Blood type B positive (B+) is determined by inheriting specific genetic factors from both biological parents. The “B” designation comes from the presence of the B antigen on the surface of red blood cells. The “positive” sign is characterized by the presence of the Rh factor protein. Understanding which blood types can produce B+ requires examining the separate genetic rules governing the ABO and Rh systems.
Understanding ABO Alleles and the ‘B’ Phenotype
The ABO blood group is controlled by a single gene locus with three common alleles: \(I^A\), \(I^B\), and \(i\) (often simplified to A, B, and O). The A and B alleles are codominant; if both are inherited, the resulting blood type is AB. The O allele (\(i\)) is recessive, meaning its trait is only expressed when two copies are inherited, resulting in Type O blood.
The Type B phenotype results from inheriting at least one \(I^B\) allele. A person with Type B blood can have the genotype \(I^B I^B\) (homozygous) or \(I^B i\) (heterozygous). In both cases, the presence of the B allele is sufficient to produce the B antigen on the red cell surface. To produce a Type B child, the parents must collectively provide at least one \(I^B\) allele.
Understanding Rh Alleles and the Positive (+) Factor
The positive (+) or negative (-) factor is determined by the Rhesus (Rh) system, specifically the presence or absence of the D antigen. This system is controlled by the \(RHD\) gene, which codes for the D antigen protein on the red blood cell surface. The Rh-positive trait is dominant, while the Rh-negative trait is recessive.
The Rh-positive phenotype results from inheriting at least one dominant D allele, represented as \(D\). An Rh-positive person can have the genotype \(DD\) (homozygous dominant) or \(Dd\) (heterozygous). Only a person who inherits two recessive alleles (\(dd\)) will be Rh negative. Therefore, for a child to be B+, they must inherit a dominant \(D\) allele from at least one parent.
The Dual Requirement for Inheriting Blood Type B+
Inheriting the B+ blood type requires two separate genetic events. The genes governing the ABO and Rh blood groups are located on different chromosomes, meaning they are inherited independently of one another. This principle, known as independent assortment, dictates that the ‘B’ (the letter) and the ‘+’ (the sign) are passed down without influencing each other.
For a child to be B+, they must receive a combination of alleles that results in the B phenotype and the Rh-positive phenotype. This requires inheriting at least one \(I^B\) allele and at least one dominant \(D\) allele from the parents. Both genetic conditions must be met for the B+ blood type to be expressed.
Specific Parental Pairings That Can Produce B+
A wide variety of parental blood types can produce a B+ child, provided they carry the necessary \(I^B\) and \(D\) alleles. The most straightforward case involves two B+ parents. However, B+ children can also be born to parents who are not B+ themselves.
Parents with Type A+ and Type B+ blood can produce a B+ child if the A+ parent is heterozygous (\(I^A i\) and \(Dd\)) and the B+ parent is also heterozygous (\(I^B i\) and \(Dd\)). In this scenario, the child could inherit the \(I^B\) allele from the B+ parent and at least one \(D\) allele from either parent.
A parent with Type O+ blood can contribute to a B+ child if their partner is Type B or AB. For example, a Type O+ parent is guaranteed to pass on the recessive \(i\) allele but can pass on a dominant \(D\) allele if they are heterozygous (\(Dd\)). If the other parent is Type AB+ (\(I^A I^B\) and \(Dd\)), the child could inherit the \(I^B\) allele from the AB+ parent and the \(D\) allele from either parent, resulting in B+.
A B+ child can also result from pairings involving Rh-negative parents, such as a B+ parent and an A- parent. The B+ parent must carry the \(I^B\) and \(D\) alleles, while the A- parent must have the \(dd\) genotype. Since the A- parent can only contribute the recessive \(d\) allele, the child must inherit the dominant \(D\) allele from the B+ parent to be Rh positive. This demonstrates that the specific combination of alleles, not the overall blood type phenotype, determines the child’s inherited blood type.