A baby’s blood type is determined by the genes inherited from both biological parents. Blood type is based on specific proteins, or antigens, present on the surface of red blood cells. This inheritance follows predictable rules of genetics, similar to how traits like eye color or height are passed down. The complete blood type, such as O positive or AB negative, is a combination of two distinct genetic systems working independently.
Understanding the ABO and Rh Blood Typing Systems
A full blood type designation combines two separate classifications: the ABO system and the Rh system. The ABO system determines if a person is type A, B, AB, or O based on the presence or absence of A and B antigens on the red blood cell surface. The Rh factor determines whether the blood type is positive (+) or negative (-), based on the presence or absence of the D antigen (Rhesus D protein). If the D antigen is present, the person is Rh-positive; if absent, the person is Rh-negative. These two systems are inherited separately.
The Genetic Rules for ABO Inheritance
The ABO system is governed by a single gene that has three variations, called alleles: A, B, and O. Every person inherits one allele from each parent, resulting in two alleles that determine their blood type. The A and B alleles are co-dominant, resulting in type AB blood if both are present. The O allele is recessive, meaning it only determines the blood type if two copies are inherited. These alleles combine into six genotypes (AA, AO, BB, BO, AB, OO), resulting in four blood types (A, B, AB, O), and allow two Type A or Type B parents to have a Type O child.
Passing Down the Positive or Negative Rh Factor
The Rh factor inheritance is simpler than the ABO system, involving a dominant-recessive relationship based on the D antigen. The presence of the Rh factor (Rh-positive) is a dominant trait (D allele), and the absence (Rh-negative) is a recessive trait (d allele). A baby will be Rh-positive if they inherit at least one dominant D allele (DD or Dd). To be Rh-negative, a baby must inherit two copies of the recessive d allele (dd genotype). Two Rh-positive parents who both carry the recessive ‘d’ allele (Dd) have a chance of having an Rh-negative child.
Potential Issues with Rh Factor Discrepancies
Understanding Rh factor inheritance is important in pregnancy due to the risk of Rh incompatibility. This situation only arises when an Rh-negative mother is carrying an Rh-positive baby. The mother’s immune system may recognize the baby’s Rh-positive red blood cells as foreign invaders if the baby’s blood enters her bloodstream, often during delivery. In response, the mother can develop antibodies that may attack the red blood cells of a subsequent Rh-positive pregnancy, causing Hemolytic Disease of the Newborn. To prevent this, a medication called Rh immune globulin (RhoGAM) is routinely administered during pregnancy and after delivery if the baby is found to be Rh-positive. RhoGAM works by destroying any fetal Rh-positive red blood cells in the mother’s circulation before her immune system can create lasting antibodies.