Blood type is a classification system used to categorize blood based on the presence or absence of specific inherited antigens on the surface of red blood cells. These antigens are molecules recognized by the immune system, determining which of the four main groups—A, B, AB, or O—a person belongs to. Understanding blood type inheritance determines the genetic possibilities for a child’s blood type, which is governed by the combination of genes passed down from both parents.
The Genetic Foundation of ABO Blood Types
The inheritance of the ABO blood group is determined by a single gene with three versions, or alleles: A, B, and O. Every person inherits two alleles, one from each parent, which determines their blood type. The A and B alleles are co-dominant; if both are inherited, both are expressed, resulting in Type AB blood.
The O allele is recessive, meaning it only determines the blood type if a person inherits two copies, resulting in Type O blood. A person’s genotype is the specific pair of inherited alleles (e.g., AA, AO, AB), while the phenotype is the resulting blood type (A, B, AB, or O).
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, a person with the genotype BB or BO will have Type B blood. Because the O allele is recessive, a parent can have Type A or Type B blood while still carrying the O allele and potentially passing it on.
Calculating the Baby’s Potential ABO Type
Determining a baby’s potential blood type requires knowing the genetic composition, or genotype, of both parents. A Type A parent might have the genotype AA or AO, and a Type B parent might have BB or BO. The “hidden” O allele is important because only two O alleles can result in a Type O child.
If both parents have Type O blood, their child must also have Type O blood, since each parent can only contribute an O allele. If one parent has Type AB blood and the other has Type O blood, the child can only inherit Type A or Type B blood, because the AB parent cannot pass on an O allele. In this pairing, the baby cannot be Type O or Type AB.
When two parents both have Type A blood, they could potentially have a child with Type O blood if both carry the recessive O allele (AO genotype). In this specific scenario, there is a one in four chance that the child will inherit the O allele from each parent. The most complex outcome occurs when one parent has Type A blood and the other has Type B blood, as this combination has the potential to produce a child with any of the four ABO blood types (A, B, AB, or O), depending on whether the parents carry the recessive O gene.
Understanding Rh Factor Inheritance
The second main classification is the Rhesus (Rh) factor, which determines if blood is positive (+) or negative (-). This is based on the presence or absence of the Rh(D) protein antigen on the red blood cell surface. Rh-positive blood means the protein is present, and Rh-negative blood means it is absent.
The Rh factor is inherited independently of the ABO system and follows a simple dominant/recessive pattern. The presence of the Rh factor (Rh+) is the dominant trait, while the absence of the factor (Rh-) is the recessive trait. A person only needs to inherit one Rh+ gene to have Rh-positive blood.
For a baby to be Rh-negative, they must inherit the Rh-negative gene from both parents. If both parents are Rh-negative, the baby will definitely be Rh-negative. If both parents are Rh-positive, they can still have an Rh-negative child, provided both parents carry a recessive Rh-negative gene in their genotype.
Why Blood Type Matters During Pregnancy
The primary medical concern related to blood type during pregnancy centers on Rh incompatibility, which occurs when an Rh-negative mother carries an Rh-positive baby. If the mother’s and baby’s blood mixes, which can happen during delivery or certain prenatal procedures, the mother’s immune system may recognize the baby’s Rh-positive red blood cells as foreign. This exposure causes the mother’s body to develop antibodies against the Rh factor.
These antibodies usually do not affect the first pregnancy, but they remain in the mother’s system and can cross the placenta in subsequent pregnancies. If a future baby is Rh-positive, the maternal antibodies can attack the fetus’s red blood cells, potentially causing severe anemia, jaundice, or hemolytic disease of the newborn. This risk is why all pregnant women are screened for their Rh status early in pregnancy.
The standard preventative treatment is an injection of Rh immune globulin, often known as RhoGAM. This injection is typically given around 28 weeks of pregnancy and again after delivery if the baby is Rh-positive. RhoGAM works by temporarily preventing the mother’s immune system from making its own antibodies, dramatically reducing the incidence of Rh-related complications.