Q.169 Zoology ⭐ 360° Option Elimination Active

NEET 2026 Zoology Question 169 Solution

Topic: Principles of Inheritance and Variation · Official NTA Booklet Code 50

Question

For a person with blood group 'O', which of the following is not a possible combination of parents' blood group genotypes?

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Verified Against NTA Final Answer Key

Option (4) : Father: I^A I^B, Mother: I^A i

📚 Principles of Inheritance and Variation Medium

📋 Given Parameters & Problem Setup

Offspring Blood Group Group 'O' (Genotype: ii)

Target Condition Parental combination that CANNOT produce this child

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The 30-Second Exam Hall Shortcut: ⚡ The "AB Cannot Have O Child" Law

⏱️ Save ~45 seconds

A parent with blood group AB (I^A I^B) can NEVER produce a child with blood group O (ii)!

1 Target offspring has blood group O, meaning homozygous recessive genotype: ii.

2 To produce an ii child, BOTH father and mother MUST contribute at least one "i" allele.

3 Scan parental combinations for any parent who has ZERO "i" alleles.

4 In Option (4): Father has genotype I^A I^B. His gametes can ONLY be I^A or I^B; he can NEVER produce a gamete with "i".

5 Therefore, an I^A I^B father can NEVER have an "O" (ii) child!

6 Option (4) is immediately identified in 5 seconds.

🛡️

Why You Can 100% Trust This Shortcut: NCERT Biology · Principles of Inheritance and Variation (Sex Linkage & Recombination: Morgan’s Drosophila crosses show tight linkage).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Genetics of ABO Blood Group System

ABO blood groups are controlled by the gene I, which has three alleles: I^A, I^B, and i. I^A and I^B are codominant with each other and completely dominant over the recessive allele i.

\text{Blood Group O} \implies \text{Genotype: } ii \implies (\text{One } i \text{ from father}) + (\text{One } i \text{ from mother})

• Any child with blood group O requires that both parents carry at least one recessive i allele.

Step 2

Evaluate Parental Combinations

Examine each given parental pair to check if both parents can produce an "i" gamete:

\begin{aligned} (1) & \text{ Father: } I^A i, \text{ Mother: } I^B i \implies i \times i \rightarrow ii \quad (\text{Possible: 25\%}) \\ (2) & \text{ Father: } I^A i, \text{ Mother: } ii \implies i \times i \rightarrow ii \quad (\text{Possible: 50\%}) \\ (3) & \text{ Father: } ii, \text{ Mother: } ii \implies i \times i \rightarrow ii \quad (\text{Possible: 100\%}) \\ (4) & \mathbf{\text{Father: } I^A I^B}, \text{ Mother: } I^A i \implies \text{Father can only donate } I^A \text{ or } I^B \implies \text{Child } \mathbf{ii} \text{ is } \mathbf{IMPOSSIBLE!} \end{aligned}

• Possible offspring of (4) are only: I^A I^A (A), I^A i (A), I^A I^B (AB), or I^B i (B). None can be O.

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360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 A

✗ Incorrect

Verdict & Scientific Reason:

Incorrect choice because both parents carry the i allele and can produce an ii (O) child with 25% probability.

⚠️

Student Fallacy (Why candidates pick this trap): Thinking parents with blood groups A and B cannot have an O child.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect choice because father provides i (50% chance) and mother always provides i, yielding 50% blood group O children.

⚠️

Student Fallacy (Why candidates pick this trap): Failing to realize heterozygous A cross with O gives O.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect choice because both parents are homozygous ii, producing 100% group O offspring.

⚠️

Student Fallacy (Why candidates pick this trap): Misunderstanding homozygous recessive crosses.

4 D

✓ 100% Correct

Verdict & Scientific Reason:

100% correct. An AB father (I^A I^B) has no i allele to transmit; hence producing an ii child is genetically impossible.

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Why this is airtight: None; rigorous application of Mendelian segregation.

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NTA Trap Alert: Confusing Phenotype with Genotype

Students sometimes write "Parent is A and parent is B, so child can only be AB". They forget that A can be I^A i and B can be I^B i. But blood group AB is ALWAYS I^A I^B without exception, leaving zero possibility for an i allele.

🛡️ How to avoid the -1 mark: Remember: Blood group AB has NO "i" allele. An AB parent can NEVER have an O child, and an O parent can NEVER have an AB child!

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Core NEET Syllabus Standard

NEET Biology Core Syllabus · Principles of Inheritance and Variation Standard Benchmark

Core Scientific Principle Tested:

"Blood group O requires two i alleles (ii); an individual with genotype I^A I^B cannot pass on an i allele."

Syllabus Focus: Multiple Alleles & ABO Blood Grouping

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

AB parent has NO "i" to give = NO "O" child possible!

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NEET 2026 Zoology Question 169 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ The "AB Cannot Have O Child" Law

Step-by-Step Rigorous Workflow & Mathematical Proof

Genetics of ABO Blood Group System

Evaluate Parental Combinations

360° Option Elimination Engine Key Feature

NTA Trap Alert: Confusing Phenotype with Genotype

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Principles of Inheritance and Variation

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Mendelian Dihybrid Cross and F2 Phenotypic Ratio (9:3:3:1)

Pedigree Probability Calculation for X-Linked Recessive Carrier Offspring

Postulates of Mendel's Law of Dominance

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