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Which parent decides the blood type of the child?

Blood type is an inherited trait that is determined by the genes passed down from a child’s biological parents. A child’s blood type depends on the specific combination of blood types of the mother and father.

How Blood Types Are Inherited

There are four main blood types: A, B, AB, and O. Each blood type is based on the presence or absence of antigens (proteins) on the surface of red blood cells:

  • Blood type A has the A antigen
  • Blood type B has the B antigen
  • Blood type AB has both A and B antigens
  • Blood type O has neither A nor B antigens

In addition to their A and B antigens, people can also have an Rh factor, which is either positive or negative. So the most common blood types are A+, A-, B+, B-, O+, O-, AB+, and AB-.

Each person inherits one blood type gene from their biological mother and one from their biological father. The blood type gene has two alleles – A and B. The inherited alleles determine someone’s blood type:

Blood Type Genotype
A AA or AO
B BB or BO

The inheritance patterns of the A and B alleles explain how a child’s blood type is determined by their parents’ blood types:

  • A and B are dominant over O
  • O is recessive
  • AB contains both A and B alleles, so it is the dominant blood type
  • If one parent is AB and the other is O, the child will inherit A or B from the AB parent
  • Two parents with O blood type can only have a child with O blood type

Predicting a Child’s Blood Type

Using knowledge of how the A and B blood type alleles are inherited, you can predict the possible blood types of a child based on their parents’ blood types.

Below is a chart showing the probability of a child having a certain blood type based on different combinations of parental blood types:

Parents’ Blood Types Possible Child Blood Types
O x O O (100%)
O x A O (50%) or A (50%)
O x B O (50%) or B (50%)
O x AB A (50%) or B (50%)
A x A A (100%)
A x B A (50%), B (50%), or AB (25%)
A x AB A (50%) or AB (50%)
B x B B (100%)
B x AB B (50%) or AB (50%)
AB x AB A (25%), B (25%), or AB (50%)

Based on the parents’ blood types, you can see which blood types the child could possibly inherit. For example, if one parent has blood type A and the other has blood type B, the child could have blood type A, B, or AB.

The Rh Blood Factor

In addition to the A and B antigens, blood also contains the Rh factor, which is inherited separately from the A/B alleles. The Rh factor refers to the presence (Rh+) or absence (Rh-) of the Rh antigen on the surface of red blood cells.

Just like with the A/B alleles, a person inherits one Rh factor gene from each biological parent. The Rh allele can be dominant (Rh+) or recessive (Rh-). Some key points about Rh inheritance:

  • If both parents are Rh+, the child will always be Rh+
  • If one parent is Rh+ and the other is Rh-, there is a 50/50 chance of the child being Rh+ or Rh-
  • If both parents are Rh-, the child can only be Rh-

The Rh blood factor does not affect the actual letter blood type. So a person with A+ blood has the same antigens on their red blood cells as someone with A- blood, but the Rh factor is positive rather than negative.

Special Cases of Inheritance

While blood type inheritance generally follows the rules described above, there are some special cases to consider:

  • Bombay Phenotype – This very rare blood type lacks the standard A and B antigens. It can occur when a person inherits two recessive hh genes, resulting in type O blood, but also lacks the H antigen that produces the O blood group antigens.
  • Rh Null Phenotype – Also extremely rare, this blood type lacks Rh antigens on red blood cells entirely. It happens when a person inherits two recessive Rh genes.
  • Cis-AB – In this case, a person has the A and B genes on the same chromosome rather than on different chromosomes. This results in the person’s blood registering as AB rather than the expected A or B type based on their parents.

These rare blood types can occur randomly due to genetic mutations on the inheritance of parents’ blood type alleles.

Blood Tests Can Determine Baby’s Blood Type Before Birth

While blood type inheritance rules can predict a baby’s likely blood type based on their parents’ types, modern medicine allows for blood typing before the baby is even born.

There are two main tests that can determine fetal blood type:

  • Amniocentesis – This test analyzes amniotic fluid surrounding the fetus for blood type. It is usually done around week 15 of pregnancy.
  • Chorionic villus sampling – In this test, a small piece of the placenta is removed and tested. It can be done earlier than amniocentesis, around week 10-12.

Determining fetal blood type from these tests allows doctors to identify any Rh incompatibility between mother and baby early in pregnancy. This allows steps to be taken to prevent complications.

Blood Type Compatibility between Mother and Baby

Blood type compatibility between mother and fetus is especially important when considering the Rh blood factor. If mother and baby have incompatible Rh statuses, it can lead to Rh isoimmunization which causes the mother’s immune system to attack fetal blood cells:

  • Rh- mother and Rh+ baby – Most concerning mismatch. Mother produces antibodies against fetal Rh+ cells.
  • Rh+ mother and Rh- baby – Not an issue, as mother does not make antibodies against the fetal blood.

If left untreated, Rh isoimmunization can cause serious complications including fetal anemia, jaundice, brain damage, or even death. Therefore, doctors recommend special prenatal care when an Rh- mother is carrying an Rh+ baby to prevent this from developing.

Treatment Options for Rh Incompatibility

There are two main treatment options used to prevent problems with an Rh incompatible pregnancy:

  • Rh immunoglobulin (RhIg) – An injection given to the mother at 28 weeks and again within 72 hours after birth prevents her body from making antibodies against fetal blood cells.
  • Intrauterine blood transfusion – If antibodies have already begun attacking the fetal blood cells, a blood transfusion can be performed through the umbilical cord to directly give the baby healthy blood.

When properly managed, Rh incompatibility generally does not cause pregnancy complications or health problems for the newborn. Discuss all options with your doctor if you have an Rh incompatible pregnancy.


In summary, a child’s blood type is determined by the combination of blood types of their biological parents. The inheritance patterns of the blood type alleles and Rh factor dictate which blood types are possible for a child from given parental blood types. While in most cases blood type can be predicted, there are some rare cases where unusual inheritance can lead to unexpected blood types. Modern medicine allows for fetal blood typing during pregnancy so any incompatibilities between mother and baby can be identified early and managed appropriately.