Question

In humans a normal allele for one of the polypeptide chains of the protein hemoglobin is incompletely dominant to the allele for an abnormal polypeptide chain. Individuals with two abnormal alleles have the disease sickle cell anemia; they only make irregular polypeptide chains. Heterozygotes are called carriers or people with the trait. Their hemoglobin contains both the normal polypeptide and the irregular one.

Problem:

Two of your friends want to marry and have children. They both are carriers for Sickle Cell Anemia. Based on this information,

What is the probability that they could have children with sickle cell anemia?

What is the probability that they could have children with sickle cell trait?

What is the probability that they could have children with normal hemoglobin (no sickle cell; no carrier)

Answer 1

Answer: In a person with sickle cell anemia (SCA), one of the polypeptides that contribute to the structure and functioning of hemoglobin (Hb) is abnormal. Since humans have two sets of each gene (known as alleles), there are two genes for Hb polypeptide as well. Thus, even if one of the alleles that codes for the polypeptide is abnormal (mutated) and produces an irregular polypeptide chain, the second allele which is normal can produce a regular functioning polypeptide chain. Thus, the normally functioning allele will give normally functioning Hb. Since one of the alleles is abnormal and the other is normal, the resultant effect is a blend of both characters. In other words, such a person will have a mixture of normal and abnormal hemoglobin in their red blood cells without having sickle cell anemia. Such a condition is called sickle cell trait (incomplete dominance).

In the scenario given in the problem, two of the friends wish to marry and have children. They both are carriers for SCA i.e. they display the sickle cell trait. The genetics of the possibilities of transfer of the alleles to their children is depicted in the figure below.

As shown in the figure above, both the parents are carriers of the trait. The chromosome in black represents a normal allele for Hb polypeptide whereas the chromosome in red represents an abnormal one. Thus, both the parents have one normal and one abnormal allele and will hence produce two types of gametes; one with a normal allele and one with abnormal allele.

As shown in the figure, after the mating there is a 50% (2 in 4) probability that the child shall have one normal (black) and one abnormal allele (red). This means, there is a 50% (2 in 4) probability that they could have children with sickle cell anemia.

Similarly, there is a 25% (1 in 4) probability that the child shall have both abnormal alleles (red). That is, there is a 25% (1 in 4) probability that they could have children with sickle cell trait.

Lastly, there is a 25% (1 in 4) probability of the child having both normal alleles, one from each parent. In other words, there is a 25% (1 in 4) probability that they could have children with normal hemoglobin (no sickle cell; no carrier).