Blood types categorize an individual’s blood based on inherited characteristics of red blood cells, determined by the presence or absence of specific antigens on their surface. This classification is important for medical procedures like transfusions and provides insights into genetic inheritance. The concept of “dominance” in blood types involves genetic principles that define how these characteristics manifest.
Understanding Genetic Dominance for Blood Types
Genes, the units of heredity, carry instructions for an organism. Different versions of a gene are called alleles. Most human cells have two alleles for each gene, one from each parent.
Alleles can interact in several ways to determine an observable trait, or phenotype. A dominant allele expresses its trait even if only one copy is present. A recessive allele, however, only expresses its trait if an individual inherits two copies of it, one from each parent. A third relationship is codominance, where both alleles are expressed equally and simultaneously, resulting in a phenotype that shows characteristics of both.
The ABO Blood Group System
The ABO blood group 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. A single gene, the ABO gene on chromosome 9, governs this system, with three common alleles: A, B, and O (IA, IB, and i).
In the ABO system, alleles A and B are codominant; inheriting both results in AB blood type, as both antigens are expressed. Both A and B alleles are dominant over the O allele. Thus, an individual with an A and an O allele (AO) has Type A blood, and one with a B and an O allele (BO) has Type B blood. Only individuals inheriting two O alleles (OO) have Type O blood, as the O allele produces no antigens. Possible genotypes and their phenotypes are: AA or AO for Type A, BB or BO for Type B, AB for Type AB, and OO for Type O.
The Rh Blood Group System
The Rh blood group system determines if blood is Rh-positive or Rh-negative, based on the presence or absence of the RhD protein (D antigen) on red blood cells. If present, blood is Rh-positive; if absent, it is Rh-negative.
The Rh factor’s inheritance involves a dominant allele (Rh-positive, D) and a recessive allele (Rh-negative, d). Inheriting at least one dominant D allele results in Rh-positive blood. Conversely, an individual must inherit two recessive d alleles to be Rh-negative. Therefore, genotypes DD and Dd result in Rh-positive blood, while only dd results in Rh-negative blood.
How Blood Types Are Inherited
Blood types are inherited from parents, with each contributing one allele for the ABO gene and one for the Rh gene. The combination of these alleles determines the child’s complete blood type.
For instance, if one parent has Type A (AO genotype) and the other has Type B (BO genotype), their children could inherit Type A, B, AB, or O blood. This demonstrates the varied outcomes possible with codominance and recessive traits. Similarly, two Rh-positive parents can have an Rh-negative child if both carry and pass on the recessive Rh-negative allele. The ABO and Rh factors are inherited independently, meaning their types and statuses are determined by separate genes.