Alleles may be dominant, recessive or codominant
Alleles are different versions of a gene. A dominant allele controls the phenotype even when only one copy is present. A recessive allele only shows its effect when both copies match. Codominant alleles both contribute to the phenotype together.
Real World
Sickle cell trait shows codominance: individuals heterozygous (HbᴬHbˢ) produce both normal and sickle-shaped red blood cells, which is why the WHO notes these carriers have partial malaria resistance without full sickle cell disease.
Exam Focus
Distinguish codominance (both alleles expressed in phenotype) from incomplete dominance (blended phenotype) — a common examiner trap.
Evaluation Scaffold
A four-step framework for high-quality evaluation. Use this for 'assess', 'evaluate', and 'to what extent' questions.
How well did you know this?