Key Fact
Mendel's laws
Law of dominance, segregation (purity of gametes), independent assortment. Monohybrid: 3:1; dihybrid: 9:3:3:1. Test cross: hybrid × recessive parent → 1:1 (Aa) or all dominant (AA).
-- NCERT Class 12 Biology, Ch. 4, p. 59Mendel Inheritance
8 MCQs4 revision cards9-step worked example
Source: NCERT Genetics and EvolutionPYQ coverage: NEET 2020, 2021, 2022, 2024, 2025, 2026Official key: NTA-verifiedLast updated: 25 Sep 2026
Mendel Inheritance, explained for NEET
The trap that costs marks in Mendel questions is not forgetting the ratios — it is confusing which ratio the question asks for. The genotypic ratio of a monohybrid F2 is 1:2:1 (AA : Aa : aa). The phenotypic ratio is 3:1 (dominant : recessive). NEET stems routinely ask for one while the cross setup nudges you toward the other. Read the last line of the stem twice.
Gregor Mendel's work on garden peas (NCERT Class 12 Biology, Chapter 4, page 66) established three principles. The Law of Dominance states that in a heterozygote, only the dominant allele expresses in the phenotype. The Law of Segregation states that two alleles of a gene separate during gamete formation so each gamete carries only one allele. The Law of Independent Assortment states that alleles of different genes assort independently during gamete formation, provided the genes are on different chromosomes (unlinked).
In a monohybrid cross (Tt × Tt), the F2 genotypic ratio is 1 TT : 2 Tt : 1 tt. With complete dominance, the phenotypic ratio collapses to 3 tall : 1 dwarf because TT and Tt look identical.
In a dihybrid cross (TtRr × TtRr), independent assortment produces 16 combinations. The F2 phenotypic ratio is 9:3:3:1 — nine showing both dominant traits, three showing first dominant only, three showing second dominant only, one showing both recessive traits. This ratio holds only when both loci are unlinked and show complete dominance.
Watch-out: When a NEET stem says "ratio of offspring," decide immediately — genotypic or phenotypic? The numbers are different, and the wrong pick is a high-frequency negative-marking trap.
Can you answer these Mendel Inheritance MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
Mendel's Law of Segregation is also known as the law of purity of gametes because:
Show answer and why every option is right or wrong
Answer: D. The Law of Segregation states that two alleles separate during gamete formation so each gamete is "pure" — carrying only one allele (NCERT Class 12 Biology, Chapter 4, page 66).
Why A is wrong: A is wrong because gametes are haploid and carry only one allele per locus, not two. Carrying two would mean no segregation occurred (trap: genotype vs phenotype ratio confusion — similar conflation of diploid parent state with haploid gamete state).
Why B is wrong: B is wrong because the recessive allele is not eliminated — it is masked in the phenotype but transmitted intact. Both alleles persist through generations (trap: dominance does not mean elimination).
Why C is wrong: C describes random fertilization, which relates to recombination of alleles at fertilization, not to the segregation event during meiosis (trap: confusing fertilization with segregation).
MCQ 2Easy RecallPractice
Which of the following is the correct phenotypic ratio in the F2 generation of a Mendelian monohybrid cross with complete dominance?
Show answer and why every option is right or wrong
Answer: B. In a monohybrid cross with complete dominance, the F2 phenotypic ratio is 3 dominant : 1 recessive. The 1:2:1 is the genotypic ratio (NCERT Class 12 Biology, Chapter 4, page 66).
Why A is wrong: A is wrong because 1:2:1 is the genotypic ratio (AA:Aa:aa), not the phenotypic ratio. With complete dominance, AA and Aa are phenotypically identical, collapsing to 3:1 (trap: genotype vs phenotype ratio confusion).
Why C is wrong: C is wrong because 9:3:3:1 is the dihybrid F2 phenotypic ratio involving two gene pairs, not a single gene (trap: applying dihybrid ratio to monohybrid cross).
Why D is wrong: D is wrong because 1:1 is the ratio of a testcross (Aa × aa), not the F2 of a monohybrid cross between two heterozygotes (trap: confusing testcross with F2 cross).
MCQ 3Easy RecallPractice
Mendel's Law of Independent Assortment is applicable when:
Show answer and why every option is right or wrong
Answer: C. Independent assortment applies when two gene pairs are on different (non-homologous) chromosomes, so alleles at one locus segregate independently of alleles at the other (NCERT Class 12 Biology, Chapter 4, page 66).
Why A is wrong: A is wrong because genes on the same chromosome are linked and tend to be inherited together, violating independent assortment (trap: linked genes do not assort independently).
Why B is wrong: B is wrong because independent assortment requires at least two gene pairs — it describes the relationship between different loci, not within a single locus (trap: confusing segregation with independent assortment).
Why D is wrong: D is wrong because incomplete dominance affects the phenotypic expression of alleles, not whether loci assort independently. Independent assortment depends on chromosomal location, not dominance type (trap: conflating dominance pattern with assortment).
MCQ 4Direct ApplicationPractice
In a monohybrid cross between two heterozygous tall pea plants (Tt × Tt), what fraction of the F2 offspring will be homozygous?
Show answer and why every option is right or wrong
Answer: B. The genotypic ratio is 1 TT : 2 Tt : 1 tt. Homozygous individuals = TT + tt = 1/4 + 1/4 = 2/4 = 1/2 (NCERT Class 12 Biology, Chapter 4, page 66).
Why A is wrong: A is wrong because 1/4 accounts for only one homozygous class (either TT or tt alone). The question asks for all homozygous offspring, which includes both TT and tt (trap: genotype vs phenotype ratio confusion — reading 'homozygous' as only the dominant or only the recessive class).
Why C is wrong: C is wrong because 3/4 is the phenotypic fraction showing the dominant trait (TT + Tt), not the fraction that is homozygous (trap: confusing dominant phenotype count with homozygous genotype count).
Why D is wrong: D is wrong because 1/3 is the fraction of the TALL offspring that are homozygous (1 TT among 1 TT + 2 Tt). The question asks about all offspring, where TT and tt together make 1/2 (trap: answering for the tall plants only).
MCQ 5Direct ApplicationPractice
A dihybrid cross (RrYy × RrYy) produces 640 offspring. How many are expected to show both recessive traits?
Show answer and why every option is right or wrong
Answer: B. In a dihybrid F2 with complete dominance and unlinked genes, the double-recessive class is 1/16 of progeny. 640 × 1/16 = 40 (NCERT Class 12 Biology, Chapter 4, page 66).
Why A is wrong: A is wrong because 160 = 640 × 1/4, which is the monohybrid recessive fraction applied incorrectly to a dihybrid cross. In a dihybrid cross, the double-recessive fraction is 1/16, not 1/4 (trap: applying monohybrid ratio to dihybrid problem).
Why C is wrong: C is wrong because 120 = 640 × 3/16, which gives the count for a single-recessive phenotypic class (dominant for one trait, recessive for the other), not the double-recessive class (trap: picking the wrong class from the 9:3:3:1 ratio).
Why D is wrong: D is wrong because 80 = 640 × 1/8, which is not a standard fraction in the 9:3:3:1 ratio. This likely comes from mistakenly doubling the double-recessive fraction (trap: arithmetic error in ratio application).
MCQ 6Direct ApplicationPractice
A testcross is performed by crossing:
Show answer and why every option is right or wrong
Answer: C. A testcross crosses an organism of unknown genotype (or F1 hybrid) with a homozygous recessive individual to determine whether the organism is homozygous dominant or heterozygous (NCERT Class 12 Biology, Chapter 4, page 66).
Why A is wrong: A is wrong because crossing two heterozygotes produces the standard F2 ratio (3:1 phenotypic), not a testcross. A testcross requires one parent to be homozygous recessive (trap: confusing F2 cross with testcross).
Why B is wrong: B is wrong because crossing with a homozygous dominant parent would mask the recessive allele in all offspring, making it impossible to distinguish the genotype of the F1 hybrid (trap: dominant parent hides the test result).
Why D is wrong: D is wrong because crossing two homozygous recessive individuals (aa × aa) produces all recessive offspring and reveals nothing about a dominant phenotype's genotype — both parents are already known (trap: no unknown genotype to test).
MCQ 7CalculationPractice
In a dihybrid cross between two heterozygous parents (AaBb × AaBb), what is the expected ratio of offspring that are heterozygous for both genes?
Show answer and why every option is right or wrong
Answer: A. For each locus independently, the probability of heterozygosity is 2/4 = 1/2 (from Aa × Aa → 1 AA : 2 Aa : 1 aa). For both loci: 1/2 × 1/2 = 1/4 = 4/16 (NCERT Class 12 Biology, Chapter 4, page 66).
Why B is wrong: B is wrong because 1/16 is the probability of being homozygous for both loci in one specific combination (e.g., AABB or aabb), not the probability of being heterozygous for both (trap: confusing homozygous with heterozygous probability).
Why C is wrong: C is wrong because 2/16 = 1/2 × 1/4 is the fraction for a class heterozygous at only one locus, such as AaBB or AABb. Heterozygous at both loci is 1/2 × 1/2 = 4/16 (trap: multiplying by a homozygote's 1/4).
Why D is wrong: D is wrong because 9/16 is the phenotypic fraction showing both dominant traits (A_B_), which includes homozygous dominant, heterozygous, and mixed genotypes — not exclusively double-heterozygous (trap: genotype vs phenotype ratio confusion — confusing phenotypic class with a specific genotypic class).
MCQ 8Concept TrapPractice
Mendel's results with dihybrid crosses would NOT have yielded a 9:3:3:1 ratio if:
Show answer and why every option is right or wrong
Answer: A. The 9:3:3:1 ratio depends on independent assortment, which requires the two genes to be on different chromosomes. Linkage (same chromosome) distorts the ratio by keeping parental allele combinations together (NCERT Class 12 Biology, Chapter 4, page 66).
Why B is wrong: B is wrong because self-pollination of F1 dihybrids is precisely the cross that generates the 9:3:3:1 ratio. Self-pollination is the method, not a disrupting factor (trap: confusing the experimental method with a condition that breaks the ratio).
Why C is wrong: C is wrong because complete dominance is one of the requirements for the 9:3:3:1 phenotypic ratio. Removing complete dominance (e.g., introducing incomplete dominance) would actually change the ratio — but the question asks what would prevent 9:3:3:1, and complete dominance supports it, not disrupts it (trap: inverting the condition — complete dominance is needed, not detrimental).
Why D is wrong: D is wrong because true-breeding parents ensure the F1 is uniformly heterozygous, which is a prerequisite for obtaining the expected F2 ratios. Non-true-breeding parents would introduce genotypic variation in F1, complicating the cross (trap: true-breeding is a requirement, not a disruption).
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Mendel Inheritance: quick recall before you leave
How do you solve a Mendel Inheritance question? A worked example
- 1
Given
In a cross between two pea plants heterozygous for seed shape (Rr × Rr), where round (R) is dominant over wrinkled (r):• Parent genotypes: Rr × Rr• Complete dominance
- 2
Required
(a) The genotypic ratio of the F2 generation.
(b) The phenotypic ratio of the F2 generation.
(c) The fraction of F2 offspring that are homozygous round. - 3
Concept
Mendel's Law of Segregation: the two alleles of a gene separate during gamete formation. Each parent produces gametes R and r in equal proportion. A Punnett square for Rr × Rr yields the standard monohybrid F2 (NCERT Class 12 Biology, Chapter 4, page 66).
- 4
Formula
Genotypic ratio = 1:2:1 (AA : Aa : aa); Phenotypic ratio = 3:1 (dominant : recessive).
- 5
Substitution
Punnett square:
| | R | r |
|-------|-------|-------|
| R | RR | Rr |
| r | Rr | rr |
Genotypes: 1 RR : 2 Rr : 1 rr - 6
Calculation
(a) Genotypic ratio: 1 RR : 2 Rr : 1 rr = 1:2:1
(b) Phenotypic ratio: RR + Rr = 3 round, rr = 1 wrinkled → 3:1
(c) Homozygous round (RR) = 1 out of 4 = 1/4
The numbers 1, 2, 3, and 4 here are exact counts from the Punnett square — they are integers representing genotype classes, not measurements. - 7
Final answer
(a) Genotypic ratio = 1:2:1
(b) Phenotypic ratio = 3:1
(c) Fraction of homozygous round = 1/4 - 8
Common trap
The high-frequency trap here is confusing genotypic and phenotypic ratios. When asked "what fraction is homozygous?", students often answer 3/4 (the dominant phenotype fraction), forgetting that 2 of those 3 are heterozygous. Similarly, when asked for the phenotypic ratio, some students give 1:2:1 (the genotypic ratio). Always identify what the question asks: genotype or phenotype.
- 9
Similar NEET-style question
In a cross Tt × Tt, what fraction of F2 offspring will be phenotypically different from the F1 parent?
Answer: The F1 parent is tall (Tt). F2 offspring that are phenotypically different are dwarf (tt) = 1/4.
---
What to remember before solving Mendel Inheritance questions
Which Mendel Inheritance formulas do you need for NEET?
Mendel's monohybrid ratio
F2 ratio in monohybrid cross — products of independent assortment of two alleles per locus.
| Symbol | Quantity | SI Unit |
|---|---|---|
| ratio | F2 progeny ratio | - |
Valid when
- Single gene with complete dominance
- Pure-bred parents
Mendel's dihybrid ratio
F2 ratio in dihybrid cross with two independently-segregating loci, complete dominance, no linkage.
| Symbol | Quantity | SI Unit |
|---|---|---|
| ratio | F2 phenotype ratio | - |
Valid when
- Two unlinked loci
- Complete dominance
Where do students lose marks on Mendel Inheritance?
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
Monohybrid: genotype 1:2:1 (AA:Aa:aa); phenotype 3:1.
When it triggers
Question asks for one ratio while presenting cross details.
How to avoid
Always note dominance: phenotype merges Aa + AA; genotype keeps them separate.
More in Genetics and Evolution: 20 exam traps and mistakes · 2 formulas · 1 question pattern from its other lessons.
Mendel Inheritance questions from past NEET papers
8 questions from NEET 2020, 2021, 2022, 2024, 2025, 2026. Answers verified against NTA official keys.
NEET 2026
1Both A and R are correct and R is the correct explanation of A
2Both A and R are correct but R is not the correct explanation of A
3A is correct but R is not correct
4A is not correct but R is correct
NTA Answer: Option 1(final)
NEET 2025
1Phenotypic ratio - 9 : 7
2Phenotypic ratio - 1 : 2 : 1
3Phenotypic ratio - 3 : 1
4Phenotypic ratio - 9 : 3 : 3 : 1
NTA Answer: Option 4(final)
NEET 2024Revised key
1A, B and C only
2A, C, D and E only
3B, C and D only
4A, B, C, D and E
NTA Answer: Option 2(revised_final)
NEET 2024Revised key
1BB
2bb
3Bb
4BB/Bb
NTA Answer: Option 2(revised_final)
NEET 2024Revised key
1A-I, B-II, C-III, D-IV
2A-II, B-I, C-III, D-IV
3A-III, B-IV, C-I, D-II
4A-IV, B-III, C-II, D-I
NTA Answer: Option 3(revised_final)
NEET 2022
1Statement I is incorrect but Statement II is correct
2Both Statement I and Statement II are correct
3Both Statement I and Statement II are incorrect
4Statement I is correct but Statement II is incorrect
NTA Answer: Option 2(final)
NEET 2021
1Net square
2Bullet square
3Punch square
4Punnett square
NTA Answer: Option 4(final)
NEET 2020
18
24
32
414
NTA Answer: Option 4(final)
Sources
NCERT refs: Class 12 Biology Chapter 4, p.66
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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