Can two O parents have a child with another blood type?

No. In the ABO system O is recessive, so two O parents can only pass on O and every child will be type O. The Rh factor is separate: two Rh positive O parents can still have an Rh negative child if both carry a hidden negative gene, giving O positive or O negative.

How is the Rh factor inherited?

Rh positive is dominant and Rh negative is recessive. A positive parent may carry two positive genes or one of each, so two positive parents can still produce a negative child if each passes on a negative gene. A child can only be Rh negative if both parents carry at least one negative gene.

Can this calculator give the exact odds?

It shows which blood types are possible rather than exact percentages, because the odds depend on each parent's hidden genotype, which a blood group alone does not reveal. It is an educational guide to inheritance and should not be used for paternity or medical decisions.

Blood Type Calculator

This blood type calculator shows the blood groups a child could inherit from two parents, using the same ABO and Rh rules taught in biology class. Blood type is set by two separate genes. The ABO gene carries the A, B and O forms, where A and B are both dominant over O, so an A parent may secretly carry an O, and only two O forms together give type O. The Rh gene is simpler: positive is dominant and negative is recessive, so a negative child needs a negative form from each parent. Choose each parent's blood group, including the plus or minus for the Rh factor, and the calculator crosses every genotype the two groups allow to list the types a child could be. Because a blood group hides whether a parent carries a recessive form, the tool shows which types are possible rather than exact odds. It is a great way to understand inheritance, check a homework answer or satisfy family curiosity. It is not a paternity test and not a medical tool: rare exceptions such as the Bombay group and subtypes exist, and real testing is the only way to confirm a blood type, so treat the result as an educational guide only.

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Mother's blood type

Father's blood type

A+, A-, O+, or O-

possible blood types for the child

Possible ABO typesA or O

Possible Rh+ or -

Combinations4

Parents who are O+ and A+ can have a child who is A+, A-, O+, or O-.

This shows possible types, not exact odds, because that depends on each parent's hidden genotype. Rare exceptions exist. Educational guide only, not a paternity or medical test.

How it works

Each parent carries two ABO forms and two Rh forms, one of each passed to the child. A blood group only tells you the outward type, so an A parent could be AA or AO, and an Rh positive parent could carry two positive forms or one positive and one negative. The calculator lists every genotype each group allows, crosses the mother's forms with the father's, and collects every blood type those crosses can produce. A child is type AB only if it inherits an A from one parent and a B from the other, and type O only if it inherits an O from both. A child is Rh negative only when both parents pass on a negative form.

Worked example

Take an O positive mother and an A positive father. The mother can only pass on O. The father is A, so he carries either two A forms or one A and one O, meaning he can pass on A or O. Crossing these gives a child who is type A or type O for the ABO group. Both parents are Rh positive but each could carry a hidden negative, so the child could be positive or negative. Putting the two genes together, the child could be A positive, A negative, O positive or O negative: four possible blood types in all.

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