Genetic inheritance explains how traits are passed from parents to offspring. A common question is whether blood type is an example of polygenic inheritance. This article explores polygenic inheritance and the genetic basis of ABO blood types to answer this question.
What is Polygenic Inheritance
Polygenic inheritance describes traits controlled by multiple genes, rather than a single gene. These genes often have small, additive effects. Environmental factors can also influence the expression of polygenic traits. This type of inheritance typically results in a continuous range of phenotypes, where characteristics vary gradually across a population.
Common examples of polygenic traits in humans include height, skin color, and eye color. Human height is influenced by numerous genes, leading to a spectrum of heights. Skin pigmentation involves the interplay of many genes. These traits do not sort into clear-cut groups but instead show a smooth distribution, often resembling a bell curve.
How ABO Blood Types Are Inherited
The ABO blood group system is determined by a single gene, the ABO gene, located on chromosome 9. This gene has three alleles: IA, IB, and i. Every person inherits two of these alleles, one from each parent, which determine their specific blood type.
The inheritance pattern of ABO blood types involves both dominance and codominance. The IA allele produces A antigens, and the IB allele produces B antigens on red blood cells. Both IA and IB alleles are dominant over the i allele, which does not produce any antigens. When both IA and IB alleles are inherited, they are codominant, meaning both A and B antigens are expressed, resulting in AB blood type. This leads to four distinct blood types: A, B, AB, and O.
Why Blood Type Is Not Polygenic
ABO blood type is not considered a polygenic trait. This is because, unlike polygenic traits that are influenced by multiple genes, ABO blood type is determined by a single gene. While this single gene has multiple alleles (IA, IB, and i), the trait itself is not the result of the additive effects of numerous genes scattered across the genome.
Polygenic traits typically exhibit continuous variation, such as the spectrum of human heights or skin tones. In contrast, ABO blood types fall into discrete, non-overlapping categories: A, B, AB, or O. There is no continuous range of blood types; an individual clearly possesses one of these four classifications. Therefore, the inheritance of ABO blood types follows a Mendelian pattern, characterized by a single gene with multiple alleles and specific dominance relationships, rather than the complex, multi-gene interactions seen in polygenic inheritance.