Blood type is a system of classification for human blood, determined by inherited factors. This classification is based on the presence or absence of specific substances, known as antigens, found on the surface of red blood cells. The unique combination of antigens a person possesses is passed down through genes from their parents, establishing the individual’s blood group from conception. A baby’s blood type relies primarily on two major genetic systems: the ABO system and the Rhesus (Rh) factor.
The Genetic Basis of the ABO System
The ABO blood group system is governed by a single gene which has three different forms, or alleles: A, B, and O. Every person inherits one allele from each biological parent, resulting in a pair that determines their blood type. The relationship between these three alleles dictates which of the four main blood types—A, B, AB, or O—a person will exhibit.
The A and B alleles are co-dominant; if an individual inherits both, both antigens are expressed equally, resulting in type AB blood. The O allele is recessive, meaning the O characteristic is only expressed if a person inherits an O allele from both parents (genotype OO). Type O blood has neither the A nor the B antigen.
If a person inherits an A allele and an O allele (AO), they will have type A blood because the dominant A masks the recessive O. Similarly, inheriting a B allele and an O allele (BO) results in type B blood. The six possible allele combinations, or genotypes (AA, AO, BB, BO, AB, OO), simplify into four possible blood type expressions, known as phenotypes (A, B, AB, O).
Predicting Blood Type Through Parental Combinations
The specific blood type a baby inherits is a matter of probability, based on the two alleles contributed by each parent. Understanding a parent’s genotype—whether they carry two identical alleles or two different ones—is necessary to predict the possible outcomes. For example, a parent with Type A blood might have the genotype AA or AO, and a parent with Type B blood might be BB or BO.
If a parent has Type O blood, their genotype must be OO, meaning they can only pass on an O allele. If an O parent partners with a Type A parent (AO), the baby has a 50% chance of inheriting Type A and a 50% chance of inheriting Type O. A Type A parent who is AA can only pass on A, ensuring the baby is Type A, regardless of the other parent’s second allele.
The ability of two parents with Type A and Type B blood to have a Type O child often surprises people. This outcome is possible only if both parents are heterozygous, meaning they each carry the recessive O allele (AO and BO genotypes). In this scenario, there is a 25% chance the child will inherit O from both parents, resulting in Type O blood.
When one parent has Type AB blood, they must pass on either an A or a B allele. A Type AB parent and a Type O parent can have a child with Type A or Type B blood, each with a 50% probability, but they cannot have a Type O child. The child’s blood type is a direct, predictable result of the genetic material contributed by both parents.
Understanding the Rhesus Factor
In addition to the ABO system, a person’s blood is classified by the Rhesus (Rh) factor, which is inherited independently. The Rh factor refers to the presence or absence of a specific protein, the D antigen, on the surface of red blood cells. If the D antigen is present, the person is Rh-positive (Rh+); if it is absent, they are Rh-negative (Rh-).
The inheritance of the Rh factor involves one gene, where the trait for Rh-positive is dominant. A person will be Rh-positive if they inherit at least one Rh-positive allele. Only individuals who inherit two Rh-negative alleles, one from each parent, will be Rh-negative.
For a baby to be Rh-negative, both parents must contribute an Rh-negative allele. If both parents are Rh-positive but are heterozygous, their child has a 75% chance of being Rh-positive and a 25% chance of being Rh-negative. The Rh factor is appended to the ABO type to form the complete classification, such as O-positive or AB-negative.
Why Blood Type Matters for Newborn Health
Knowing a newborn’s complete blood type is significant for medical purposes, primarily regarding the safety of blood transfusions and the potential for incompatibility between mother and baby. If a baby requires a transfusion shortly after birth, the correct blood type must be administered to prevent a dangerous immune reaction.
The Rhesus factor is particularly important during pregnancy when an Rh-negative mother carries an Rh-positive baby. This situation, known as Rh incompatibility, can cause the mother’s immune system to produce antibodies that treat the baby’s Rh-positive red blood cells as foreign. While this rarely causes issues during a first pregnancy, the antibodies can persist and attack the red blood cells of a subsequent Rh-positive fetus.
This attack can lead to Hemolytic Disease of the Newborn (HDN), causing the baby to develop anemia and jaundice. Medical advances allow for preventative treatment, such as the administration of Rh immunoglobulin to the mother during and after pregnancy, which prevents her body from forming these harmful antibodies. Early identification of potential Rh incompatibility is a standard part of prenatal care, allowing doctors to monitor and intervene.