Blood type refers to the classification of blood based on the presence or absence of inherited substances, called antigens, on the surface of red blood cells. These antigens are determined by an individual’s genetic makeup, passed down from their parents. Understanding the genetic principles behind blood type inheritance helps predict possible blood types in offspring and informs various medical practices, such as blood transfusions and organ transplantation.
Fundamentals of Genetic Inheritance
Genetic inheritance begins with genes, segments of DNA providing instructions for building specific molecules. Humans inherit two copies of most genes, one from each parent. Different versions of the same gene are called alleles, which reside at the same location on homologous chromosomes.
Alleles interact in different ways, influencing observable traits. A dominant allele expresses its trait even if only one copy is present. A recessive allele only expresses its trait if an individual inherits two copies. The combination of alleles an individual possesses is their genotype, while the physical characteristic that results from this genotype is their phenotype.
ABO Blood Group Inheritance
The ABO blood group system provides a clear example of multiple alleles and codominance. This system involves three alleles for the ABO gene: IA, IB, and i. The IA allele leads to the production of A antigens on red blood cells, while the IB allele results in B antigens. The ‘i’ allele does not produce any antigens.
The IA and IB alleles exhibit codominance; when both are present (genotype IAIB), both A and B antigens are expressed equally, resulting in AB blood type. Both IA and IB alleles are dominant over the ‘i’ allele. Individuals with genotypes IAIA or IAi have A blood type, and those with IBIB or IBi have B blood type. Only individuals who inherit two ‘i’ alleles (genotype ii) will have O blood type, as no A or B antigens are produced.
Rh Factor Inheritance
The Rh factor, commonly referred to as positive (+) or negative (-), represents another blood group system with a simpler inheritance pattern. This trait is determined by a single gene with two alleles: D (dominant) and d (recessive). The presence of the D allele results in the Rh-positive phenotype, meaning the D antigen is present on the red blood cells.
Individuals who are homozygous dominant (DD) or heterozygous (Dd) will be Rh-positive. The ‘d’ allele is recessive, indicating the absence of the D antigen. A person will only be Rh-negative if they inherit two copies of the ‘d’ allele (genotype dd).
Combining Blood Type Inheritance
A person’s full blood type is a combination of their ABO blood group and their Rh factor, inherited independently. For example, someone might be A positive (A+) or O negative (O-). The principles of inheritance for both systems allow for the prediction of possible blood types in offspring based on the parents’ genotypes.
Parents each contribute one allele for the ABO system and one for the Rh system to their child. Understanding these inheritance patterns is important for medical purposes, such as ensuring blood compatibility for transfusions and managing Rh incompatibility in pregnancies, where an Rh-negative mother carrying an Rh-positive fetus can sometimes face complications.