Blood type is a system of classification based on the presence or absence of specific protein structures, called antigens, found on the surface of red blood cells. These antigens are molecules that can trigger an immune response if they are foreign to the body, which is why blood typing is important for safe transfusions. The most widely known classification system is the ABO blood group system, discovered in the early 1900s. This system categorizes human blood into four main types: A, B, AB, and O. The expression of these blood types is determined by the genetic information passed down from parents.
Understanding Alleles and Phenotype Expression
The determination of blood type involves the inheritance of genetic instructions from both parents. These instructions are carried by variations of a gene, known as alleles, which dictate what kind of antigen, if any, is produced on the red blood cell surface. The ABO system involves three possible alleles: A, B, and O. Every person inherits two of these alleles, one from each biological parent, which together form their genetic code for blood type.
The concepts of dominance and recessiveness govern how these inherited alleles are expressed. The A allele and the B allele are dominant, meaning that if either is present, its corresponding antigen will be produced. The O allele is recessive and does not code for the production of A or B antigens. This distinction separates a person’s genotype (the specific pair of alleles they possess) from their phenotype (the physically expressed blood type).
The Two Possible Genotypes for Type A Blood
A person is classified as having Type A blood when their red blood cells display only the A antigen on their surface; this is the Type A phenotype. To express this phenotype, an individual must have inherited at least one A allele. This requirement leads to two distinct genetic possibilities, or genotypes, for Type A blood: AA and AO.
The first possible genotype is AA, which occurs when a person inherits an A allele from both parents. Because both alleles are the same, this is known as a homozygous Type A genotype. This genotype ensures a strong expression of the A antigen on all red blood cells.
The second possible genotype is AO, which results from inheriting an A allele from one parent and an O allele from the other. This combination creates a heterozygous Type A genotype. The dominant A allele is fully expressed, while the recessive O allele remains masked, still resulting in the Type A blood phenotype.
How Type A Blood is Inherited
Understanding whether a person is homozygous (AA) or heterozygous (AO) is important for predicting the blood types of their offspring. Since every child receives one allele from each parent, the parents’ genotypes directly determine the probability of inheritance.
For example, if both parents have the heterozygous AO genotype, they can each pass on either the A or the O allele. This scenario creates a 75% chance for their child to have Type A blood (AA or AO) and a 25% chance of having Type O blood (OO).
A parent with the homozygous AA genotype can only pass on an A allele. If that parent mates with a person who has Type O blood (genotype OO), all children will inherit the AO genotype and have Type A blood. Conversely, two parents with Type A blood can still have a child with Type O blood if both carry the recessive O allele (genotypes are both AO).