AB positive blood is relatively rare because it requires inheriting both the A and B alleles from your parents, combined with the Rh positive factor. The infrequency of these specific combinations makes AB+ the least common blood type in many populations.
Here's a breakdown of why:
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Inheritance: Blood type is determined by the genes inherited from your parents. You receive one allele for blood type from each parent. The alleles are A, B, and O.
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AB Blood Type: For someone to have AB blood, they must inherit an A allele from one parent and a B allele from the other. If either parent donates an O allele, the offspring won't have AB blood (unless the other parent donates both an A and B, which is not possible).
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Rh Factor: The Rh factor is a separate genetic trait. To be Rh positive (AB+), you need to inherit at least one Rh positive allele. Since Rh-positive is dominant to Rh-negative, someone who is Rh+ could have either two Rh+ alleles or one Rh+ and one Rh- allele.
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Low Probability Combination: The scarcity of AB positive blood arises because it requires the specific combination of:
- Inheriting both the A and B alleles for AB blood type.
- Inheriting at least one Rh positive allele.
Since the genes for ABO blood group and Rh factor are inherited independently, we can deduce why AB+ is rare. Both A and B alleles need to be present and at least one Rh+ allele. The lower the frequency of all the A, B, and Rh+ alleles, the lower will be the chance that both A and B alleles exist together in a person with Rh+ allele.
In Summary: The relatively low global prevalence of the A allele combined with the B allele and then adding the criteria of possessing the Rh+ factor is what contributes to the rarity of the AB positive blood type. The mathematical probability of inheriting these combinations is simply lower than for other blood types.
