ABO blood type inheritance (A, B, AB, O)
This calculator predicts possible child blood types and possible child genotypes from two parents using the
ABO system. The ABO system has three alleles: IA, IB, and i.
The probabilities come from standard Mendelian gamete formation (each parent contributes one allele) plus the ABO dominance rules.
This calculator is ABO-only (no Rh factor in this topic).
1) Alleles and dominance rules
ABO has a special dominance pattern:
IA and IB are codominant with each other (both expressed in AB),
and both are dominant over i.
\[
I^{A}\ \text{and}\ I^{B}\ \text{are codominant},\qquad
I^{A} > i,\ \ I^{B} > i
\]
2) Genotype → phenotype mapping
Blood type (phenotype) is determined by genotype:
| Genotype |
Blood type |
| \(I^{A}I^{A}\) or \(I^{A}i\) |
A |
| \(I^{B}I^{B}\) or \(I^{B}i\) |
B |
| \(I^{A}I^{B}\) |
AB |
| \(ii\) |
O |
3) From blood type to possible parent genotypes
Some blood types correspond to one genotype, and some correspond to two genotypes:
| Parent blood type |
Possible genotype(s) |
| A |
\(I^{A}I^{A}\) or \(I^{A}i\) |
| B |
\(I^{B}I^{B}\) or \(I^{B}i\) |
| AB |
\(I^{A}I^{B}\) only |
| O |
\(ii\) only |
If a parent is type A or B and the genotype is unknown, there are two compatible genotypes.
The calculator’s default assumption (when genotype is not specified) is:
it treats the compatible genotypes as equally likely.
If you know the genotype, you can select it and remove this assumption.
4) Gametes and allele probabilities
Each parent produces gametes that carry one allele. The allele probabilities depend on the genotype:
\[
\begin{aligned}
I^{A}I^{A} &\rightarrow P(I^{A}) = 1 \\
I^{A}i &\rightarrow P(I^{A}) = \frac{1}{2},\ \ P(i)=\frac{1}{2} \\
I^{B}I^{B} &\rightarrow P(I^{B}) = 1 \\
I^{B}i &\rightarrow P(I^{B}) = \frac{1}{2},\ \ P(i)=\frac{1}{2} \\
I^{A}I^{B} &\rightarrow P(I^{A}) = \frac{1}{2},\ \ P(I^{B})=\frac{1}{2} \\
ii &\rightarrow P(i)=1
\end{aligned}
\]
5) Cell probabilities (AND rule) and totals (OR rule)
Combine one allele from Parent 1 with one allele from Parent 2. For each allele combination, multiply gamete probabilities
(AND rule). Then group outcomes that lead to the same genotype or blood type by adding probabilities (OR rule).
Example idea (symbolic form): if Parent 1 produces allele \(x\) with probability \(P_1(x)\) and Parent 2 produces allele \(y\)
with probability \(P_2(y)\), then that cell probability is:
\[
P(xy) = P_1(x)\cdot P_2(y)
\]
If a blood type can occur through multiple genotypes (for example A can be \(I^{A}I^{A}\) or \(I^{A}i\)),
then the probability of blood type A is the sum of the probabilities of all genotypes that map to A:
\[
P(\text{type A}) = P(I^{A}I^{A}) + P(I^{A}i)
\]
6) What the calculator outputs
The calculator reports two layers:
(1) child blood type probabilities for A, B, AB, O, and
(2) child genotype probabilities for the six possible ABO genotypes.
It also provides simple factual “possible?” checks (for example, “Is an O child possible?”) based on whether the computed probability is greater than 0.
7) Common outcomes (quick intuition)
These are helpful as a sanity check:
| Parents |
Key possible child types |
Why |
| O × O |
O only |
Both parents are \(ii\), so all children are \(ii\). |
| AB × O |
A or B (not AB, not O) |
AB makes \(I^{A}\) or \(I^{B}\), O makes \(i\). |
| AB × AB |
A, AB, or B (not O) |
No \(i\) allele is available from either parent. |
| A × B |
Could be A, B, AB, or O |
If both parents are heterozygous \(I^{A}i\) and \(I^{B}i\), then O becomes possible. |
If your results look surprising, the first thing to check is whether a parent’s genotype is truly known.
For A and B parents, choosing \(I^{A}I^{A}\) vs \(I^{A}i\) (or \(I^{B}I^{B}\) vs \(I^{B}i\)) can change whether blood type O is possible.
