BiologyChapter 12: Inheritance
Incomplete Dominance and Codominance
Incomplete: heterozygote is in-between (red x white snapdragon -> pink school).
Learning objectives
- Explain incomplete dominance and codominance in clear biological language.
- Distinguish 1. incomplete from 2. blood group.
- Use the key terms to answer a short Federal Board exam question.
Definition
Incomplete: heterozygote is in-between (red x white snapdragon -> pink school). F2: 1 red : 2 pink : 1 white (phenotype follows genotype). Codominance: both alleles show (roan cattle; AB blood antigens both present). ABO blood: IA, IB, i. IA and IB codominant; both dominant to i. O is ii. These are exam examples, not a transfusion protocol. Incomplete dominance and codominance extend Mendel's rules beyond simple dominant-recessive pairs. ABO blood groups are the classic codominance exam example.
Important terms
- INCOMPLETE DOMINANCE AND CODOMINANCE
- Incomplete: heterozygote is in-between (red x white snapdragon -> pink school).
- 1. INCOMPLETE
- Neither allele hides fully.
- 2. BLOOD GROUP
- A: IAIA or IAi.
Overview
Blend look vs both look:
Incomplete
Neither allele hides fully. 1 : 2 : 1 of three phenotypes. Do not call pink a 'mix of paint' in a chemistry way; it is gene expression. Snapdragon: RR = red, rr = white, Rr = pink (blend). Phenotype ratio 1:2:1 matches genotype ratio — unlike complete dominance where heterozygote looks dominant.
Examples RR red · Rr pink · rr white
Blood Group
A: IAIA or IAi. B: IBIB or IBi. AB: IAIB. O: ii. Universal donor/recipient is a school shorthand with limits (Rh exists). No matching-how-to for a clinic. IA and IB codominant (AB type shows both antigens). i recessive (O type: ii only). Rh is a separate gene. Universal donor/recipient is exam shorthand with real-world limits.
Examples AB both antigens · O neither A nor B · Rh named as another gene
WHAT — INCOMPLETE DOMINANCE AND CODOMINANCE
Incomplete: heterozygote is in-between (red x white snapdragon -> pink school). F2: 1 red : 2 pink : 1 white (phenotype follows genotype). Codominance: both alleles show (roan cattle; AB blood antigens both present). ABO blood: IA, IB, i. IA and IB codominant; both dominant to i. O is ii. These are exam examples, not a transfusion protocol. Incomplete dominance and codominance extend Mendel's rules beyond simple dominant-recessive pairs. ABO blood groups are the classic codominance exam example.
WHY — Why this topic matters
INCOMPLETE DOMINANCE AND CODOMINANCE is an independent FBISE learning unit because students must connect its structures and processes with their biological functions and examples.
HOW — How it works
INCOMPLETE: Neither allele hides fully. 1 : 2 : 1 of three phenotypes. Do not call pink a 'mix of paint' in a chemistry way; it is gene expression. Snapdragon: RR = red, rr = white, Rr = pink (blend). Phenotype ratio 1:2:1 matches genotype ratio — unlike complete dominance where heterozygote looks dominant. Examples: RR red; Rr pink; rr white. BLOOD GROUP: A: IAIA or IAi. B: IBIB or IBi. AB: IAIB. O: ii. Universal donor/recipient is a school shorthand with limits (Rh exists). No matching-how-to for a clinic. IA and IB codominant (AB type shows both antigens). i recessive (O type: ii only). Rh is a separate gene. Universal donor/recipient is exam shorthand with real-world limits. Examples: AB both antigens; O neither A nor B; Rh named as another gene. Worked: IA i x IB i. Gametes: IA or i, and IB or i. Boxes: IAIB, IAi, IBi, ii. Children: AB, A, B, O possible (1:1:1:1 school).
Worked example
- IA i x IB i.
- Gametes: IA or i, and IB or i.
- Boxes: IAIB, IAi, IBi, ii.
- Children: AB, A, B, O possible (1:1:1:1 school).
In simple words
Blood groups for genetics marks. Not a transfusion manual.
Key points
- pink is incomplete
- AB is codominant
- three phenotypes
- IA, IB, i
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Incomplete Dominance and Codominance
Sticky Notes · Quick Revision
Core Idea
- Incomplete: heterozygote is in-between (red x white snapdragon -> pink school). F2: 1 red : 2 pink : 1 white (phenotype follows genotype). Codominance: both alleles show (roan cattle; AB blood antigens both present). ABO blood: IA, IB, i. IA and IB codominant; both dominant to i. O is ii. These are exam examples, not a transfusion protocol. Incomplete dominance and codominance extend Mendel's rules beyond simple dominant-recessive pairs. ABO blood groups are the classic codominance exam example.
1. INCOMPLETE
- Neither allele hides fully. 1 : 2 : 1 of three phenotypes. Do not call pink a 'mix of paint' in a chemistry way; it is gene expression. Snapdragon: RR = red, rr = white, Rr = pink (blend). Phenotype ratio 1:2:1 matches genotype ratio — unlike complete dominance where heterozygote looks dominant.
2. BLOOD GROUP
- A: IAIA or IAi. B: IBIB or IBi. AB: IAIB. O: ii. Universal donor/recipient is a school shorthand with limits (Rh exists). No matching-how-to for a clinic. IA and IB codominant (AB type shows both antigens). i recessive (O type: ii only). Rh is a separate gene. Universal donor/recipient is exam shorthand with real-world limits.
Exam Example
- Worked: IA i x IB i.
- Gametes: IA or i, and IB or i.
- Boxes: IAIB, IAi, IBi, ii.
- Children: AB, A, B, O possible (1:1:1:1 school).
Common Misconception
- Do not treat 1. INCOMPLETE and 2. BLOOD GROUP as the same idea; each explains a different part of the topic.
FBISE Exam Check
- Define incomplete dominance and codominance, explain the two key parts, and use the correct biological terms.
- Exam reminder: Worked: IA i x IB i.
60-second revision
- Incomplete: heterozygote is in-between (red x white snapdragon -> pink school).
- 1. INCOMPLETE: Neither allele hides fully.
- 2. BLOOD GROUP: A: IAIA or IAi.
- Exam reminder: Worked: IA i x IB i.
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