What kind of gene expression are the ABO blood types an example of?

Question

BlackTom AI · Accepted Answer

The question asks about the pattern of gene expression demonstrated by ABO blood types.
Option 1: complete dominance. This would mean one allele completely masks the other in heterozygotes, producing a single phenotype for both AA and Aa. That is not how ABO works, since Aa individuals display either A, B, or AB phenotypes rather than a single dominant phenotype, so this option is not accurate for ABO.
Option 2: codominance. In this case, both alleles in a heterozygote are expressed, producing a phenotype that shows traits of both alleles. This matches ABO blood groups, where I^A and I^B are both expressed in individuals with AB blood type, yielding the AB phenotype. Thus, this option aligns with the true expression pattern.
Option 3: incomplete dominance. Here, heterozygotes show an intermediate phenotype between the two homozygotes (for example, a blending of traits). ABO blood types do not show a simple intermediate phenotype; AB is not an intermediate between A and B, but a combination expressing both traits, so this option mischaracterizes ABO expression.
Option 4: recessiveness. This would imply that one allele is completely masked by the other in heterozygotes, resulting in only the dominant phenotype unless the genotype is homozygous recessive. For ABO, heterozygotes can express A or B or AB traits depending on the combination, so calling it recessive is incorrect.
Option 5: polymorphism. Polymorphism refers to the existence of multiple phenotypes or alleles within a population. While ABO is indeed polymorphic at the genetic level with multiple alleles in the population, the question focuses on the mode of gene expression, not the population-level variation. Therefore this option speaks to diversity rather than the expression pattern itself and is thus not the best descriptor of the expression mechanism.
In summary, the reasoning points to codominance as the description that best captures how ABO alleles are expressed in heterozygotes, particularly the AB phenotype where both A and B products are produced.

What kind of gene expression are the ABO blood types an example of?

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