If the allele for blood type O is recessive, how come the majority of people have O as their blood type?

Answer

Dear Nina,

Why couldn’t a recessive gene be universal by definition? This is a common misinterpretation (as is the idea that a dominant allele should a priori be general… which is also incorrect). No, the status as a dominant or recessive phenotype of an allele is not directly related to the frequency of that allele in a population. Thus, a recessive allele can be very common, while a dominant allele can be rare. The concepts of dominance and recessivity are often misinterpreted and can really only be interpreted “relatively”. In other words, an allele is not in itself dominant or recessive, it is the phenotypic effects of that gene that are dominant or recessive. For example, if we look at the gene that causes sickle cell anemia, we see that this gene shows a whole range of phenotypic effects, each with its own dominance/recessivity. Under normal atmospheric conditions, the sickle cell anemia gene (producing the so-called hemoglobin S) behaves like a recessive allele, because no symptoms of sickle cell anemia will occur in heterozygous individuals (they only appear in homozygotes for the sickle cell gene or in heterozygotes when they are under conditions come from low oxygen pressure, e.g. in the high mountains). BUT: the sickle cell allele also offers an increased resistance to malaria and for that increased resistance the sickle cell allele behaves as a dominant gene, because heterozygous individuals for the sickle cell allele (which therefore do not exhibit sickle cell anemia under normal circumstances) do have an increased resistance to malaria ( as are individuals homozygous for the sickle cell allele). In short, one and the same allele can have a different dominance/recessivity status depending on the phenotypic effect being looked at.
However, even apart from the relative interpretation of dominance and recessivity, there is no relationship between dominance or recessivity and the frequency of an allele. There are many other examples where a dominant allele is rare, or a recessive allele is most frequent. For example, a certain form of polydactyly (an abnormality in which extra fingers and toes develop) is caused by a dominant allele, but this allele only occurs in about one in 400 births (in the US). This means that the vast majority of people are homozygous for the recessive (normal) allele.

Hopefully this explanation was a bit clear.

Kind regards,
Thierry Backeljau