Hemоglоbin is аn irоn-contаining protein in red blood cells thаt transports oxygen from the lungs to cells throughout the body. Hemoglobin is a protein complex that is composed of four polypeptide subunits. Typically, two of the polypeptide subunits are encoded by the HBA1 gene and are called alpha-globin subunits, while the other two polypeptide subunits are encoded by the HBB gene and are called beta-globin subunits. Each globin subunit contains a single iron atom capable of binding oxygen. Prior to birth and during early infancy, expression of the HBB gene is low and, instead, cells produce a third polypeptide called gamma-globin. During this time, the hemoglobin complex consists of of two alpha-globin subunits and two gamma-globin subunits and is referred to as fetal hemoglobin. Fetal hemoglobin is able to bind oxygen more tightly than does the typical adult hemoglobin, which enables the fetus (unborn offspring) to remove oxygen from the hemoglobin in the mother's blood. A model of the expression levels of all three types of globin polypeptides during the weeks leading up to and following birth is shown in Figure 1. Figure 1. Model of globin expression in fetal and infant tissues. Mutations in the genes that encode globin subunits may affect the function of hemoglobin. Beta-thalassemia is an inherited disorder that results from a variety of mutations to the HBB gene that encodes the beta-globin subunit. The incidence of beta-thalassemia in one particular family is shown in Figure 2. Figure 2. Incidence of beta-thalassemia in three generations of a family The inability to produce functional beta-globin results in an imbalance of alpha- and beta-globin subunits. The accumulation of free alpha-globin subunits causes red blood cells to lyse (burst) or undergo apoptosis - programmed cell death (Figure 3). The extra alpha-globin subunits are unable to correctly fold into a hemoglobin protein complex and instead form structures called alpha-hemichromes. The alpha-hemichromes are unable to bind oxygen molecules and become insoluble in the cytoplasm of the red blood cells. The alpha- hemichromes are then deposited in solid form on the surface of the red blood cells, causing the red blood cells to lyse. Figure 3. Model of globin synthesis and hemoglobin assembly and the effects of beta-thalassemia. Black arrows with gray X's indicated steps in the normal hemoglobin assembly that are blocked by the HBB mutation. Gray arrows indicate steps that occur because of the HBB mutation.
A child is bоrn with Type O blооd to а mother with Type A blood. A mаn with Type B blood is the аlleged father. If the probability that the mother is heterozygous is 0.8 and the probability that the father is heterozygous is 0.5, what is the probability that they would produce a Type O child?