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Incomplete Dominance, Codominance, and Linkage – NEET Genetics Simplified

Incomplete dominance, codominance and linkage are important concepts in Genetics for NEET Biology. This NCERT-focused guide explains each concept in simple language with examples, ratios, comparison tables, mnemonics and practice MCQs for NEET Biology 2027.

Study Note: Use this article along with NCERT Biology. Understand the inheritance pattern first, then practise questions based on genotype, phenotype and ratios.

Quick Answer

The three concepts can be remembered very simply:

  • Incomplete dominance: Heterozygote shows an intermediate phenotype.
  • Codominance: Both alleles express themselves in the heterozygote.
  • Linkage: Genes located close together on the same chromosome tend to be inherited together.

In This Article

  • Incomplete Dominance
  • Codominance
  • Linkage
  • Types of Linkage
  • Incomplete Dominance vs Codominance
  • Linkage and Independent Assortment
  • Important NEET Ratios
  • Quick Revision Table
  • NEET Biology MCQs
  • Preparation Strategy

1. Incomplete Dominance

Incomplete dominance is an inheritance pattern in which neither allele completely dominates the other. As a result, the heterozygote shows an intermediate phenotype between the two homozygous phenotypes.

Classic Example: Snapdragon Flower Colour

Flower colour in snapdragon is a commonly used example of incomplete dominance. When a red-flowered plant is crossed with a white-flowered plant, the heterozygous offspring may produce pink flowers.

Genotype Phenotype
RR Red flowers
rr White flowers
Rr Pink flowers

Important NEET Ratio

When two heterozygotes are crossed:

Rr × Rr → RR : Rr : rr = 1 : 2 : 1

Because the heterozygote has a distinct intermediate phenotype, the phenotypic ratio is also 1 : 2 : 1.

🧠 NEET Trick:

Incomplete = Intermediate
The heterozygote shows an intermediate phenotype.

2. Codominance

Codominance occurs when both alleles of a gene pair are expressed simultaneously in the heterozygous condition.

There is no blending of the two alleles. Instead, the characteristics associated with both alleles can be observed.

Classic Example: ABO Blood Group

The human ABO blood group system provides an important example of codominance.

The alleles IA and IB are codominant.

Therefore:

IAIB → Blood Group AB

Both A and B antigens are expressed in the phenotype.

Codominance vs Incomplete Dominance

Feature Incomplete Dominance Codominance
Heterozygote Intermediate phenotype Both phenotypes/features expressed
Blending Intermediate appearance occurs No blending; both expressions remain detectable
Example Snapdragon flower colour AB blood group
Typical F2 phenotype ratio 1 : 2 : 1 1 : 2 : 1 for the corresponding genotypes in a simple heterozygous cross
🧠 NEET Trick:

Co = Coexist
Both alleles express themselves together.

3. Linkage

Linkage refers to the tendency of genes located close together on the same chromosome to be inherited together.

Linked genes do not behave exactly like genes located on different chromosomes that undergo independent assortment.

Key Idea

Closer genes → stronger linkage → lower chance of recombination.

4. Linkage and Independent Assortment

Mendel's law of independent assortment applies to genes that assort independently. However, genes located close together on the same chromosome tend to be transmitted together because of linkage.

Crossing over during meiosis can produce recombinant combinations. The frequency of recombination generally increases as the distance between two linked genes increases.

Gene Relationship Expected Behaviour
Genes very close together Strong linkage; fewer recombinants
Genes farther apart on same chromosome More recombination
Genes on different chromosomes Can assort independently

5. Types of Linkage

Complete Linkage

In complete linkage, linked genes are inherited together with no detectable recombinant offspring in the particular cross being considered.

A classical example discussed in genetics is linkage involving genes in male Drosophila, where crossing over does not occur in males.

Incomplete Linkage

In incomplete linkage, crossing over can occur between homologous chromosomes, producing some recombinant combinations along with parental combinations.

Female Drosophila provides a classical example used to explain recombination and linkage.

🧠 NEET Trick:

Closer genes = stronger linkage.
Greater distance between genes generally means a greater opportunity for crossing over and recombination.

6. Linkage vs Independent Assortment

Feature Linkage Independent Assortment
Location Genes are on the same chromosome Genes may be on different chromosome pairs or sufficiently far apart
Inheritance Tends to be together Alleles assort independently
Recombination Can be reduced because of linkage Independent combinations are produced

7. Important Genetics Ratios for NEET

Incomplete dominance:
Phenotypic ratio in a typical F2 cross = 1 : 2 : 1

Genotypic ratio:
1 : 2 : 1

Important: Do not automatically apply the classic Mendelian 3 : 1 phenotypic ratio to incomplete dominance.

8. NEET Genetics Quick Revision Table

Concept Key Idea Example Memory Trick
Incomplete Dominance Heterozygote shows intermediate phenotype Snapdragon Incomplete = Intermediate
Codominance Both alleles are expressed AB blood group Co = Coexist
Linkage Genes on same chromosome tend to be inherited together Drosophila genetics Closer = Stronger

9. NEET Biology MCQs

1. In incomplete dominance, the heterozygote generally shows:

A. Only the dominant phenotype
B. Only the recessive phenotype
C. An intermediate phenotype
D. No phenotype

✅ Answer: C. An intermediate phenotype

2. Which is a commonly cited example of incomplete dominance?

A. ABO blood group
B. Snapdragon flower colour
C. Haemophilia
D. Colour blindness

✅ Answer: B. Snapdragon flower colour

3. In a typical F2 cross involving incomplete dominance, the phenotypic ratio is:

A. 3 : 1
B. 9 : 3 : 3 : 1
C. 1 : 2 : 1
D. 1 : 1

✅ Answer: C. 1 : 2 : 1

4. In codominance:

A. One allele completely masks the other
B. Both alleles express themselves in the heterozygote
C. Neither allele is expressed
D. Both alleles disappear

✅ Answer: B. Both alleles express themselves in the heterozygote

5. The genotype IAIB produces:

A. Blood group A
B. Blood group B
C. Blood group O
D. Blood group AB

✅ Answer: D. Blood group AB

6. Linkage refers to:

A. Genes located on different chromosomes always separating
B. Genes close together on the same chromosome tending to be inherited together
C. Mutation of a gene
D. Duplication of chromosomes

✅ Answer: B. Genes close together on the same chromosome tending to be inherited together

7. The probability of recombination generally decreases when two genes:

A. Are very far apart
B. Are close together on the same chromosome
C. Are on different organisms
D. Are both recessive

✅ Answer: B. Are close together on the same chromosome

8. Which statement is correct?

A. Incomplete dominance means both alleles are fully expressed separately
B. Codominance produces an intermediate phenotype
C. In codominance, both alleles can be expressed
D. Linkage occurs only between genes on different chromosomes

✅ Answer: C. In codominance, both alleles can be expressed

9. Which concept is correctly matched with its example?

A. Incomplete dominance — Snapdragon
B. Codominance — AB blood group
C. Linkage — Drosophila genetics
D. All of the above

✅ Answer: D. All of the above

10. Which mnemonic is most useful for remembering codominance?

A. Incomplete = Intermediate
B. Co = Coexist
C. Closer = Stronger
D. Many = One

✅ Answer: B. Co = Coexist

NEET Biology for Odisha Students

Students preparing for NEET in Odisha can use this NCERT-focused Genetics resource for concept learning, revision and MCQ practice.

For effective NEET Biology preparation in Odisha, focus on NCERT concepts, practise questions regularly and maintain a consistent revision schedule.

10. How to Study Genetics for NEET

Step 1 — Read NCERT: Understand the basic terminology before attempting MCQs.

Step 2 — Compare concepts: Learn the difference between incomplete dominance and codominance.

Step 3 — Memorise examples: Snapdragon, ABO blood group and Drosophila linkage are useful examples.

Step 4 — Practise ratios: Pay special attention to the 1 : 2 : 1 ratio associated with incomplete dominance.

Step 5 — Revise mistakes: Keep a short list of concepts that you repeatedly get wrong.

11. Test Your Preparation

Try all 10 MCQs without looking at the answers.

  • 9–10 correct: Excellent concept clarity.
  • 7–8 correct: Good preparation; revise weak points.
  • 5–6 correct: Revisit the concepts and examples.
  • Below 5: Read the topic again and practise another set of questions.

12. Final NEET Revision

Incomplete Dominance → Heterozygote shows an intermediate phenotype.

Codominance → Both alleles express themselves in the heterozygote.

Linkage → Genes close together on the same chromosome tend to be inherited together.

Remember:

  • Incomplete = Intermediate
  • Co = Coexist
  • Closer genes = Stronger linkage

📚 Related NEET Biology Resources

➜ Gene Interaction: Epistasis, Pleiotropy & Polygenic Inheritance

➜ Mendelian Genetics and Inheritance

➜ Genetics MCQs for NEET Biology

➜ Class 12 Biology NCERT Notes

➜ NEET Biology Resources for Odisha Students

Prepare Genetics the NCERT-First Way

Understand the concept → Learn the example → Practise MCQs → Analyse mistakes → Revise.

Explore NEET Biology →

Disclaimer: This article is provided for educational and exam-preparation purposes. Students should refer to the prescribed NCERT Biology textbook for the official curriculum and terminology.

This website is an independent educational resource and is not affiliated with or endorsed by NCERT, NTA, NEET or any examination authority.

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