Blood types are categorized primarily by the ABO and Rh systems, including classifications like A, B, AB, O, and whether one is Rh-positive or Rh-negative. These define characteristics of an individual’s red blood cells. A common question is whether a child’s blood type can naturally differ from that of their parents. Understanding the genetic principles behind blood type inheritance helps explain these patterns.
Understanding Blood Types
Blood types are determined by specific protein markers, called antigens, on the surface of red blood cells. In the ABO system, individuals possess A antigens, B antigens, both, or neither (Type O). The Rh factor refers to the presence (Rh-positive) or absence (Rh-negative) of the RhD antigen. These combinations define common blood types, such as A+, B-, O+, or AB-.
How Blood Types Are Inherited
Blood type inheritance follows Mendelian genetics, where genes and their alleles are passed from parents to offspring. The ABO blood group is controlled by a single gene with three primary alleles: A, B, and O. A and B alleles are codominant, meaning both A and B antigens are expressed if inherited together, resulting in AB blood type. Conversely, the O allele is recessive, expressed only if an individual inherits two O alleles.
Each parent contributes one ABO allele to their child. For instance, if both parents have type A blood but carry the O allele (genotype AO), they can have children with type A or type O blood. Similarly, parents with type A (AO) and type B (BO) blood can potentially have children with any of the four ABO blood types: A, B, AB, or O. The Rh factor is inherited separately, with Rh-positive (D) dominant over Rh-negative (d). An Rh-negative child must inherit an Rh-negative allele from both parents.
When Your Blood Type Differs from Your Parents’
It is entirely possible and a normal genetic outcome for a child to have a blood type different from one or both parents. This occurs due to the various combinations of dominant and recessive alleles inherited. For example, two parents with Type A blood can have a Type O child if both carry the recessive O allele (genotype AO). In this scenario, the child inherits an O allele from each parent, expressing the Type O phenotype.
Similarly, if one parent has Type A blood (genotype AO) and the other has Type B blood (genotype BO), their child could inherit an O allele from each, resulting in Type O blood. This illustrates the diverse genetic possibilities within a family.
Beyond Typical Inheritance: Rare Blood Types
While typical Mendelian inheritance explains most blood type patterns, extremely rare scenarios can lead to unexpected results. One such example is the Bombay phenotype, also known as the hh blood group. Individuals with the Bombay phenotype lack the H antigen, which is a precursor molecule necessary for the formation of A and B antigens.
Even if a person inherits A or B alleles, these antigens cannot be expressed without the H antigen, causing their blood to type as O. This rare phenotype occurs when an individual inherits two recessive ‘h’ alleles, preventing H antigen production. A child with the Bombay phenotype might appear to have Type O blood, even if their ABO genotype suggests A, B, or AB, seemingly defying parental inheritance. Other rare occurrences, such as chimerism, where an individual has two different sets of DNA, or acquired blood type changes due to certain medical conditions, can also lead to unusual blood typing results.