Blood types classify individuals based on specific markers on their red blood cells. These markers, called antigens, are inherited from parents, like eye or hair color. Understanding blood types is important for safe blood transfusions and managing pregnancy incompatibilities.
Understanding O Positive Blood
The “O” in O positive blood refers to the absence of both A and B antigens on the surface of red blood cells. The “positive” aspect indicates the presence of the Rh factor (RhD antigen), a protein on the red blood cell surface. O positive blood is common, with approximately 38% of the American population having this blood type. People with O positive blood can donate to all positive blood types (A+, B+, AB+, and O+) because their red blood cells lack A and B antigens. However, individuals with O positive blood can only receive transfusions from O positive or O negative donors.
Principles of Blood Type Inheritance
Blood type inheritance follows genetic patterns, with each parent contributing one allele for the ABO group and one for the Rh factor.
ABO Blood Group Inheritance
The ABO gene, located on chromosome 9, has three alleles: A, B, and O. Alleles A and B are codominant, meaning both are expressed if present, while the O allele is recessive. This means a person with an A allele and an O allele will have type A blood, and similarly for type B.
Rh Factor Inheritance
The Rh factor is determined by a separate gene on chromosome 1, with two main alleles: Rh-positive (D) and Rh-negative (d). The Rh-positive allele (D) is dominant, meaning that if an individual inherits at least one Rh-positive allele, they will have Rh-positive blood. Conversely, a person must inherit two Rh-negative alleles (dd), one from each parent, to have Rh-negative blood. This independent inheritance of ABO and Rh factors determines a child’s complete blood type.
Parental Blood Type Combinations for an O Positive Child
For a child to have O positive blood, they must inherit an O allele from each parent for the ABO group and at least one Rh-positive allele. This means the child’s genotype will be OO for the ABO system and either DD or Dd for the Rh factor.
Parents with O positive (O+) and O positive (O+) blood can have an O positive child. For the ABO type, both O+ parents must have the OO genotype and pass an O allele. For the Rh factor, if both parents are heterozygous (Dd), they can pass on a D allele, resulting in an O positive child. Similarly, if one or both O+ parents are homozygous dominant (DD), they will also pass on a D allele.
If one parent is O positive (O+) and the other is O negative (O-), they can also have an O positive child. Both parents contribute an O allele. The O- parent has a dd genotype, so they contribute a d allele. The O+ parent must be heterozygous (Dd) for the Rh factor to contribute the dominant D allele, leading to an O positive (Dd) child.
Parents with A positive (A+) and O positive (O+) blood can produce an O positive child. The A+ parent must carry the O allele (genotype AO) and pass it to the child, while the O+ parent contributes their O allele. For the Rh factor, at least one parent must contribute a dominant D allele, which is possible if either or both A+ and O+ parents are Rh-positive.
An A positive (A+) parent and a B positive (B+) parent can also have an O positive child. Both parents must be heterozygous for their ABO blood type (AO and BO, respectively) to each contribute an O allele. For the Rh factor, both A+ and B+ parents must be Rh-positive (DD or Dd) to pass on a D allele, resulting in an O positive child.
Parents with A positive (A+) and A positive (A+) blood can have an O positive child if both are heterozygous (AO) and both contribute an O allele. For the Rh factor, both A+ parents must be Rh-positive (DD or Dd) to pass on a D allele. Likewise, two B positive (B+) parents can produce an O positive child if both are heterozygous (BO) and both contribute an O allele, and both are Rh-positive (DD or Dd).
It is not possible for two AB blood type parents to have an O child, as AB parents cannot contribute an O allele. Other combinations are possible, such as an A positive parent and a B negative parent, or an A negative parent and an O positive parent, provided the necessary recessive O alleles and dominant Rh-positive alleles are passed on.