Blood typing is a common test used for both identity and health purposes, establishing a clear profile of the molecules present on an individual’s red blood cells. While many believe a child’s blood type is easily predicted, the inheritance of these traits can sometimes seem counterintuitive. Blood type is determined by specific genes passed down from each parent. The combination of these genes can result in a child displaying a trait that neither parent outwardly expresses. Understanding these genetic rules reveals how two parents with the same blood type, such as O Positive, can produce a child with a different, seemingly hidden blood type.
Understanding the O Blood Type
The familiar A, B, AB, and O blood groups are determined by specific protein structures, known as antigens, on the surface of red blood cells. These antigens are governed by the ABO gene system, which involves three possible variations, or alleles: A, B, and O. A person inherits one allele from each parent, resulting in a pair of alleles that determines their blood type. The A and B alleles are dominant, meaning only one copy is needed for the corresponding antigen to be expressed. The O allele is recessive, and an individual will only have Type O blood if they inherit the O allele from both parents, resulting in the genotype OO.
The Inheritance of Positive and Negative Status
The positive or negative designation refers to the Rhesus, or Rh, factor, a separate genetic system from the ABO group. This designation specifically relates to the presence or absence of the D antigen, a protein found on the red blood cell surface. The trait for Rh-positive status is dominant ($D$), and the absence of the D antigen, resulting in Rh-negative status, is the recessive trait ($d$). A person who is Rh-positive can have the genotype $DD$ or $Dd$. An individual can only be Rh-negative if they inherit the recessive $d$ allele from both parents, resulting in the homozygous recessive genotype $dd$.
The Genetic Mechanism for an O Negative Child
The question of whether two O Positive parents can have an O Negative child is answered by combining the inheritance rules of the ABO system and the Rh factor. For the child to be O Negative, their complete genotype must be $OO dd$, requiring the recessive $O$ and $d$ alleles from each parent. For the parents to be O Positive yet carry these necessary recessive genes, their full genotype must be $OO Dd$. Since both parents are $Dd$, each has a 50% chance of passing on the recessive $d$ allele to their offspring. This results in a 25% chance that the child will inherit $d$ from both, leading to the O Negative $OO dd$ genotype.
Medical Significance of Rh Status
The difference between Rh-positive and Rh-negative blood is medically significant during blood transfusions and, most notably, pregnancy. If an Rh-negative person is exposed to Rh-positive blood, their immune system may recognize the D antigen as foreign and begin producing antibodies against it. This immune response can lead to complications if the person is exposed to Rh-positive blood again later. In pregnancy, this leads to Rh incompatibility if an Rh-negative mother carries an Rh-positive fetus. To prevent this immune sensitization, Rh-negative expectant mothers are routinely given an injection of Rh immunoglobulin (RhoGAM).