It's all about the alleles of the hemoglobin beta gene. Sickle cell anemia is inherited in an autosomal recessive pattern, which means that both copies of the gene in each cell have mutations. Those who are heterozygous for the sickle cell allele produce both normal and abnormal hemoglobin so the inheritance pattern of the trait is considered to be codominant . Two different types of relationship between the same alleles. Why?
I think it is because in the sickle cell anemia the important thing is: To determine whether the individual is 'HEALTHY' or 'SICK'. But in the sickle cell trait we go deeper and are concerned with the presence of 'NORMAL' or 'SICKLE' RBCs inside the body. Did I get it right?
• HbAHbA: Healthy (only normal RBCs can be seen)
• HbAHbS: Healthy (Both normal and sickle RBCs can be seen)
• HbSHbS: Sick (Only sickle RBCs can be seen)
As you can see here, the inheritance pattern for anemia is an autosomal recessive pattern, but for the trait the pattern is codominance. The question is: Why are there two different types of relationship between the same alleles?
Consider this question:
What is the type of relationship between the normal and mutant alleles (which causes sickle cell anemia) of the hemoglobin beta gene? Autosomal recessive pattern or codominance?
Can one choose one of the options? I think this question is vague. Because if the sickle cell anemia was mentioned, the answer would be autosomal recessive pattern. But if the sickle cell trait (the presence or absence of sickle RBCs) was mentioned, the answer would be codominance. Isn't it so?
With thanks to Bryan Krause, I searched a little more and saw this in Wikipedia just now.
The sickle cell trait can be used to demonstrate the concepts of co-dominance and incomplete dominance. An individual with the sickle cell trait shows incomplete dominance when the shape of the red blood cell is considered. This is because the sickling happens only at low oxygen concentrations. With regards to the actual concentration of hemoglobin in the circulating cells, the alleles demonstrate co-dominance as both 'normal' and mutant forms co-exist in the bloodstream. Thus it is an ambiguous condition showing both incomplete dominance and co-dominance.
Before this, I believed that only two phenotypes can be seen in HbAHbS individuals: Normal RBC and Sickle-shaped RBC
But now I doubt it. Is there one more phenotype? A red blood cell that is not normal nor sickle-shaped but rather something in between? A red blood cell that is sickled incompletely? Is this phenotype really observed, studied or defined scientifically?