The inheritance of blood type is a classic demonstration of complex genetic rules, moving beyond the simple dominant-recessive patterns often taught in introductory biology. Blood type is determined by the presence or absence of specific surface markers, known as antigens, on the outside of red blood cells. These antigens are coded for by genes inherited from both biological parents, and the way these genes interact dictates the final blood type. Understanding the genetics of blood type requires a detailed look at how multiple variations of a single gene can exist within a population.
The Genetic Basis: Multiple Alleles
The primary determinant of the A, B, AB, or O blood type is a single gene located on chromosome 9, often called the \(I\) gene. While most inherited traits are governed by a gene with only two versions, the ABO blood system involves three common alleles: \(I^A\), \(I^B\), and \(i\). This scenario, where three or more versions of a gene exist within a species’ gene pool, is known as multiple allelism.
The \(I^A\) allele instructs the red blood cell to produce the A antigen, and the \(I^B\) allele codes for the B antigen. The third common allele, \(i\), is recessive and does not code for any functional antigen. Although the population maintains a pool of three possible alleles, any single person still only inherits two copies of this gene, receiving one allele from each parent. The combination of these two inherited alleles determines the individual’s specific blood type.
Codominance and Recessiveness in the ABO System
The ABO system demonstrates two different modes of inheritance simultaneously: codominance and simple recessiveness. The \(I^A\) and \(I^B\) alleles exhibit codominance, meaning that when an individual inherits both, they are both fully expressed. The result is Type AB blood, where red blood cells display both the A antigen and the B antigen on their surface.
The \(i\) allele is recessive to both \(I^A\) and \(I^B\). A person who inherits \(I^A\) and \(i\) will express only the A antigen, resulting in Type A blood. Similarly, inheriting \(I^B\) and \(i\) results in Type B blood, as the \(I^B\) allele dominates the expression of \(i\). Type O blood only occurs if an individual inherits two copies of the recessive \(i\) allele, which produces neither the A nor the B antigen.
Mapping Inheritance: Genotypes and Phenotypes
The combination of the two inherited alleles constitutes the individual’s genotype. The resulting blood type that is expressed is called the phenotype. Because of the dominance and codominance rules, there are six possible genotypes that translate into only four common phenotypes.
An individual with Type A blood can have the genotype \(I^A I^A\) or \(I^A i\). Likewise, a person with Type B blood can be \(I^B I^B\) or \(I^B i\). The Type AB phenotype requires the heterozygous genotype \(I^A I^B\), and the Type O phenotype is exclusively the homozygous recessive genotype \(ii\).
For example, if two heterozygous Type A parents (both \(I^A i\)) cross, they each have a chance to pass the \(i\) allele to their child. The resulting genotypes are 25% \(I^A I^A\), 50% \(I^A i\), and 25% \(ii\). This means the child has a 75% chance of having the Type A phenotype and a 25% chance of having the Type O phenotype. A cross between a heterozygous Type A parent (\(I^A i\)) and a heterozygous Type B parent (\(I^B i\)) can produce offspring with any of the four blood types (A, B, AB, or O) with equal 25% probability.
Inheritance of the Rh Factor
The positive or negative designation that follows the ABO blood type, known as the Rhesus (Rh) factor, represents a second, separate genetic system. The Rh status is determined by the presence or absence of the D antigen on the red blood cells, controlled by a different gene than the ABO system. This inheritance follows a straightforward pattern of simple dominance.
The allele for the presence of the D antigen (Rh-positive) is dominant, while the allele for its absence (Rh-negative) is recessive. An individual who inherits even one dominant Rh-positive allele will be Rh-positive. A person must inherit two copies of the recessive Rh-negative allele to be Rh-negative. This simple dominant-recessive rule makes the Rh factor a less complex inheritance pattern than the ABO system.