Blood types are inherited from one’s biological parents and are defined by specific markers, known as antigens, found on the surface of red blood cells. Understanding an individual’s blood type is important for various medical procedures, particularly for ensuring compatibility during blood transfusions. The precise genetic makeup passed down from parents determines a person’s unique blood type.
Understanding Blood Type Systems
Blood typing relies on two primary systems: the ABO system and the Rh factor system. The ABO system categorizes blood into four main types: A, B, AB, and O. These types are determined by the presence or absence of A and B antigens on the red blood cell surface. For example, type A blood has A antigens, type B has B antigens, type AB has both, and type O blood has neither A nor B antigens.
The Rh factor, also known as the Rhesus factor, is another protein that may be present on the surface of red blood cells. If this protein is present, an individual is considered Rh positive (Rh+); if it is absent, they are Rh negative (Rh-). This factor adds a positive or negative designation to the ABO blood types, resulting in eight common blood types such as A+, O-, or B+.
The Inheritance of O Negative Blood
ABO blood type inheritance follows specific genetic rules. Alleles for A and B antigens are dominant, while the O blood type allele is recessive. An individual must inherit two copies of the O allele, one from each parent, to have type O blood. If an A or B allele is present, it will be expressed, overriding the O allele.
The Rh factor also exhibits dominant and recessive inheritance patterns. The Rh positive allele is dominant, and the Rh negative allele is recessive. For an individual to be Rh negative, they must inherit a recessive Rh negative allele from both parents. If even one dominant Rh positive allele is inherited, the individual will be Rh positive.
Combining these two systems, a person with O negative blood has inherited two recessive O alleles and two recessive Rh negative alleles. This genetic combination means their red blood cells lack A, B, and Rh factor antigens. Both biological parents must contribute an O allele and an Rh negative allele for their child to have O negative blood.
Can Two O Negative Parents Have an O Positive Child?
Given the genetic principles of blood type inheritance, two parents who are both O negative cannot have an O positive child. Since O negative is a recessive trait for both the ABO and Rh systems, each O negative parent possesses only the recessive alleles for type O blood (oo) and for Rh negative blood (dd), meaning they can only pass on ‘o’ and ‘d’ alleles to their offspring.
Consequently, any child born to two O negative parents will inherit an ‘o’ allele from each parent, resulting in type O blood, and a ‘d’ allele from each parent, making them Rh negative. This means the child will invariably be O negative. The presence of an Rh positive factor in a child would require at least one parent to contribute a dominant Rh positive allele, which is not possible if both parents are truly Rh negative.
While extremely rare exceptions can occur, they are not part of standard Mendelian inheritance. Genetic mutations are also theoretically possible but are considered highly improbable causes for a blood type change of this nature.