ABO Gene (Chromosome 9) — 3 Alleles:
  Iᴬ (A antigen) — Co-dominant
  Iᴮ (B antigen) — Co-dominant
  i  (no antigen) — Recessive

Genotype → Phenotype (Blood Type):
  Iᴬ Iᴬ  or  Iᴬ i   →  Type A
  Iᴮ Iᴮ  or  Iᴮ i   →  Type B
  Iᴬ Iᴮ              →  Type AB
  i  i               →  Type O

Rh Factor (Chromosome 1):
  RHD gene → D antigen
  D+ (Rh positive) = Dd or DD
  D− (Rh negative) = dd (recessive)

Result: 8 common blood types:
  A+, A−, B+, B−, AB+, AB−, O+, O−

A and B alleles are co-dominant (both expressed in AB). Both are dominant over O (recessive). Rh positive is dominant over Rh negative.

Parent × Parent → Possible Children
─────────────────────────────────────
O   ×  O   →  O only
O   ×  A   →  O, A
O   ×  B   →  O, B
O   ×  AB  →  A, B
A   ×  A   →  O, A
A   ×  B   →  O, A, B, AB (all!)
A   ×  AB  →  A, B, AB
B   ×  B   →  O, B
B   ×  AB  →  A, B, AB
AB  ×  AB  →  A, B, AB (never O)

Rh Factor:
Rh+ × Rh+  →  Rh+ or Rh−
Rh+ × Rh−  →  Rh+ or Rh−
Rh− × Rh−  →  Rh− only

Note: Cannot predict with certainty
without knowing parents' genotypes.

Two type A parents can have a type O child (if both carry the recessive i allele: Iᴬi × Iᴬi → ii possible). Two AB parents can never have a type O child.

Blood   RBC         Plasma       Can give   Can receive
Type    Antigens    Antibodies   RBCs to    RBCs from
──────────────────────────────────────────────────────
A       A antigen   Anti-B       A, AB      A, O
B       B antigen   Anti-A       B, AB      B, O
AB      A and B     None         AB only    All (universal)
O       None        Anti-A,B     All        O only

Key principle:
  Your body makes antibodies against
  antigens you DON'T have.

  Type A blood has Anti-B antibodies
  → giving B blood to type A patient
  → Anti-B attacks B antigens
  → hemolytic transfusion reaction
  → potentially fatal

This natural antibody system is why type O negative is the 'universal donor' (no antigens to trigger reactions) and AB positive is the 'universal recipient' (no antibodies to attack donor cells).

Risk: Rh− mother + Rh+ father
  → Baby may be Rh+
  → Mother's immune system sees Rh+
    baby blood as "foreign"
  → Produces anti-D antibodies

First pregnancy: Usually safe
  (antibodies develop late)

Subsequent pregnancies with Rh+ baby:
  Mother's anti-D antibodies cross
  placenta → attack baby's RBCs
  → Hemolytic Disease of Newborn (HDN)

Prevention:
  RhoGAM (anti-D immunoglobulin)
  Given at 28 weeks + within 72 hrs
  of delivery
  Success rate: >99%

Before RhoGAM (pre-1968):
  HDN affected ~1% of all pregnancies
  10,000+ infant deaths/year in USA

RhoGAM was one of the most impactful medical interventions of the 20th century, virtually eliminating a major cause of infant mortality and disability.