Codominance, multiple alleles & incomplete dominance
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Question
Define incomplete dominance.
Answer
Neither allele is fully dominant, so the **heterozygote shows a new, intermediate (blended)** phenotype (e.g. red × white → **pink**).
Question
Define codominance.
Answer
Both alleles are **fully expressed at the same time** in the heterozygote — you see **both** phenotypes together (not a blend).
Question
What are multiple alleles?
Answer
A gene with **more than two** alleles in the population (e.g. ABO has **I^A, I^B and i**), though each individual still carries only **two**.
Question
F2 phenotype ratio from two pink (incomplete-dominance) flowers?
Answer
**1 red : 2 pink : 1 white** — the phenotype ratio equals the 1 : 2 : 1 genotype ratio because every genotype is visible.
Question
How do you tell incomplete dominance from codominance?
Answer
**Incomplete dominance** = a single **blended** phenotype. **Codominance** = **both** phenotypes shown **together** (side by side).
Question
Why are codominant/incomplete alleles written as capitals with superscripts (e.g. C^R, C^W)?
Answer
Because **neither allele is recessive**, so neither should be written lower case — superscripts keep them equal.
Question
Which ABO alleles are codominant, and which is recessive?
Answer
**I^A and I^B are codominant** (both expressed → group AB); both are **dominant to i**, which is **recessive** (group O = i i).
Question
Give the genotype(s) for each ABO blood group.
Answer
A = I^A I^A or I^A i; B = I^B I^B or I^B i; **AB = I^A I^B**; **O = i i**.
Question
Why is ABO blood group an example of discrete variation?
Answer
Each genotype maps to one of only **four separate groups** (A, B, AB, O) with **no in-betweens** — distinct categories, not a continuous range.
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