Rh Factor - Why Two Positive Parents Can Have a Negative Child

Here I hope to clear up any confusion about Rhesus factor blood type inheritance.

The Rhesus factor in blood type is determined by a pair of alleles, one inherited from each parent. The positive (+) allele is dominant, while the negative (–) allele is recessive.

• Individuals with the genotype +/+ are homozygous dominant and are Rhesus positive.

• Individuals with the genotype +/– are heterozygous and are also Rhesus positive, but they carry one hidden (recessive) negative allele.

• Individuals with the genotype –/– are homozygous recessive and are Rhesus negative.

Because of this:

• Two parents who are heterozygous (+/–) — that is, considered Rhesus positive but carriers of the negative allele — can produce an Rhesus negative child if each passes on their “–” allele.

• Two homozygous positives (+/+), however, cannot have an Rhesus negative child, as they do not possess a negative allele to pass on.

In short, people who are “positive” are not all genetically identical — some are “fully positive” (+/+) while others are “carriers” (+/–), which explains how two Rhesus positive parents can have an Rhesus negative child.