Question

With the sickle cell allele of the hemoglobin gene, a heterozygous individual (one normal and one sickle allele) has a phenotype in between the homozygous normal and homozygous sickle genotypes. What is this relationship between alleles called?

Answer 1

The relationship between alleles shown by a heterozygous individual with a phenotype intermediate between homozygous dominant and recessive conditions is called incomplete dominance, as exemplified by sickle cell anemia.

Step-by-step explanation:

The relationship between alleles where a heterozygous individual expresses a phenotype that is intermediate between the homozygous dominant and homozygous recessive conditions is called incomplete dominance. In the context of sickle cell anemia, individuals heterozygous for the sickle cell allele (HbA/HbS) have a mix of normal and sickled red blood cells, leading to a phenotype that is neither completely normal nor as severe as the homozygous sickle cell disease (HbS/HbS) phenotype. This inheritance pattern exemplifies incomplete dominance because both the normal hemoglobin allele (HbA) and the sickle cell allele (HbS) partially influence the phenotype, resulting in red blood cells that are only sometimes sickle-shaped.

A person who is heterozygous (Ss) for the sickle cell trait will have some misshapen cells and some normal cells. The sickle cell trait provides some resistance to malaria, which explains the high frequency of the trait in regions where malaria is common.