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Blood Type Inheritance: Can O+ Mother & A- Father Have A- or O- Children?

Genetic Background

ABO blood type is determined by three alleles: IA, IB (codominant), and i (recessive). Rh factor is determined by a separate gene: Rh+ (D) dominant over Rh– (d).

Parental Genotypes

  • Mother (O+): ABO genotype ii. Rh genotype: DD or Dd (phenotype Rh+).
  • Father (A–): ABO genotype IAi (since type A can be IAIA or IAi; but to have an O child, he must carry i). Rh genotype: dd (Rh–).

Possible Offspring Genotypes & Phenotypes

Mother's GameteFather's GameteChild ABOChild RhPhenotype
iIAIAiDd (if mom Dd) or dd (if mom dd? but mom is Rh+ so at least one D)A+ or A–
iiiiDd or ddO+ or O–

Correction: The mother is Rh+, so she has at least one D allele. She could be DD or Dd. The father is dd. Therefore:

  • If mother is DD: all children are Dd (Rh+) → phenotypes A+ or O+ only.
  • If mother is Dd: 50% chance child gets d from mom + d from dad = dd (Rh–). Then possible phenotypes: A– (IAi dd) or O– (ii dd).

Answer to the Specific Question

Yes, it IS possible for this couple to have a child with A negative or O negative blood type—but only if the mother is heterozygous for Rh (Dd). If the mother is DD, all children will be Rh+.

Sex of the child (boy/girl) is irrelevant—blood type genes are autosomal.

How to Know Mother’s Rh Zygosity

  • If her parents are known: one Rh– parent guarantees she is Dd.
  • Rh zygosity testing (DNA-based) is available but rarely done routinely.
  • Population frequency: ~55% of Rh+ Caucasians are Dd; higher in some groups.

Key Takeaway

The original statement that it’s "not possible" is incorrect. A negative Rh child is possible if the Rh+ mother carries a recessive Rh– allele.

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