Understanding a baby’s blood type involves genetics, where inherited characteristics from both parents combine to determine this unique biological marker. Just like eye color, a child’s blood type is passed down through genes. Blood types represent specific proteins and sugars present on the surface of red blood cells.
The ABO Blood Group System
The ABO blood group system categorizes blood into four main types: A, B, AB, and O. These classifications depend on the presence or absence of specific protein molecules, known as antigens, on the surface of red blood cells. Type A blood has A antigens, type B blood has B antigens, and type AB blood has both. Type O blood has neither A nor B antigens.
The ABO blood type is controlled by a single gene on chromosome 9, which has three alleles: A, B, and O. The A and B alleles are co-dominant, meaning both are expressed if inherited together. The O allele is recessive, expressed only if two O alleles are inherited. For example, a person with type A blood could have inherited two A alleles (AA) or one A and one O allele (AO), as A dominates over O.
The Rh Factor
Beyond the ABO system, the Rh factor designates a blood type as either positive (+) or negative (-). This is based on the presence or absence of the RhD antigen on red blood cells. If the RhD antigen is present, the blood type is Rh-positive; if absent, it is Rh-negative. About 85% of people are Rh-positive.
The Rh factor is inherited independently of the ABO system, with its genes on a different chromosome. Specific alleles dictate Rh status. The Rh-positive allele is dominant, while the Rh-negative allele is recessive. An individual will be Rh-positive if they inherit at least one Rh-positive allele.
Parental Genes and Inheritance
For the ABO system, each parent passes on one of their two ABO alleles to their child. For example, if a parent has type A blood (genotype AO), they can pass on either an A or an O allele. The combination of alleles from both parents determines the child’s ABO blood type.
The Rh factor inheritance follows a similar pattern. Each parent contributes one Rh allele, and the Rh-positive allele is dominant over the Rh-negative allele. A child will be Rh-positive if they receive at least one Rh-positive allele from either parent. Conversely, a child will only be Rh-negative if they inherit an Rh-negative allele from both parents. This explains how two Rh-positive parents can have an Rh-negative child if both carry a recessive Rh-negative allele.
Predicting Your Baby’s Blood Type
Knowing the parents’ blood types allows for the prediction of possible blood types for their baby. For instance, if both parents have type O blood, their child will exclusively have type O blood, as O is a recessive allele and both parents must carry two O alleles (OO). If one parent has type A blood (genotype AA or AO) and the other has type B blood (genotype BB or BO), the child could potentially have A, B, AB, or O blood, depending on the specific alleles each parent passes on.
For the Rh factor, if both parents are Rh-negative, their child will always be Rh-negative. If both parents are Rh-positive, the child can be either Rh-positive or Rh-negative. This occurs if both Rh-positive parents happen to carry the recessive Rh-negative allele and both pass it on. When one parent is Rh-positive and the other is Rh-negative, the baby can be either Rh-positive or Rh-negative.