For blood type possibilities based on parents, ABO and Rh inheritance narrows a child’s possible blood types.
Maybe you’re filling out a school form, planning a blood donation, or checking a medical record. Blood type feels like a small detail, yet it can raise big questions when you line up family members.
A parents-to-child chart won’t predict one exact result unless you know each parent’s hidden gene pair. It can still do something useful: show which child blood types can happen and which ones can’t.
This page walks through the ABO letters (A, B, AB, O) and the Rh sign (+ or −) in plain language. You’ll learn a simple way to narrow the options, then you’ll see where real life can throw curveballs.
| Parents’ ABO Types | Child ABO Types Possible | Child ABO Types Not Possible |
|---|---|---|
| A + A | A, O | B, AB |
| A + B | A, B, AB, O | — |
| A + AB | A, B, AB | O |
| A + O | A, O | B, AB |
| B + B | B, O | A, AB |
| B + AB | A, B, AB | O |
| B + O | B, O | A, AB |
| AB + AB | A, B, AB | O |
| AB + O | A, B | AB, O |
| O + O | O | A, B, AB |
What The ABO Letters Mean On A Lab Report
ABO blood type is about markers on the surface of red blood cells. Type A has A markers, type B has B markers, type AB has both, and type O has neither. Your immune system also makes antibodies that react to the markers you don’t have, which is why ABO matching matters for transfusions.
When people say “I’m type O,” they’re talking about the ABO part only. To get the full label, you add the Rh sign. O+ and O− are not the same thing.
ABO Comes From One Gene With Three Common Alleles
Each parent passes one ABO allele to a child. The common alleles are A, B, and O. The pair you inherit is your ABO genotype, and the letter you show on paperwork is your ABO type (your phenotype).
A And B Can Both Show Up Together
A and B are codominant, so if you inherit A from one parent and B from the other, you show AB. O acts recessive, so it only shows when you inherit O from both parents. That single detail explains why type O can appear in a family even when neither parent is type O on paper.
Blood Type Possibilities Based On Parents With A Simple Method
Table charts work best when you understand the hidden genotypes behind each visible blood type. Here are the common genotype options for ABO:
- Type A can be AA or AO.
- Type B can be BB or BO.
- Type AB is AB.
- Type O is OO.
Step 1: List Each Parent’s Possible Genotypes
Start with what you know: the parents’ ABO types. If a parent is type AB or type O, there’s only one genotype choice. If a parent is type A or type B, there are two choices, and you don’t know which one is true unless you test family members or run a genetic test.
The one-allele-from-each-parent rule is laid out in the NIH NCBI book chapter on the ABO blood group, along with the six common ABO genotypes.
Step 2: Pair One Allele From Each Parent
Write the first parent’s two alleles across the top of a simple 2×2 grid and the second parent’s two alleles down the side. Fill the four boxes by pairing the alleles. Each box is one child genotype that can occur.
Say one parent is type A and the other is type O. The type O parent must be OO. The type A parent could be AA or AO. If the type A parent is AA, each child gets an A and an O and will be type A. If the type A parent is AO, a child can get AO (type A) or OO (type O).
Step 3: Convert Genotypes Into Blood Types
Once you have the child genotypes, swap them for the visible blood type. Any box with an A and a B becomes AB. Any box with at least one A and no B becomes A. Any box with at least one B and no A becomes B. OO becomes O.
Why Two Type A Parents Can Have A Type O Child
That surprise result happens when both parents are AO. Each parent can pass the O allele, so OO is on the table. If either parent is AA, type O drops off the list.
Rh Factor Adds The Plus Or Minus
The Rh sign is most often about the D antigen. If you have it, you type as Rh positive. If you don’t, you type as Rh negative. The NHS blood groups page notes the four main ABO groups and that each one can be RhD positive or RhD negative.
Rh matters in transfusions, and it can matter in pregnancy. Labs treat it with care, because an RhD negative person can form antibodies after exposure to RhD positive blood.
Rh Inheritance Uses A Dominant Pattern
For a simplified inheritance model, treat Rh positive as having at least one “D” allele, and Rh negative as “dd.” A person who is Dd types as positive, yet they carry one negative allele.
That means two Rh positive parents can have an Rh negative child if both parents are Dd. A single Rh negative parent can still have an Rh positive child if the other parent has a D allele.
Blood Type Possibilities From Parents With Rh Pairings
Once you know the parents’ Rh signs, you can rule some outcomes in or out. The table below uses the simplified D/d model that most family charts rely on.
| Parents’ Rh Types | Child Rh Types Possible | What This Means |
|---|---|---|
| + + | +, − | Rh negative is possible only if both parents carry d. |
| + − | +, − | Either sign can occur, based on whether the + parent is DD or Dd. |
| − − | − | All children will be Rh negative (dd). |
Combine The Two Lists Into Full Types
After you run the ABO and Rh steps, you’ll have two short lists: letters and signs. Pair each letter with each sign to get the full set of options.
If your letters are A or O and your signs are + or −, the full list is A+, A−, O+, and O−. If your signs list has only −, cross out the plus types right away.
When Family Charts Don’t Match What A Lab Finds
Most of the time, ABO and Rh charts line up with real typing. Still, a chart is a model, and labs run into edge cases.
Hidden Alleles And Missing Family Data
The most common “mystery” is simply an unseen allele. A parent typed as A may carry O (AO). A parent typed as B may carry O (BO). If you assume AA or BB without proof, you can cross out child types that are still possible.
Family details can be incomplete. Adoption, donor conception, and misattributed parentage exist, and a chart can’t settle those questions.
Rare ABO And Rh Variants
Some people have uncommon ABO variants that affect typing. The Bombay phenotype is one well-known case where a person types as O on standard tests, yet their ABO genes differ. On the Rh side, “weak D” and other variant patterns can lead to results that need a lab note, since the D antigen can show up in different strengths.
If a blood type result drives a medical decision, labs rely on direct testing, not a family chart.
Can Blood Type Rule Out A Biological Parent?
Blood type can sometimes rule out a parent-child pairing. It cannot prove a pairing on its own. Many unrelated people share the same blood type, and hidden alleles leave room for overlap.
A clear rule-out happens when the genetics make a child type impossible. One classic case: a parent with type AB cannot pass an O allele, so an AB parent and any partner cannot have a type O child.
If you need a firm answer about biological relationships, DNA testing is the standard approach. Blood type can be a clue, not a verdict.
A Practical Checklist For Narrowing A Child’s Type
Many people use blood type possibilities based on parents to fill out paperwork before a test is available. Treat that list as a placeholder for now, then confirm it with typing when you can.
If you want to use a chart the right way, stick to a routine. It keeps you from guessing a genotype you don’t know.
- Write down each parent’s full type, including + or −.
- Use the ABO table to list the child’s possible letters.
- Use the Rh table to list the child’s possible signs.
- Combine the lists into full types (like A+ or O−).
- Circle the types that are impossible for the child based on the parents’ letters or signs.
- If you still need certainty, get the child typed by a lab or blood donation record.
Blood typing answers the question with no guesswork. Until you have that result, treat any chart as a way to narrow choices, not a promise.
References & Sources
- National Library of Medicine (NIH), NCBI Bookshelf.“The ABO Blood Group.”Explains ABO alleles, genotypes, and how one allele comes from each parent.
- National Health Service (NHS).“Blood groups.”Lists ABO groups and notes that each group can be RhD positive or RhD negative.
Mo Maruf
I created WellFizz to bridge the gap between vague wellness advice and actionable solutions. My mission is simple: to decode the research and give you practical tools you can actually use.
Beyond the data, I am a passionate traveler. I believe that stepping away from the screen to explore new environments is essential for mental clarity and physical vitality.