A child with B-positive blood inherits two things from their parents: at least one copy of the B allele in the ABO system, and at least one copy of the RhD gene that makes them Rh-positive. In practice, this means at least one parent must have type B or type AB blood, and at least one parent must be Rh-positive (or, less commonly, a silent carrier of the RhD gene). The full picture involves more combinations than most people expect, along with a handful of rare genetic quirks that occasionally break the standard rules.
The ABO Side of B-Positive
Your ABO blood type depends on which sugar molecules sit on the surface of your red blood cells. The gene responsible has three common versions: A, B, and O. You inherit one version from each parent, giving you two copies. A and B are codominant with each other, and both are dominant over O. So a person with B-type blood carries either two B copies or one B and one O. Either way, at least one B allele had to come from a parent, and that parent must have expressed B or AB on their own blood type.
This constraint narrows the list of possible parental ABO pairings. If neither parent carries a B allele, a B-type child is essentially off the table under normal genetics. Two type-A parents, two type-O parents, or an A-and-O couple cannot produce a B child through standard inheritance, because none of them has a B allele to pass along.
Every Parental Combination That Can Produce B-Positive
The table below covers all ABO pairings where at least one parent carries a B allele. For the child to be B-positive rather than just B, the Rh-positive requirement (covered in the next section) also has to be met. Each combination lists what the child’s ABO type could be, not just B.
- B × B: Children can be type B or type O. Both parents carry B, so B is the most likely outcome, but if both parents are BO carriers, one in four children will be OO and type O.
- B × O: Children can be type B or type O. The B parent passes either B or O; the O parent always passes O.
- B × A: All four blood types are possible. If the B parent is BO and the A parent is AO, children can end up A, B, AB, or O. This is the only common pairing that can produce every ABO type.
- B × AB: Children can be type A, type B, or type AB, but not type O. The AB parent always contributes either an A or a B allele.
- AB × O: Children can be type A or type B, never AB and never O. The AB parent passes one allele (A or B); the O parent passes O. So the child ends up AO (type A) or BO (type B), roughly a coin flip.
- AB × A: Children can be type A, type B, or type AB. If the A parent is AO, type B children are possible because the child can inherit B from the AB parent and O from the A parent.
- AB × AB: Children can be type A, type B, or type AB. Type O is essentially impossible here because neither parent has an O allele to contribute (each parent has one A and one B).
A couple of patterns stand out. The AB × O pairing often surprises people because neither parent is type B themselves, yet the AB parent’s hidden B allele can show up in the child as straightforward type B. And the B × A cross is the only standard combination that can theoretically produce a child of any ABO type, which is why it sometimes causes confusion during paternity discussions or blood bank screenings.
How the Rh Factor Gets Inherited
The “positive” in B-positive refers to the RhD protein on the surface of red blood cells. The genetics here are simpler than ABO in one sense: there are two main possibilities at the RHD gene. You either have a functional copy that produces the D protein (making you Rh-positive) or you lack it (making you Rh-negative). Rh-positive is dominant, so you only need one working copy from either parent to test positive.
The RHD and RHCE genes sit close together on chromosome 1 and likely arose from a duplication of the same ancestral gene.1Blood Reviews. Defining the Rh blood group antigens: Biochemistry and molecular genetics Most Rh-negative people simply lack the RHD gene entirely, while Rh-positive people have one or two functional copies. For a child to be B-positive, at least one parent has to pass along a working RHD gene. That parent will usually test as Rh-positive themselves, though not always.
Two Rh-positive parents can have either Rh-positive or Rh-negative children, depending on whether each parent carries one functional copy or two. If both parents carry one working copy and one non-functional copy, about one in four of their children will inherit the non-functional version from both sides and be Rh-negative. Conversely, if one parent is Rh-positive and the other is Rh-negative, roughly half the children will be Rh-positive.
Can Two Rh-Negative Parents Have an Rh-Positive Child?
Under textbook genetics, two Rh-negative parents should only produce Rh-negative children, because neither parent has a functional RHD gene to pass on. But rare exceptions exist. Some individuals test as Rh-negative on standard blood-bank serology yet still carry parts of the RHD gene or weakly expressed variants that can produce a small amount of the D protein. When this happens, the parent appears Rh-negative on routine testing but is genetically capable of passing on a functional or partially functional RHD allele.2Al- Anbar Medical Journal. Rh Blood Group Positive Newborn of Rh Blood Group Negative Parents, Why, and How?
These cases are uncommon but well-documented. The phenomenon called “weak D” accounts for a small fraction of routine Rh typings, roughly 0.2 to 1 percent, where the D antigen is present but at levels too low to be picked up by standard reagents.3PubMed Central. Serological weak D phenotypes: a review and guidance for interpreting the RhD blood type using the RHD genotype For decades, blood banks managed this ambiguity by treating donors with weak D as Rh-positive (to protect Rh-negative recipients) and treating patients and pregnant women with weak D as Rh-negative (to be safe). Molecular genotyping now allows more nuanced management, particularly for the three most common weak D types in people of European descent, which can safely be classified as Rh-positive.
So the short version: two parents who both test as Rh-negative on a standard blood card will almost never have an Rh-positive child. But in rare cases involving weak D or other RHD gene variants, it can happen, and DNA-level testing can explain why.
Putting ABO and Rh Together
Since ABO and Rh are inherited independently (they sit on different chromosomes), you can combine any ABO-valid pairing with any Rh-valid pairing. A child’s B-positive result requires meeting both criteria at once: inheriting at least one B allele and at least one functional RHD gene. For example, a type B-negative father and a type O-positive mother could have a B-positive child if the father passes his B allele and the mother passes her RHD gene. Or a type AB-positive mother and a type O-negative father could produce a B-positive child if the child gets B from the mother and RHD from the mother as well.
The key insight is that each system follows its own inheritance pattern. The ABO gene is on chromosome 9, while the RH genes are on chromosome 1, so they shuffle independently during reproduction. A common source of confusion is thinking that a parent’s combined blood type (like “A-negative”) travels as a package. It does not. The A and the negative are passed separately, and a child can get any mix of ABO and Rh from each parent.
Rare Genetic Exceptions That Bend the Rules
Standard inheritance tables work in the vast majority of cases, but genetics has well-documented exceptions that can produce results that look impossible on paper.
Cis-AB
In typical ABO genetics, the A and B alleles always come from separate chromosomes, one from each parent. But in the rare variant called cis-AB, both A and B activities come from a single allele on one chromosome. The underlying cause is a gene mutation that produces a single enzyme capable of adding both the A sugar and the B sugar to red blood cells.4PubMed Central. Cis-AB, the Blood Group of Many Faces, Is a Conundrum to the Novice Eye In at least one documented case, the mechanism was traced to unequal chromosomal crossing-over rather than a point mutation, yielding a chromosome that carried instructions for both A and B enzymes side by side.5PubMed Central. Genetic mechanism of cis-AB inheritance. I. A case associated with unequal chromosomal crossing over
Why does this matter for B-positive inheritance? A parent with cis-AB can pass the combined A-and-B allele to a child, meaning the other parent does not need to carry any B allele at all. This can produce children whose blood types seem impossible given their parents’ apparent types, and it occasionally causes confusion during paternity testing or prenatal blood typing.
Chimerism
Chimerism occurs when a person carries two genetically distinct cell populations, usually as a result of twin embryos merging very early in development. A chimeric person can have two different blood types circulating at once, sometimes producing results that contradict both parents’ types. In one reported case, a child born to an A-positive father and an A-negative mother tested as AB-positive, a result that standard genetics cannot explain from two type-A parents.6PubMed Central. A case of chimerism-induced paternity confusion: what ART practitioners can do to prevent future calamity for families In another case, a child’s blood typed as having both AB and O characteristics; molecular analysis confirmed the child was a tetragametic chimera, carrying cells from what would have been two fraternal twins.7PubMed. Congenital tetragametic blood chimerism explains a case of questionable paternity
Chimerism is rare enough that most people will never encounter it. But it is worth knowing about, especially in the era of assisted reproduction, because it can produce blood typing results that make families question parentage when no infidelity or sample mix-up occurred.
Bombay and Para-Bombay Phenotypes
There is an even more exotic exception involving the H antigen, which is the molecular foundation that A and B sugars are built on. People with the Bombay phenotype lack the enzyme needed to make H antigen on their red cells, so even if they carry A or B alleles, those alleles have nothing to attach to and the person tests as type O. Their children, however, can inherit the A or B allele and, if they also inherit a working H-antigen gene from the other parent, express a blood type that looks like it came from nowhere.8Hematology, Transfusion and Cell Therapy. AB para-Bombay phenotype: a rare blood group variant and its clinical significance A Bombay parent who carries a hidden B allele could produce a B-positive child with a partner who has no B allele at all, something that would look completely baffling on a standard blood card.
When Rh Mismatch Matters in Pregnancy
If you are reading about blood type inheritance because of pregnancy planning, the Rh factor deserves special attention. When an Rh-negative mother carries an Rh-positive baby (a scenario that can happen any time the father contributes an RHD gene), the mother’s immune system may recognize the baby’s Rh-positive red blood cells as foreign and develop antibodies against them.9PubMed Central. Anti-D administration in pregnancy for preventing Rhesus alloimmunisation This usually is not a problem in a first pregnancy, because the antibody response takes time to build. But in subsequent pregnancies with another Rh-positive baby, those antibodies can cross the placenta and attack the baby’s red cells, causing anemia that can become severe.
The standard prevention is an injection of anti-D immunoglobulin (often called RhoGAM in the United States), given to Rh-negative mothers during pregnancy and again within 72 hours after delivering an Rh-positive baby. A review of trials involving over 10,000 women found that post-delivery anti-D significantly reduced the chances of the mother developing Rh antibodies, both within six months and in subsequent pregnancies.10Cochrane Database of Systematic Reviews. Anti‐D administration after childbirth for preventing Rhesus alloimmunisation If you are an Rh-negative woman whose partner could be Rh-positive (or whose partner’s Rh status is unknown), this is the most clinically relevant piece of the blood type inheritance puzzle.
B Blood Type and Disease Susceptibility
People who look into their B-positive status sometimes wonder whether it affects their health beyond transfusion compatibility. Research on ABO and disease susceptibility is a sprawling field, but one of the better-studied associations involves norovirus, the stomach bug notorious for tearing through cruise ships and daycare centers. A meta-analysis found that people with type O blood had about 28 percent higher odds of norovirus infection compared to other groups, while people with type B showed no significant increase or decrease in susceptibility.11PubMed. ABO blood group-associated susceptibility to norovirus infection: A systematic review and meta-analysis The association between ABO type and norovirus depends partly on the specific strain of virus, since different strains bind to different sugar structures on gut cells. Having B-type sugars on your cells does not grant blanket protection, but the data suggest it is not a disadvantage either, at least for this particular pathogen.
Other disease associations with ABO type (cardiovascular risk, certain cancers, malaria susceptibility) have been studied extensively, but effect sizes tend to be modest, and your overall lifestyle and genetic background matter far more than which sugar decorates your red blood cells.
Can B-Type Blood Be Converted to Universal Donor Blood?
One of the more fascinating research threads related to the B antigen involves stripping it off red blood cells entirely. The B antigen is just an extra sugar (galactose) sitting on top of the H antigen that all blood types share. In theory, an enzyme that clips off that extra galactose should convert B-type red cells into something that behaves like type O, the universal donor type. This was first demonstrated back in 1982, though the process required enormous amounts of enzyme. Early proof-of-principle transfusions in humans showed no adverse effects, and researchers continued searching for better enzymes over the following decades.12PubMed Central. Toward universal donor blood: Enzymatic conversion of A and B to O type
By 2000, a recombinant enzyme was being tested for converting group B red cells to group O in transfusion-dependent patients.13PubMed. Transfusion to blood group A and O patients of group B RBCs that have been enzymatically converted to group O The conversion of B-type cells has historically been easier than converting A-type cells, because the B sugar is structurally simpler to remove. More recent work has identified bacterial enzymes from gut microbes that can strip both A and B antigens more efficiently, bringing the idea of universal enzymatic conversion closer to practical use, though it still has not entered routine clinical practice. For B-positive individuals, this research is a reminder that the antigen defining your blood type is not some deep, immutable feature of your cells. It is a single sugar molecule that, in principle, can be snipped off.
The Evolutionary History of the B Allele
If you have B-positive blood and wonder where that B allele came from in the grand sweep of evolution, the answer is surprisingly complex. Other great apes also have ABO blood group variation, but their A and B types did not descend directly from the same A and B alleles in humans. Phylogenetic analyses of primate ABO genes have shown that A and B blood group antigens in chimpanzees and gorillas arose independently through convergent evolution, not from a shared ancestral A/B polymorphism passed down to all species.14PubMed. Evidence for convergent evolution of A and B blood group antigens in primates A broader analysis found at least three independent appearances of B alleles from ancestral A-type alleles across primate lineages.15Molecular Biology and Evolution. Evolution of primate ABO blood group genes and their homologous genes The same pattern holds in Old World monkeys, where A and B reactivity evolved separately from the hominoid versions.16PubMed. Characterization of the ABO blood group genes in macaques: evidence for convergent evolution
In other words, nature has invented the B blood group antigen multiple times. The mutation required to shift an A-type enzyme into a B-type enzyme is relatively small, involving just a handful of amino acid changes in the glycosyltransferase enzyme. This makes the A-to-B switch an evolutionary path that different primate lineages have stumbled onto repeatedly, suggesting there may be selective pressures (possibly related to pathogen resistance) that favor maintaining both A and B alleles in a population. Your B allele is not a quirk of random drift; it is a solution that evolution has converged on again and again.