A person’s blood type, categorized into the A, B, AB, or O system, is important medical information. The ABO type is determined by the presence or absence of specific protein molecules, called antigens, on the surface of red blood cells. Receiving an incompatible type can trigger a dangerous immune reaction. The positive or negative sign attached to the blood type is determined by a separate protein, the Rhesus (Rh) factor. The child’s complete blood type is a genetic outcome resulting from the contributions of both biological parents.
The Genetic Components of Blood Type
The determination of the four main blood groups—A, B, AB, and O—is controlled by a single gene located on chromosome 9. This gene exists in three possible forms, known as alleles: A, B, and O. Each person inherits one allele from the mother and one allele from the father, resulting in a pair of alleles that determines their specific blood type.
The A allele and the B allele are considered co-dominant, meaning that if a person inherits both (AB), both antigens are produced and expressed equally on the red blood cells, resulting in Type AB blood. The O allele is recessive, meaning it is only expressed if a person inherits an O allele from both parents.
This system creates six possible combinations of alleles, known as genotypes. For example, a person with the genotype AA or AO will have Type A blood, as the A allele is dominant over the recessive O allele. Similarly, genotypes BB or BO result in Type B blood, while only the OO genotype results in Type O blood. The genotype AB is the only combination that results in Type AB blood.
How Both Parents Contribute to Inheritance
The child’s blood type depends on the pair of alleles provided by each parent. During reproduction, each parent passes down only one of their two blood type alleles to the child. The combination of these two single alleles determines the child’s final genotype and blood type.
For instance, consider a pairing where one parent has Type A blood with the genotype AO, and the other has Type B blood with the genotype BO. The AO parent can pass on either A or O, and the BO parent can pass on either B or O. This specific combination creates four equally possible outcomes for the child’s genotype: AB, AO, BO, or OO.
These four genotypes translate to four possible blood types for the child: AB, A, B, or O, each with a 25% chance of occurring. This example shows that even when both parents have A or B blood types, they can still have a child with Type O blood if both happen to pass on their recessive O allele.
The Separate Inheritance of the Rhesus Factor
The Rhesus (Rh) factor is inherited separately from the ABO alleles. It is determined by the presence or absence of a different protein, the D antigen, on the surface of the red blood cells. Most individuals, about 85%, are Rh-positive, meaning they possess this protein.
The Rh-positive trait is dominant over the Rh-negative trait. A person only needs to inherit one copy of the Rh-positive allele from either parent to be Rh-positive. To be Rh-negative, a child must inherit the Rh-negative allele from both parents.
The Rh factor is particularly relevant in pregnancy when an Rh-negative mother is carrying an Rh-positive fetus. In this scenario, the mother’s immune system can potentially develop antibodies against the fetal red blood cells, leading to complications in later pregnancies. Medical interventions, such as an injection of Rh immune globulin, are routinely used to prevent the mother from becoming sensitized, protecting future Rh-positive fetuses.