Blood type is determined by inherited substances, called antigens, found on the surface of red blood cells. These antigens act as markers, allowing the body to recognize its own blood cells. An individual’s unique combination of antigens is based on genetic information passed down from their biological parents, dictating their blood type before birth.
The Basics of Blood Types
Human blood is categorized using two systems: ABO and Rh. The ABO system classifies blood into four main types—A, B, AB, and O—based on the presence or absence of A and B antigens on red blood cells. For instance, type A blood has A antigens, type B has B antigens, type AB has both, and type O has neither.
The Rh system determines if blood is positive (+) or negative (-), based on the presence or absence of the Rh(D) antigen (also known as the D antigen) on red blood cells. If present, blood is Rh-positive; if absent, it is Rh-negative. These two systems combine to form the eight common blood types, such as A+, O-, or AB+.
Genetic Building Blocks of Blood Type
Blood type inheritance involves concepts of genes, alleles, genotypes, and phenotypes. A gene is a segment of DNA that carries instructions for a specific trait, while alleles are different versions of that gene. Each individual inherits two alleles for each gene, one from each parent.
Genotype refers to the specific combination of alleles an individual possesses, whereas phenotype is the observable trait resulting from that genotype, such as having A or O blood. For the ABO system, there are three main alleles: A, B, and O. The A and B alleles are codominant, meaning that if both are inherited, both A and B antigens are expressed. The O allele is recessive, meaning it is only expressed if two O alleles are inherited.
For the Rh system, inheritance involves two alleles, D and d. The D allele, which leads to Rh-positive blood, is dominant over the d allele, resulting in Rh-negative blood. A single D allele is sufficient for a person to be Rh-positive.
Predicting ABO Blood Type Inheritance
Understanding how ABO alleles interact allows for predicting potential blood types in offspring. Since A and B alleles are codominant and O is recessive, various combinations can occur. For example, a person with type A blood could have either an AA or AO genotype, while a person with type B blood could have a BB or BO genotype. Type AB individuals always have an AB genotype, and type O individuals always have an OO genotype.
If a parent with type A blood (genotype AO) and a parent with type B blood (genotype BO) have children, their offspring could inherit A (AO), B (BO), AB (AB), or O (OO) blood types. Genetic tools like a Punnett square can visually represent these allele pairings and predict the probability of each blood type outcome.
When both parents have type O blood (OO genotype), all their children will also have type O blood. Similarly, if one parent has AB blood and the other has O blood, their children can only be type A or type B, as they will inherit either an A or B allele from one parent and an O allele from the other.
Predicting Rh Factor Inheritance
The inheritance of the Rh factor follows a dominant-recessive pattern. An individual who is Rh-positive can have two possible genotypes: DD (homozygous dominant) or Dd (heterozygous). Someone who is Rh-negative must have the genotype dd (homozygous recessive), as they lack the D antigen.
If both parents are Rh-negative (dd), all their children will also be Rh-negative (dd). However, if two Rh-positive parents who are both heterozygous (Dd) have a child, there is a possibility for their child to be Rh-negative (dd). If one parent is homozygous Rh-positive (DD) and the other is Rh-negative (dd), all their children will be Rh-positive (Dd).
Why Understanding Blood Type Inheritance Matters
Understanding blood type inheritance is important, particularly in medical contexts. One application is in blood transfusions, where matching blood types is necessary to avoid severe immune reactions. Transfusing incompatible blood can trigger a dangerous immune response, potentially leading to life-threatening complications.
Another area is during pregnancy, concerning Rh incompatibility. If an Rh-negative mother carries an Rh-positive baby, her immune system might produce antibodies against the baby’s red blood cells, which can cause complications in subsequent Rh-positive pregnancies. Medical interventions, such as RhoGAM injections, can prevent the mother’s body from forming these antibodies, safeguarding future pregnancies.