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Rh Factor Inheritance: Why Two O-Negative Parents Can Only Have O-Negative Children

The Rh blood group system is determined by the RHD gene on chromosome 1. The D antigen (Rh factor) is either present (Rh-positive) or absent (Rh-negative).

Genetics

  • Rh-Positive: At least one functional RHD allele (DD or Dd).
  • Rh-Negative: Two deleted/nonfunctional RHD alleles (dd).

O-Negative Parents (Genotype: OO dd)

  • ABO: Both ii → only i allele to transmit.
  • Rh: Both dd → only d allele to transmit.

Offspring Genotype

ABO:  i × i  →  ii (Type O) — 100%
Rh:   d × d  →  dd (Rh-negative) — 100%

Result: All children will be Type O, Rh-negative (O negative).

Exceptions (Vanishingly Rare)

  • RHD Gene Rearrangement: Partial D, weak D, Del — serologically Rh-negative but genetically variant.
  • Chimerism: Two zygotes fuse → two cell lines.
  • Somatic Mutation / Gene Conversion: Theoretical.
  • Lab Error / Non-Paternity: Most common "explanation."

Clinical Note

Rh-negative mothers carrying Rh-positive fetuses (from Rh-positive father) risk alloimmunization → HDFN. Rho(D) immune globulin (RhoGAM) at 28 weeks and postpartum prevents this. Two Rh-negative parents eliminate this risk entirely.

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