How to use it
- Choose the ABO group and the Rh type of the mother.
- Do the same for the father.
- Read the Possible blood groups of the child: each row is a group and the chance it has.
How the chances are worked out
The ABO blood group is set by one gene with three alleles: IA (group A), IB (group B) and i (group O). A and B are dominant over i, and A and B together give group AB. Group A is IAIA or IAi, group B is IBIB or IBi, group AB is IAIB, and group O is ii. The gene is on chromosome 9.
The Rh type is set by the RHD gene on chromosome 1. The D antigen is dominant: a person with DD or Dd is Rh positive, and only dd is Rh negative. Each child gets one allele of each gene from each parent, with an equal chance for each of the parent's two alleles, and the two genes are treated as independent.
Where a parent's genotype is not known, the calculator gives each genotype of that blood group the same weight. This is an assumption; in real populations the frequencies differ. This is a genetic estimate for learning, not medical advice, and not a paternity test.
A = IAIA or IAi, B = IBIB or IBi, AB = IAIB, O = ii
Rh positive = DD or Dd, Rh negative = dd
Chance of a child's group = chance from ABO x chance from RhWhat these rules do not predict
- Rare variants. The Bombay phenotype and cis AB can give results these rules do not predict.
- Subgroups of A and B, which are not in this simple model.
- Paternity. A blood group can rule out a parent in some cases, but it cannot confirm one; a paternity test is needed for that.
Worked example: mother O negative, father AB positive
The mother is ii, so every child gets an O from her. The father is IAIB, so the child gets an A or a B from him, each with a chance of one half. The child is therefore group A or group B, 50 % each.
The mother is dd and always gives d. The father is positive with DD or Dd, each half of the time; he gives a d with chance 1/2 only if he is Dd, which is 1/4 overall. The child is Rh negative only when the father gives a d, so 25 % of children are negative and 75 % positive.
Multiplying the two gives A+ 37.5 %, A- 12.5 %, B+ 37.5 % and B- 12.5 %.
| Child's blood group | Chance |
|---|---|
| A+ | 37.5 % |
| A- | 12.5 % |
| B+ | 37.5 % |
| B- | 12.5 % |
Frequently asked questions
Can two O parents have a child with another blood group?
No, not for the ABO group. Two O parents are ii, so the child is ii and always group O. The Rh type can still differ if one parent carries a D allele.
Can two Rh-negative parents have a positive child?
No. A child is Rh negative only if both parents pass on a d. Two Rh-negative parents are dd, so the child is dd and negative.
Why does the calculator give a chance and not one answer?
Each child inherits one allele from each parent by chance. The chance shows how often each group would appear across many such children, not what any one child will be.
Can this show who the father is?
No. Blood groups can rule some people out, but they cannot confirm a parent. A paternity test uses DNA and is the only reliable way to confirm a parent.
Are the blood groups I enter stored?
No. The chances are calculated in your browser and nothing you enter is sent to us.
Sources
Figures and text last checked on Tuesday, November 10, 2026.