Blood Type Calculator

The results that surprise people follow directly from recessive alleles. Two type A parents can have a type O child if both carry AO, because each may pass the O. Two Rh-positive parents can have an Rh-negative child for exactly the same reason. Conversely, two Rh-negative parents can only have Rh-negative children, since neither has a positive allele to pass on — one of the few genuinely absolute rules here. That asymmetry is why blood type can sometimes exclude a possible parent but can never confirm one: many different people share any given blood type, so exclusion is informative while a match proves nothing.

ABO is controlled by three alleles: A and B are co-dominant while O is recessive, so each parent passes one allele and the child’s group follows from the pair. Rh positive is dominant, so a positive parent may still carry a recessive negative allele. Combined probability is the ABO chance multiplied by the Rh chance.

Probabilities assume each possible parental genotype is equally likely, which is the standard textbook simplification — a type A parent may carry AA or AO and there is no way to know which without genetic testing, and real population frequencies are not equal. Rare exceptions exist, including the Bombay phenotype, cis-AB and weak D variants, any of which can produce results this model calls impossible. This is educational only and cannot establish or rule out parentage.

A type A parent and a type B parent can have a child of any ABO group — A, B, AB or O — because both may carry a recessive O allele. By contrast, an AB parent and an O parent can only have A or B children, never AB and never O.

1. Select each parent’s ABO blood group.
2. Select each parent’s Rh factor, positive or negative.
3. Read the possible child blood types and their probabilities.
4. Note which types are listed as not possible.

• Assuming a child must share a blood type with one parent.
• Thinking two positive parents cannot have a negative child.
• Treating a blood type match as proof of parentage.
• Forgetting that A and B parents can produce all four ABO groups.

The Blood Type Calculator works out which blood types a child can inherit, given both parents’ ABO group and Rh factor. It lists every possible outcome with its probability, and — often more usefully — which types are genetically impossible for that pairing.

Biology students learning Mendelian inheritance, expectant parents who are curious, and anyone studying genetics.

• Covers ABO and Rh together, not just one of them.
• Shows which blood types are impossible, which is the genuinely informative part.
• Gives probabilities rather than just a list of possibilities.
• Explains the reasoning, so it works as a teaching aid.

• Checking a genetics homework answer.
• Understanding how two positive parents can have a negative child.
• Learning why blood type A parents can have a type O child.
• Exploring inheritance patterns for a biology class.