Commonly confused
Incomplete dominance vs codominance
Both break the simple dominant-recessive rule. The only difference is this: does the in-between form mix the two, or show both at once?
Incomplete dominance
Red flower × white flower gives pink. Pink is not a new gene. It is red and white mixing. Cross two pinks and red and white come back (1 : 2 : 1). The mix is the clue.
Codominance
Blood group AB. The A marker and the B marker both sit on the red cells, fully, side by side. Nothing mixes into a middle form. You see both, clearly.
The trap
Students swap these because both give a 1 : 2 : 1 ratio and both break simple dominance. One question decides it: is the middle form a mix, or both traits together? Pink is a mix, so incomplete dominance. AB shows both, so codominance.
Quick check
RR is red, rr is white, and Rr is pink. This is:
- ACodominance
- BIncomplete dominance
- CComplete dominance
- DA test cross