sickled hemoglobin cells in their blood.

When such individuals are exposed to a longer duration of low oxygen, the regular hemoglobin in their blood is converted to a sickle shape.Thus, almost all hemoglobin molecules become sickle-shaped.

As a result, heterozygous individuals show symptoms of sickle-cell disease under reduced blood oxygen.