A person’s blood type is an inherited characteristic, but the outcome is not always a direct match with a parent. Blood type is determined by two separate genetic systems: the ABO group and the Rhesus (Rh) factor. Because of how genes are passed down and expressed, a child often has a blood type different from both parents. Understanding the basic rules of genetic inheritance is the only way to predict the possible blood types for any given family.
Understanding the ABO System
The ABO blood group is determined by the presence or absence of specific carbohydrate molecules, called antigens, on the surface of red blood cells. Type A blood has the A antigen, Type B has the B antigen, Type AB has both, and Type O has neither antigen. This system is controlled by a single gene on chromosome 9, which has three possible forms, known as alleles: \(A\), \(B\), and \(O\).
Every person inherits two alleles for the ABO gene, one from each parent, creating their specific genotype. The \(A\) and \(B\) alleles are codominant, meaning that if both are present (genotype \(AB\)), both antigens are expressed, resulting in Type AB blood. The \(O\) allele is recessive to both \(A\) and \(B\), requiring a person to inherit two \(O\) alleles (genotype \(OO\)) to express the Type O phenotype. Genotypes \(AO\) or \(AA\) result in Type A blood, while \(BO\) or \(BB\) result in Type B blood.
Predicting Your Child’s Blood Type
The interaction of these alleles means that the child’s blood type, or phenotype, may not visibly resemble either parent’s, even though the genes, or genotype, came directly from them. For instance, two parents can both have Type A blood, but if their genotypes are both \(AO\), they can pass the recessive \(O\) allele to their child. If the child inherits an \(O\) allele from each parent, the resulting genotype is \(OO\), and the child will have Type O blood, a type that neither parent expresses.
A complex scenario occurs when a parent with Type A blood (genotype \(AO\)) and a parent with Type B blood (genotype \(BO\)) have a child. This combination means the child has a 25% chance of inheriting any of the four ABO blood types: Type AB, Type A, Type B, or Type O. The ability to produce a Type O child from two Type A or Type B parents demonstrates why a child’s blood type often differs from their parents.
The only way to guarantee a child has the same blood type as both parents is if both parents are Type O, as they can only pass on the \(O\) allele, resulting in a Type O child. Similarly, two Type AB parents cannot have a Type O child because neither parent carries the recessive \(O\) allele. In every other combination, there is a possibility for the child’s blood type to be different from one or both parents.
The Rh Factor and Positive/Negative Inheritance
The second part of a person’s complete blood type designation is the Rhesus (Rh) factor, which is indicated by the plus or minus sign. This factor is determined by the presence or absence of the D antigen protein on the surface of red blood cells. If the D antigen is present, the person is Rh-positive (\(Rh+\)); if it is absent, the person is Rh-negative (\(Rh-\)).
The Rh factor is inherited separately from the ABO system. The allele for Rh-positive is dominant, while the allele for Rh-negative is recessive. An Rh-positive person has at least one dominant Rh-positive allele, meaning their genotype is either \(Rh+/Rh+\) or \(Rh+/Rh-\). An Rh-negative person must inherit two copies of the recessive Rh-negative allele, resulting in a genotype of \(Rh-/Rh-\).
Because the Rh-positive allele is dominant, two Rh-positive parents can have an Rh-negative child if both parents carry the recessive Rh-negative allele (genotype \(Rh+/Rh-\)). In this situation, there is a 25% chance that the child will inherit the recessive \(Rh-\) allele from each parent, resulting in an Rh-negative blood type. This is another common way a child’s blood type designation can differ from their parents.
The Rh factor is particularly important during pregnancy if an Rh-negative mother is carrying an Rh-positive fetus. The mother’s immune system may recognize the Rh-positive cells as foreign, potentially leading to the production of antibodies that could affect the fetus. Medical monitoring and preventative treatment, such as Rh immune globulin, are used to manage this incompatibility and ensure the baby’s health.