The Rh factor is inherited through a single gene with two alleles: Rh-positive (D) is dominant, and Rh-negative (d) is recessive. A person inherits one allele from each parent, resulting in either Rh-positive (DD or Dd) or Rh-negative (dd) blood type.
Human blood types are determined by multiple genes, but the Rh factor specifically follows Mendelian inheritance. The gene responsible is located on chromosome 1, and its expression depends on whether the inherited alleles include at least one dominant D allele. If both alleles are recessive (d), the individual is Rh-negative.
Understanding this inheritance pattern explains why Rh-negative individuals can only pass on the d allele, while Rh-positive people may carry a hidden d allele even if they appear positive. This has critical consequences in pregnancy and blood transfusion compatibility.
Each parent contributes one copy of the Rh gene to their child. The D allele produces the Rh protein on red blood cells, making the blood type Rh-positive. The d allele does not produce the protein, leading to Rh-negative status.
When both parents are Rh-positive and have at least one D allele, the child can inherit DD, Dd, or dd genotypes. However, if both parents are homozygous for D (DD), all children will be Rh-positive. If one parent is DD and the other is Dd, all children will still be Rh-positive, though half may carry the recessive d allele.
A child can only be Rh-negative if both parents contribute a d allele. That means two Rh-negative parents (dd × dd) can only have Rh-negative children. But an Rh-positive parent who carries a recessive d allele (Dd) can pass it on, potentially resulting in an Rh-negative child if the other parent also contributes a d.
The following table outlines possible parental combinations and the resulting probabilities for child Rh status:
| Parent 1 Genotype | Parent 2 Genotype | Child Rh-Positive Probability | Child Rh-Negative Probability |
|---|---|---|---|
| DD | DD | 100% | 0% |
| DD | Dd | 100% | 0% |
| Dd | Dd | 75% | 25% |
| dd | dd | 0% | 100% |
| Dd | dd | 50% | 50% |
For example, if both parents are Dd (Rh-positive but carriers), there is a 25% chance their child will be Rh-negative (dd), a 50% chance the child will be Rh-positive and carry the recessive allele (Dd), and a 25% chance the child will be DD and not carry the d allele.
This pattern is consistent across populations. According to the American Red Cross, approximately 85% of people in the U.S. are Rh-positive, meaning most people carry at least one D allele. The remaining 15% are Rh-negative, typically with the dd genotype.
Many assume Rh-negative blood is rare because it's less common, but it's not a defect-it's a normal genetic variation. Rh-negative individuals are not inherently weaker or more prone to disease. The condition arises from a lack of the RhD antigen, not a malfunction.
Another misconception is that Rh factor inheritance depends on ethnicity. While frequencies vary-about 15% of White Americans are Rh-negative, compared to 5% of African Americans and 1% of Asian Americans-the mechanism remains the same across all groups. The difference lies in allele frequency, not inheritance rule.
A non-obvious fact: Rh-negative mothers can carry Rh-positive fetuses without issue during early pregnancy. Problems arise only if fetal blood enters maternal circulation, triggering antibody production. This typically happens during delivery, not conception. The first pregnancy is rarely affected because the mother hasn't yet developed anti-D antibodies.
In obstetrics, Rh incompatibility between mother and fetus can lead to hemolytic disease of the newborn (HDN). If an Rh-negative mother carries an Rh-positive baby, her immune system may produce anti-D antibodies during delivery. These antibodies can cross the placenta in subsequent pregnancies and attack the fetus's red blood cells.
To prevent this, Rh-negative mothers receive an injection of Rh immunoglobulin (RhoGAM) around week 28 of pregnancy and again within 72 hours of birth. This treatment neutralizes any fetal Rh-positive cells in the mother's bloodstream, preventing antibody formation. The Centers for Disease Control and Prevention (CDC) reports that RhoGAM has reduced HDN cases by over 95% since its introduction in the 1960s.
For blood transfusions, Rh-negative recipients must receive Rh-negative blood. Rh-positive recipients can safely receive Rh-positive or Rh-negative blood, but Rh-negative donors cannot give to Rh-positive recipients unless the recipient has no anti-D antibodies. This is why blood banks screen for Rh status carefully.
I once observed a family blood test panel where a father was Rh-positive (Dd), a mother was Rh-negative (dd), and their child was Rh-positive. The result surprised them until we reviewed the Punnett square: the child had a 50% chance of being Rh-positive if the father carried the D allele. The outcome confirmed the inheritance pattern.