Genetic determinants of blood Rh factor

Your blood's Rh factor is either positive (Rh+) or negative (Rh-). The Rh+ gene is more common than the Rh- gene, and is dominant.

• If one of your two Rh genes is Rh+ and the other is Rh-, your blood type is Rh+.
• It takes a pair of Rh- genes to make your blood type Rh-.

When an Rh- woman conceives with an Rh+ man, it is probable that their offspring will be Rh+.

• If the father has two Rh+ genes, the fetus will inherit an Rh+ gene. That Rh+ gene, paired with the mother's Rh- gene, will result in an Rh+ fetal blood type.
• If the father has one Rh+ and one Rh- gene, the fetus will inherit one of them, either:
  • An Rh+ gene, resulting in an Rh+ fetal blood type, or
  • An Rh- gene which, when paired with the mother's Rh- gene, results in an Rh- fetal blood type.

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