Blood types categorize blood based on inherited characteristics, primarily antigens on red blood cells. These classifications are crucial for medical procedures, such as blood transfusions, to ensure compatibility. The ABO and Rh systems are the two most important blood grouping systems, determined by genetic factors.
Genetic Fundamentals
Genetic traits are determined by genes, segments of DNA that provide instructions for characteristics. Each gene has different versions, known as alleles. An individual inherits two alleles for each gene, one from each parent, which together determine a particular trait. The interaction between these alleles dictates how the trait is expressed.
Alleles can be categorized as either dominant or recessive. A dominant allele expresses its trait even when only one copy is present. Conversely, a recessive allele only expresses its trait if an individual inherits two copies of that specific allele, one from each parent. If a dominant allele is also present, the recessive trait will not be observable.
Recessiveness in the ABO System
The ABO blood group system is governed by a gene with three common alleles: A, B, and O. The A and B alleles are dominant over the O allele. If an individual inherits an A allele and an O allele (genotype AO), their blood type will be A. Similarly, an individual with a B allele and an O allele (genotype BO) will have blood type B.
For an individual to have blood type O, they must inherit two copies of the O allele, one from each parent. This genotype (OO) signifies the absence of both A and B antigens on the red blood cells. Thus, O blood type is the recessive trait in the ABO system, expressed only when no dominant A or B alleles are present. The AB blood type represents codominance, where both A and B alleles are expressed equally.
Recessiveness in the Rh Factor
The Rh blood group system is determined by the presence or absence of the D antigen on red blood cells. If the D antigen is present, a person’s blood is considered Rh-positive (Rh+). If the D antigen is absent, the blood is Rh-negative (Rh-). The inheritance of the Rh factor follows a dominant-recessive pattern.
The allele for the Rh-positive trait is dominant. An individual is Rh-positive if they inherit at least one copy of this allele. Conversely, the allele for the Rh-negative trait is recessive. For an individual to be Rh-negative, they must inherit two copies of the recessive Rh-negative allele, one from each parent. This genotype is typically represented as ‘dd’. The Rh factor system demonstrates how a recessive trait appears only when both inherited alleles are recessive.