Blood types are a fascinating aspect of human biology, with their inheritance and characteristics often sparking curiosity. Understanding basic genetics helps clarify common questions about blood, including the characteristics of Type O blood.
What Are Blood Types?
Blood types classify human blood based on specific markers, called antigens, on the surface of red blood cells. The ABO blood group system categorizes blood into four main types: A, B, AB, and O. Type A blood has A antigens, Type B blood has B antigens, and Type AB blood possesses both. Type O blood is distinguished by the absence of both A and B antigens.
Blood plasma also contains antibodies that react against foreign antigens. For example, a person with Type A blood has anti-B antibodies, and someone with Type B blood has anti-A antibodies. Type O blood contains both anti-A and anti-B antibodies, whereas Type AB blood has neither. These antibodies are important for immune responses, ensuring compatible blood transfusions.
Understanding Genetic Dominance and Recessiveness
Genetic traits are passed down through segments of DNA called genes, with different versions known as alleles. Each individual inherits two alleles for most genes, one from each parent. The interaction between these alleles determines the observable characteristic, or phenotype. The underlying genetic makeup, including the specific combination of alleles, is called the genotype.
One allele can mask the effect of another, a phenomenon known as dominance. A dominant allele expresses its trait even when only one copy is present. The allele whose effect is masked is called recessive; its trait only appears if an individual inherits two copies of this allele, one from each parent. When an individual has two identical alleles for a gene, their genotype is homozygous; if they have two different alleles, it is heterozygous.
How ABO Blood Types Are Inherited
ABO blood types are inherited through a single gene with three alleles: IA, IB, and i. The IA allele produces A antigens, and the IB allele produces B antigens. The ‘i’ allele does not produce A or B antigens.
The IA and IB alleles exhibit codominance; if both are inherited (IAIB genotype), both A and B antigens are produced, resulting in Type AB blood. Both IA and IB alleles are dominant over the ‘i’ allele. This means that individuals with genotypes IAIA or IAi will have Type A blood, while those with IBIB or IBi genotypes will have Type B blood.
Why Type O Blood Is Not Dominant
Type O blood is recessive. For an individual to have Type O blood, they must inherit two copies of the ‘i’ allele, resulting in the genotype ii. This is because the ‘i’ allele is non-functional in terms of producing A or B antigens. Consequently, Type O red blood cells lack both A and B antigens.
Because Type O is recessive, if an ‘i’ allele is paired with an IA or IB allele, the dominant A or B antigen will be expressed. This is why a person with one IA allele and one ‘i’ allele will have Type A blood, not Type O. The common misconception that Type O is dominant might stem from its prevalence in the population or its status as a “universal donor.” Type O negative blood is considered a universal donor because its red blood cells lack A, B, and Rh antigens, making them less likely to trigger an immune reaction.