Final answer:
The heterozygous type for sickle cell trait is an example of codominance, where both the normal allele (S) and sickle cell allele (s) are fully expressed in a person's phenotype, resulting in a mix of normal and sickle-shaped blood cells.
Step-by-step explanation:
The type of heterozygous for the sickle cell trait is an example of codominance. In the case of someone who is heterozygous (Ss), where 'S' stands for the normal hemoglobin allele and 's' stands for the sickle cell hemoglobin allele, both alleles are expressed. This means that the person will have some normal hemoglobin cells (HbA) and some sickle-shaped hemoglobin cells (HbS).
In codominance, unlike incomplete dominance, both alleles are fully and separately expressed, resulting in a phenotype that includes both traits distinctly. The hemoglobin example in sickle cell disease is a classic case where a person with genotype SS will have normal red blood cells, a person with genotype ss will have sickled red blood cells, and a person with genotype Ss will have a mixture of normal and sickled red blood cells.
To answer the Rev questions:
An allele is a variant form of a gene. The type of allele that only affects the phenotype in the homozygous condition is a recessive allele. Two individuals can have the same phenotype but different genotypes, for example, a phenotype for a dominant trait can be produced by both homozygous dominant (AA) and heterozygous (Aa) genotypes. A tall, green plant that is homozygous for each trait would have a genotype of TTGG and a phenotype of being tall and green.