Mendelian inheritance · USMLE Step 1

In codominance, which statement is true?

  1. Only the dominant allele is expressed in the phenotype
  2. Both alleles are fully expressed in the phenotype
  3. Neither allele is expressed in the resulting phenotype
  4. The phenotype skips generations in the pedigree
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Correct answer: Both alleles are fully expressed in the phenotype

Codominance occurs when both alleles are fully and simultaneously expressed, as in ABO blood types. This differs from incomplete dominance, where the two phenotypes blend. Codominance does not skip generations.

Difficulty: Medium Question 15 of 20

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