A child with type AB blood, with a type-A father and a type-B mother, exhibits:

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BlackTom AI · Accepted Answer

The question states: a child with type AB blood, with a type-A father and a type-B mother, exhibits: and provides four answer options. 
Option 1: polygenic inheritance. This would involve many genes contributing to a trait, typically producing a wide range of phenotypes (as with height or skin color). Blood type ABO is determined by a single gene with multiple alleles (IA, IB, i), and the phenotype AB results specifically from the presence of both IA and IB alleles. Therefore, this option does not fit the ABO inheritance pattern in this case.
Option 2: codominance. In ABO blood groups, IA and IB alleles are codominant, meaning both are expressed when present together in an individual. An individual with IA from one parent and IB from the other shows the AB phenotype, displaying both antigens on the surface of red blood cells. This directly explains why a child can be type AB when one parent is type A and the other is type B, making this option consistent with the genetic mechanism involved.
Option 3: mutation. A mutation refers to a change in DNA sequence that creates a new allele. While mutations can alter traits, the ABO AB phenotype arises from the standard IA and IB alleles functioning as codominant, not from a rare or new mutation. Thus, this option mischaracterizes the typical explanation for AB blood type.
Option 4: a genetic abnormality. Labeling the AB blood type as a genetic abnormality would be inaccurate in the context of normal, common ABO genetics. AB blood type is a normal, expected phenotype produced by codominant IA and IB alleles, not an abnormality.
In summary, the reasoning aligns with codominance to explain the AB phenotype in offspring from an A and a B parent, while the other options either describe a different inheritance pattern or mischaracterize the ABO system.

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A child with type AB blood, with a type-A father and a type-B mother, exhibits:

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