Understanding how blood types are inherited can be confusing, especially when a child’s blood type appears different from their parents’. Exploring the fundamental principles of blood typing and genetic inheritance helps clarify these patterns.
Understanding Blood Types
Blood typing relies on identifying specific markers, known as antigens, on the surface of red blood cells. The two primary systems for classifying human blood are the ABO system and the Rh factor. The ABO system categorizes blood into four main types: A, B, AB, and O, based on the presence or absence of A and B antigens. Individuals with O blood type do not have A or B antigens on their red blood cells.
Separate from the ABO system, the Rh factor determines whether blood is positive or negative. This factor refers to the presence or absence of the D antigen on red blood cells. If the D antigen is present, a person is Rh positive; if it is absent, they are Rh negative. Approximately 85% of people are Rh-positive, making it the more common type.
The Basics of Genetic Inheritance
Blood types are inherited from parents through genes, which are segments of DNA that carry instructions for specific traits. Each person inherits two copies, or alleles, for each blood type gene, receiving one allele from each biological parent. Some alleles are dominant, meaning their trait is expressed even if only one copy is present, while others are recessive, requiring two copies for the trait to appear.
In the ABO system, the A and B alleles are codominant, meaning both are expressed if inherited together. However, the O allele is recessive, so a person will only have O blood type if they inherit an O allele from both parents. Similarly, for the Rh factor, the allele for Rh-positive (presence of the D antigen) is dominant over the allele for Rh-negative (absence of the D antigen). This means an individual can be Rh-positive by inheriting one or two dominant Rh-positive alleles. To be Rh-negative, an individual must inherit two recessive Rh-negative alleles, one from each parent.
The O Positive and O Negative Connection
Given these genetic principles, two O positive parents can have an O negative child. For this to happen, both parents must carry the recessive alleles for both the O blood type and the Rh-negative factor.
Regarding the Rh factor, an O positive parent can be genetically heterozygous, meaning they have one dominant Rh-positive allele and one recessive Rh-negative allele. This combination results in them expressing the Rh-positive trait. If both O positive parents are heterozygous for the Rh factor, each parent has a chance to pass on their recessive Rh-negative allele to their child. If the child inherits the Rh-negative allele from both parents, they will be O negative. This outcome highlights how recessive traits can remain unexpressed in parents but appear in their offspring.