Blood types represent specific characteristics of a person’s red blood cells, determined by the presence or absence of protein markers called antigens. The main system is the ABO group, classifying blood as Type A, Type B, Type AB, or Type O. The Rhesus (Rh) factor is the second classification, designating blood as either positive (+) or negative (-). Blood type is a trait inherited entirely from biological parents. Understanding the rules of this inheritance allows parents to predict the range of possible blood types their child may have.
How the ABO System is Inherited
The ABO blood group system is governed by a single gene that has three possible forms, known as alleles: A, B, and O. Every person inherits one allele from each parent, resulting in a pair of alleles that determines their blood type. The A and B alleles are co-dominant, meaning that if a person inherits both, they will express both characteristics, resulting in Type AB blood.
The O allele is recessive, meaning its trait is only expressed if it is paired with another O allele. For instance, Type A blood results from inheriting two A alleles (genotype AA) or one A and one O allele (genotype AO). In both cases, the A antigen is present, and the resulting blood type (phenotype) is Type A. Similarly, Type B blood can result from the genotypes BB or BO.
Only a person who inherits two O alleles (genotype OO) will have Type O blood, as the recessive O allele does not produce the A or B antigens. The combination of the two inherited alleles is the genotype, while the resulting blood type expressed is the phenotype.
Determining Your Baby’s Possible Blood Types
The specific combination of parental blood types dictates the possible outcomes for a child. If both parents have Type O blood, their child must also have Type O blood, as they can only pass on the O allele. When a Type AB parent partners with a Type O parent, the child inherits either A or B from one parent and O from the other, resulting in a 50% chance of Type A and a 50% chance of Type B.
A pairing between two Type A parents can produce a Type O child only if both parents carry the recessive O allele (AO genotype). In this case, there is a 25% chance of Type O and a 75% chance of Type A. If one Type A parent (AO) and one Type B parent (BO) both carry the hidden O allele, they have a 25% chance for every blood type: A, B, AB, and O.
When two Type AB parents reproduce, their child can inherit Type A, Type B, or Type AB blood, but cannot inherit Type O blood. This is because Type AB parents lack the O allele required to pass on two copies. Certain outcomes are guaranteed, such as when both parents are Type O, or when one parent is Type O and the other is homozygous Type A (AA) or Type B (BB), guaranteeing a Type A or Type B child.
Understanding the Rh Factor
The Rh factor is the second major blood group system, and its inheritance is separate from the ABO system. This factor is determined by the presence or absence of the D antigen protein on the surface of red blood cells. If the protein is present, the person is Rh positive (+); if absent, the person is Rh negative (-).
The gene for the Rh factor follows a simple dominant and recessive pattern. The trait for Rh positive is dominant, while the trait for Rh negative is recessive. A person is Rh negative only if they inherit the recessive gene from both parents.
If both parents are Rh negative, their child will be Rh negative. If one parent is Rh negative and the other is Rh positive, the child has a high chance of being Rh positive, but can still be Rh negative if the Rh positive parent is a carrier. Two Rh positive parents have a 75% chance of an Rh positive child and a 25% chance of an Rh negative child, provided both parents carry the recessive gene.
Why Blood Type Matters During Pregnancy
Knowing the Rh factor of both the mother and the fetus is important for prenatal care. Rh incompatibility occurs when an Rh negative mother is carrying an Rh positive baby. During pregnancy or delivery, some of the baby’s Rh positive blood cells can enter the mother’s bloodstream.
The mother’s immune system recognizes the Rh positive cells as foreign and may begin to produce antibodies to attack them. These antibodies typically do not affect the first pregnancy. However, once the mother has produced them, they can cross the placenta in subsequent pregnancies and attack the red blood cells of any future Rh positive fetus. This leads to a condition called hemolytic disease of the newborn.
This complication is preventable. Rh negative mothers are routinely given an injection of Rh immunoglobulin around 28 weeks of pregnancy and again after delivery if the baby is Rh positive. This shot prevents the mother’s immune system from developing the antibodies, protecting future pregnancies from Rh-related complications.