Blood Type Parent Checker
Match babies to parents by ABO and Rh blood type. Enter 2–4 couples and babies to see which pairings work — solve the classic baby-swap genetics problem.
A × O → A or O
AB × O → A or B
O × O → O
a baby fits a couple only if its ABO group AND Rh factor are both possible
Suggested by Mrs Aftab (MMG student)
The babies were swapped
- Baby 1 → Couple 2, Baby 2 → Couple 1
| Baby | Couple 1A+ × O+ | Couple 2AB+ × O+ |
|---|---|---|
| Baby 1A+ | Possible | Possible |
| Baby 2O+ | Possible | AB+ × O+ can only have A or B children |
Each square uses the carrier genotype for an A or B parent (Iᴬi, Iᴮi) and Rh-positive parent (Dd), which covers every child type the couple could have.
| Parent 1 allele \ Parent 2 allele | i | i |
|---|---|---|
| Iᴬ | IᴬiA · 25% | IᴬiA · 25% |
| i | iiO · 25% | iiO · 25% |
| Parent 1 allele \ Parent 2 allele | D | d |
|---|---|---|
| D | DDRh+ · 25% | DdRh+ · 25% |
| d | DdRh+ · 25% | ddRh− · 25% |
| Parent 1 allele \ Parent 2 allele | i | i |
|---|---|---|
| Iᴬ | IᴬiA · 25% | IᴬiA · 25% |
| Iᴮ | IᴮiB · 25% | IᴮiB · 25% |
| Parent 1 allele \ Parent 2 allele | D | d |
|---|---|---|
| D | DDRh+ · 25% | DdRh+ · 25% |
| d | DdRh+ · 25% | ddRh− · 25% |
A genetics learning tool based on standard ABO and Rh inheritance. Rare variants (Bombay phenotype, cis-AB, weak or partial D) can break these rules, so a “not possible” result is not proof of non-parentage and this is not a paternity test. Only DNA testing can establish parentage.
How to use the Blood Type Parent Checker
Enter your valueType the number you want to work with into the Blood Type Parent Checker input field.
Choose your units or optionsSelect the units or options that match your figure. Calculy handles the conversion or formula for you.
Read the resultThe result updates instantly — there is no button to press and no sign-up required.
What is the blood type parent checker?
It solves the classic genetics question: which baby can belong to which couple? You enter the blood groups of two to four couples and of the same number of babies. The checker works out which ABO groups and Rh factors each couple can pass on, then tells you which babies fit which couples — and whether the babies were given to the right families.
It uses the same inheritance rules as our baby blood type predictor, but works in the other direction: from babies who are already born back to their possible parents.
The rule it applies
Iᴬ and Iᴮ are codominant and both dominant over i, so group A can be IᴬIᴬ or Iᴬi, B can be IᴮIᴮ or Iᴮi, AB is IᴬIᴮ and O is ii. For Rh, D is dominant: Rh+ is DD or Dd and Rh− is dd. A baby fits a couple only if both its ABO group and its Rh factor are possible for that couple.
Which children can each couple have?
| Parents | Possible children | Not possible |
|---|---|---|
| O × O | O | A, B, AB |
| O × A | A, O | B, AB |
| O × B | B, O | A, AB |
| O × AB | A, B | AB, O |
| A × A | A, O | B, AB |
| A × B | A, B, AB, O | none |
| A × AB | A, B, AB | O |
| B × B | B, O | A, AB |
| B × AB | A, B, AB | O |
| AB × AB | A, B, AB | O |
Rh: two Rh− parents can only have an Rh− child; if either parent is Rh+, the child can be Rh+ or Rh−.
Worked example: were the babies swapped?
Two babies are suspected of being exchanged in hospital. Blood tests show:
| Couple | Parents | Baby they took home | Baby's group |
|---|---|---|---|
| Mr & Mrs John | A × O | Baby 1 | A |
| Mr & Mrs Syman | AB × O | Baby 2 | O |
The John couple (A × O) can have A or O children — O is possible if Mr John is Iᴬi. The Syman couple (AB × O) can only have A or B children, so baby 2 (O) cannot be theirs. Baby 1 (A) fits either couple. The only consistent matching is baby 2 → John and baby 1 → Syman: the babies were swapped.
How to use this checker
- Choose how many families to compare (2 to 4), or press Load the hospital example.
- For each couple, enter both parents' ABO group and Rh factor.
- For each baby, enter its blood group — Baby 1 is the baby given to Couple 1, and so on.
- Read the verdict and the grid: every baby × couple cell says whether it is possible and, if not, why. Punnett squares for each couple appear below.
Limitations
- A learning tool, not a paternity test. Blood type can only rule couples in or out under standard inheritance; only DNA testing can establish parentage.
- Rare variants exist. The Bombay phenotype (hh), cis-AB alleles and weak or partial D can make a real parent look impossible.
- “Can't decide” is common. Many couples can have children of several types, so blood type often leaves more than one matching.
FAQs
- How can blood type show that two babies were swapped?
- Each couple can only have children with certain blood types. If a baby's blood type is impossible for the couple it went home with, but possible for another couple, the only consistent explanation is that the babies were exchanged. For example, an AB × O couple can only have A or B children, so an O baby cannot be theirs.
- What does the parent checker actually calculate?
- For every couple it works out which ABO groups and Rh factors their children can have, marks each baby as possible or not possible for each couple, and then lists every one-to-one matching of babies to couples in which every baby fits. One matching means blood type settles the question; more than one means it can't.
- Why can an A × O couple have an O baby?
- A type A parent can be IᴬIᴬ or Iᴬi. If they carry the hidden i allele (Iᴬi) and the O parent is ii, each child has a 50% chance of inheriting i from both parents and being type O.
- Can blood type prove who a baby's parents are?
- No. Blood type can only rule a couple in or out under standard inheritance, many unrelated people share the same type, and rare variants such as the Bombay phenotype or cis-AB break the usual rules. This is a genetics learning tool, not a paternity test — only DNA testing can establish parentage.
- What does 'blood type can't decide' mean?
- It means more than one matching of babies to couples is consistent with the blood types. For example, two A × B couples can have a baby of any ABO group, so blood type alone can't tell which baby belongs to which couple.
- Does the Rh factor matter for matching?
- Yes. Two Rh-negative parents can only have an Rh-negative child, so an Rh-positive baby cannot belong to them. If either parent is Rh-positive, the baby can be either. The checker tests ABO and Rh together.