When an Rh-negative female conceives a child with an Rh-positive male, the fetus could be either Rh-positive or Rh-negative, depending on the father's Rh genotype. Since Rh-positive is dominant and Rh-negative is recessive, if the father is heterozygous (one Rh-positive allele and one Rh-negative allele), there is about a 50% chance the fetus will be Rh-positive and 50% chance Rh- negative. If the father is homozygous Rh-positive (both alleles Rh-positive), then the fetus will definitely be Rh-positive. Statistics indicate that about half of the children born to an Rh-negative mother and an Rh-positive father will be Rh-positive, reflecting the typical inheritance pattern. Therefore, the chance the fetus will be Rh-positive is approximately 50% if the father carries one Rh-negative allele, higher if he's homozygous Rh-positive.
Inheritance of Rh Factor and Fetus Rh Status
- Rh factor is inherited genetically from both parents.
- Rh-positive is dominant over Rh-negative.
- An Rh-negative mother (genotype is two Rh-negative alleles) can have an Rh-positive fetus if the father contributes the Rh-positive allele.
- If the father is heterozygous Rh-positive (one positive, one negative allele), the fetus has about 50% chance of being Rh-positive.
- If the father is homozygous Rh-positive, the fetus will be Rh-positive (100% chance).
Clinical Importance
- Rh incompatibility occurs when an Rh-negative mother carries an Rh-positive fetus.
- About half of such pregnancies will involve an Rh-positive fetus.
- Rh incompatibility can lead to maternal immune response affecting the fetus in subsequent pregnancies if not managed.
- Preventive treatment with Rh immune globulin (RhoGAM) is recommended for Rh-negative mothers pregnant with Rh-positive fetuses to prevent complications.
Thus, the chance the fetus will be Rh-positive is approximately 50%, assuming the Rh-positive father is heterozygous for the Rh gene, which is common. If more genetic details of the father are known, the probability can be refined.
