Blood types, determined by specific markers on red blood cells, are inherited from parents and influence various aspects of health, including transfusion compatibility. This inheritance helps clarify potential outcomes for offspring.
Decoding Your Blood Type: ABO and Rh Factors
Blood typing relies on two systems: the ABO system and the Rh factor. The ABO system categorizes blood into four groups—A, B, AB, and O—based on the presence or absence of A and B antigens. Antigens are protein molecules that can trigger an immune response. Type A blood possesses A antigens, type B has B antigens, and type AB blood contains both A and B antigens. In contrast, type O blood lacks both A and B antigens on its red blood cells.
The Rh factor is an inherited protein on red blood cells. Individuals are classified as Rh positive (Rh+) if present, and Rh negative (Rh-) if absent. The combination of the ABO blood group and the Rh factor results in the eight common blood types, such as A+, O-, or O+. For example, O positive blood means the individual has no A or B antigens, but they do have the Rh factor protein.
How Blood Types Are Inherited
Blood types are determined by genes inherited from each parent, with each parent contributing one allele for blood type to their child. The ABO gene has three alleles: A, B, and O. A and B alleles are codominant, meaning both A and B antigens are expressed if present, resulting in AB blood type. The O allele is recessive, requiring two O alleles to be inherited for type O blood to be expressed.
For instance, a person with type A blood could have inherited an A allele from both parents (genotype AA) or an A allele from one parent and an O allele from the other (genotype AO). Similarly, type B blood can result from genotypes BB or BO. Type O blood, however, requires the genotype OO, indicating an O allele was received from each parent. The Rh factor also follows a dominant-recessive inheritance pattern, where the Rh positive allele (Rh+) is dominant over the Rh negative allele (Rh-). This means that a person will be Rh positive if they inherit at least one Rh+ allele.
The Genetic Outcome for O Positive Parents
When both parents are O positive, the child’s blood type is determined by both ABO and Rh systems. For the ABO system, since O is a recessive blood type, both parents must have the genotype OO. Each parent passes an O allele to their child. Therefore, a child born to two O positive parents will always have type O blood.
For the Rh factor, Rh positive parents can have varying genetic makeup, being either homozygous dominant (Rh+/Rh+) or heterozygous (Rh+/Rh-). If both O positive parents are heterozygous (Rh+/Rh-), there is a 25% chance their child will inherit an Rh- allele from each parent, resulting in an Rh negative blood type (Rh-/Rh-). The child would then be O negative. If either parent is homozygous dominant (Rh+/Rh+), the child will be Rh positive. Thus, a child of two O positive parents will have O blood, but could be Rh positive or Rh negative, with O positive being the more probable outcome.