Sibling Blood Types: Why They Don't Prove Shared Parentage

Blood types follow Mendelian inheritance but cannot definitively prove or disprove full sibling relationships due to genetic complexity.

ABO System Basics

• Alleles: Iᴬ, Iᴮ (codominant), i (recessive).
• Genotypes: IᴬIᴬ or Iᴬi = Type A; IᴮIᴮ or Iᴮi = Type B; IᴬIᴮ = AB; ii = O.

Why Sibling Blood Types Are Inconclusive

• Multiple allele combinations from same parents can produce different blood types in siblings.
• Example: Both parents Type A (genotype Iᴬi) → children can be A (75%) or O (25%).
• Rh factor: D/d alleles; same logic.
• Rare shared type (e.g., AB‑negative) increases probability but doesn't prove shared parents.

What Blood Typing Can Do

• Exclude paternity if child's type is impossible from alleged parents (e.g., O mother + O father → AB child).
• Support (not prove) relationship when combined with other evidence.

Definitive Testing: DNA

• Sibling DNA test: compares STR profiles; calculates sibling index (full vs. half vs. unrelated).
• >99% accuracy for full vs. half distinction with sufficient markers.
• Legal chain‑of‑custody required for court.