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This is because the trait is recessive, meaning that only one correctly functioning copy of the blood clotting factor gene is necessary for normal clotting. Females have two X chromosomes, and hence redundant copies of the blood clotting factor gene located on them.
Overdominance is a phenomenon in genetics where the phenotype of the heterozygote lies outside the phenotypical range of both homozygous parents. Overdominance can also be described as heterozygote advantage regulated by a single genomic locus, wherein heterozygous individuals have a higher fitness than homozygous individuals.
In persons with sickle cell disease, high levels of fetal hemoglobin as found in a newborn or as found abnormally in persons with hereditary persistence of fetal hemoglobin, the HbF causes the sickle cell disease to be less severe. In essence the HbF inhibits polymerization of HbS. A similar mechanism occurs with persons who have sickle cell trait.
When combined with Hemoglobin S (β^6Glu → Val) it causes a severe form of Sickle cell disease known as Hemoglobin S/O-Arab. Detection of Hb O-Arab can be carried out with a blood test, identifying the carries of hemoglobinopathies, so as to inform patients their chances of producing an affected child and ensure appropriate guidance is given.
If he has microcytosis (mean cell hemoglobin < 27 pg or mean red cell volume < 80 fl), the woman is tested. When both are microcytic, their hemoglobin A2 concentrations are measured. If both have a concentration above 3.5% (diagnostic of thalassemia trait) they are referred to the local designated health post for genetic counseling. [77]
In addition, there is sickle cell trait (HbAS) which is defined by having HbA and HbS. This makes the individual heterozygous for sickle cell. Of the world population, it is estimated that there are about 300 million individuals with the sickle cell trait and about 100 million of those are in sub-Saharan Africa. [18]