Q17.5-2CC - Campbell Biology

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.

Step-by-Step Solution

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 1: 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.

Step 2: 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.

Step 3: Effect of sickle-cell trait on heterozygous individual

Individuals heterozygous for sickle-cell allele are said to have sickle-cell traits. This is because the 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.