6. A newborn baby got mixed up with other babies in a hospital. If the mother is of O blood group and is Rh +veand the father is of AB blood group and is Rh -ve, which one of the following can be their baby?
(1) AB and Rh +ve       (2) O and Rh -ve
(3) A and Rh +ve         (4) B and Rh –ve

Step-by-Step Genetic Analysis

Step 1: ABO Blood Group Possibilities

Mother: O → genotype ii [web:151][web:152].

Father: AB → genotype I^AI^B.

Gametes:

• Mother: always i
• Father: I^A or I^B

Possible child genotypes:

• I^Ai → phenotype A
• I^Bi → phenotype B

Key conclusion: Child can be A or B only. Cannot be O (needs ii) or AB (needs I^AI^B) [web:122].

Step 2: Rh Factor Possibilities

Mother: Rh+ (DD or Dd)

Father: not specified, but evaluating child Rh status.

• If mother DD: all children Dd → Rh+
• If mother Dd: children Dd (Rh+) or dd (Rh-)

Both Rh+ and Rh- possible.

Step 3: Evaluate Each Option

Option 1: AB and Rh+

ABO: IMPOSSIBLE (O×AB cannot produce AB)

Result: ❌ Not their baby

Option 2: O and Rh-

ABO: IMPOSSIBLE (O×AB cannot produce O)

Result: ❌ Not their baby

Option 3: A and Rh+ ✅ CORRECT

• ABO: ✅ Possible (I^Ai = Type A)
• Rh: ✅ Always possible (Rh+ mother guarantees at least some Rh+ children)

Result: ✅ Perfectly compatible [web:125].

Option 4: B and Rh-

• ABO: ✅ Possible (I^Bi = Type B)
• Rh: ⚠️ Possible only if mother Dd (not guaranteed if DD)

Result: Compatible but not certain [web:123].

Why A, Rh+ is the Best Answer

Exam convention: Choose the option that is always genetically possible regardless of parental genotypes within phenotypes.

• A Rh+: Always possible from O Rh+ × AB
• B Rh-: Only possible if mother is heterozygous Dd

Final Conclusion

The uniquely safe match is A, Rh+ (option 3) – guaranteed compatible regardless of exact maternal Rh genotype [web:152].