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Inherited Disorders and Birth Defects

Blood Type

12/08/2010

Question:

Is it possible to have a child with type o blood if both parents are not type O? Thanks.

Answer:

Yes, it is possible for parents who are not type O to have a child with type O blood.

ABO blood types are inherited. A person’s blood type is determined by the inheriting 1 of 3 different forms of a gene (called alleles) from each parent - in this case - A, B, or O.

If a person has 2 A alleles – they have A blood type. If they have 1 A allele and 1 O allele they have A blood type, because A is dominant over O.

If a person has 2 B alleles – they have B blood type. If they have 1 B allele and 1 O allele they have B blood type, because B is dominant over O.

If a person has 1 A allele and 1 B allele – they have AB blood type. AB alleles are co-dominant, so they both show up.

If a person has 2 O alleles – they have O blood type.

Both A and B forms of the gene are dominant over O. If you have a child who has O blood type, then you would expect the parents to be either A blood type (which means the parent has AO genotype) or B blood type (which means the parent has BO genotype). In this case, the parents each have a 50-50 chance of passing on the O genotype. If they child receives both O alleles (one from each parent), the child will then have the O blood type.

For more information:

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Response by:

Anne Matthews, RN, PhD Anne Matthews, RN, PhD
Associate Professor of Genetics
School of Medicine
Case Western Reserve University