Principles of Inheritance and Variation — NEET Botany MCQs with answers

Principles of Inheritance and Variation is a Class 12 Botany chapter in the NEET (UG) syllabus. NEET720 has 1,250 reviewed practice questions on it, each with a quick answer and a step-by-step explanation. The 8 questions below are free and fixed, so you can bookmark this page; the full chapter, plus mistake tracking and spaced revision, is in the app.

Topics covered

Mendelian Inheritance · Deviations from Mendelism · Chromosomal Theory and Linkage · Sex Determination · Mutation, Pedigree and Genetic Disorders · Linkage and Crossing Over · Mutations · Inheritance of two genes · Population genetics · History of Genetics · Incomplete dominance · Sex determination in plants · Polyploidy · Cytoplasmic inheritance · Multiple alleles · Genetics · Principles of Inheritance and Variation · Mendelian Genetics · Deviation from Mendelism · Chromosomal Theory of Inheritance · Mutation and Genetic Disorders · Linkage and gene mapping · Pedigree analysis · Mutation and inheritance pattern · ABO blood groups and pedigree exclusion · Polygenic inheritance and environment · Sex determination and X-linked probability · Linkage phase and dihybrid outcome · Incomplete dominance with independent assortment · Chromosomal theory and X-linkage evidence · Test cross application · ABO paternity application · Sex determination application · X-linked recessive carrier probability · Pleiotropy application · Aneuploidy from nondisjunction · Linkage phase inference · Sickle cell anaemia mechanism · Meiosis I versus meiosis II nondisjunction · Gene mapping accuracy at large distances

8 free Principles of Inheritance and Variation practice questions with answers

Choose an answer in your head before opening it. Each explanation says why the correct option is right and, where relevant, why the tempting wrong option is wrong.

1. Question 1 · easy · Chromosomal Theory and Linkage

   Walter Sutton and Theodor Boveri independently proposed the chromosomal theory of inheritance based on the observation that chromosomes and Mendelian factors (genes) show a striking parallelism in their behaviour. Which of the following is the physical basis on which this theory rests?

   • A.Chromosomes are visible under a light microscope, so their number could be counted to match Mendel's ratios directly.
   • B.Chromosomes replicate semi-conservatively, which was assumed to prove that genes must also replicate semi-conservatively.
   • C.Chromosomes and genes both occur in pairs, and both segregate during gamete formation so that each gamete receives only one member of each pair.
   • D.Chromosomes were shown to be made of DNA before genes were shown to be made of DNA.
   Show answer and explanation

   Answer: C. Chromosomes and genes both occur in pairs, and both segregate during gamete formation so that each gamete receives only one member of each pair.

   Sutton and Boveri noted that chromosomes, like Mendelian factors, occur in pairs and segregate during gamete formation, giving each gamete only one member of each pair — this parallel behaviour is the foundation of the chromosomal theory.

   The chromosomal theory of inheritance rests entirely on cytological observation of parallel behaviour, not on molecular biology. Option A correctly states the core parallelism: pairing and segregation. Option B is wrong because semi-conservative replication of DNA was discovered decades later (Meselson-Stahl) and is unrelated to Sutton and Boveri's argument. Option C wrongly reduces the theory to simple visibility. Option D is chronologically and conceptually wrong — the chemical nature of the gene was unknown in Sutton and Boveri's time.

   Common mistake: Students conflate the chromosomal theory (cytological, early 1900s) with later molecular proofs of DNA as genetic material.

   Key point: The chromosomal theory is built on parallel behaviour: pairing, segregation, and individuality of chromosomes and genes — not molecular chemistry.

2. Question 2 · medium · Chromosomal Theory and Linkage

   Consider the following statements regarding the parallelism between chromosome behaviour and gene behaviour that underlies the chromosomal theory of inheritance: I. Paternal and maternal chromosomes pair during meiosis and then separate, matching the segregation of a pair of alleles. II. Chromosomes retain their individuality throughout the life cycle, matching the stability of genes across generations. III. The number of chromosomes doubles at every meiotic division, matching the doubling of gene number in each generation. IV. Only one member of each chromosome pair (and correspondingly one allele of each gene pair) is transmitted to a gamete. How many of the above statements are correct?

   • A.One
   • B.Two
   • C.Three
   • D.Four
   Show answer and explanation

   Answer: C. Three

   Statements I, II and IV correctly describe the parallel behaviour of chromosomes and genes; statement III is false because meiosis halves chromosome number, it does not double it.

   Statement I is correct: homologous (paternal/maternal) chromosomes pair and separate during meiosis I, mirroring allele segregation. Statement II is correct: chromosomes maintain individuality and structural continuity across generations, just as genes remain stable entities. Statement III is incorrect: meiosis reduces chromosome number by half in gametes (reductional division), it does not double it — this statement reverses the actual mechanism. Statement IV is correct: only one chromosome of each homologous pair, and correspondingly one allele of each gene pair, enters a given gamete, which is the cytological basis of Mendel's law of segregation. Hence three of the four statements are correct.

   Common mistake: Students misremember meiotic reduction as an increase, especially when statements are phrased with confident-sounding but incorrect numerical claims.

   Key point: Meiosis HALVES chromosome number in gametes; this reduction is what parallels allelic segregation, not any doubling event.

3. Question 3 · easy · Chromosomal Theory and Linkage

   Thomas Hunt Morgan chose Drosophila melanogaster to experimentally verify the chromosomal theory of inheritance and to establish the concept of linkage. Which feature of Drosophila was NOT a reason for its selection as an experimental organism?

   • A.It has a very short life cycle, allowing several generations to be studied within a short time.
   • B.It could be grown easily on simple synthetic medium in the laboratory and produces a large number of offspring.
   • C.It has clear differentiation between sexes and giant, easily distinguishable salivary gland chromosomes.
   • D.It has an unusually large number of chromosome pairs, providing many independent linkage groups to study.
   Show answer and explanation

   Answer: D. It has an unusually large number of chromosome pairs, providing many independent linkage groups to study.

   Drosophila has only four pairs of chromosomes, a small number that made tracking linkage groups manageable — not a large number as stated in option D.

   Morgan's choice of Drosophila melanogaster was based on several practical advantages genuinely described in NCERT: short life cycle, ease of laboratory rearing on simple medium, large numbers of offspring, clear sexual dimorphism, and giant, easily visible polytene salivary gland chromosomes. Option D is the false statement because Drosophila actually has a small chromosome number (n=4), which was an advantage precisely because it kept the number of linkage groups low and manageable, the opposite of what the option claims.

   Common mistake: Students assume 'more chromosomes' always means 'more useful for genetics,' when a small, well-characterized set is actually more tractable.

   Key point: Drosophila's small chromosome number (4 pairs) simplified linkage studies — a large chromosome number would have complicated them.

4. Question 4 · medium · Chromosomal Theory and Linkage

   Morgan crossed white-eyed male Drosophila with red-eyed (wild-type) females and observed that the white-eye trait reappeared in the F2 generation exclusively in males. What did this pattern of inheritance experimentally establish?

   • A.That eye colour genes are located on autosomes and show simple dominant-recessive inheritance.
   • B.That the gene for eye colour is located on the X chromosome, providing direct experimental evidence that genes are carried on chromosomes.
   • C.That white eye colour arises from cytoplasmic inheritance transmitted only through the mother's cytoplasm.
   • D.That the white-eye allele is located on the Y chromosome and is transmitted only from father to son.
   Show answer and explanation

   Answer: B. That the gene for eye colour is located on the X chromosome, providing direct experimental evidence that genes are carried on chromosomes.

   The sex-linked reappearance of the white-eye trait in F2 males allowed Morgan to correlate a specific phenotype with the X chromosome, providing the first solid experimental proof that genes are physically located on chromosomes.

   Morgan's white-eye experiment is the landmark experimental verification of the chromosomal theory. Because the recessive white-eye allele was inherited in a pattern that tracked the X chromosome (present in F2 only in males, since males are hemizygous XY and express any X-linked recessive allele they carry), Morgan correlated a specific gene with a specific chromosome for the first time. Option A is wrong because the pattern is sex-linked, not simple autosomal inheritance. Option C wrongly invokes cytoplasmic inheritance, which is chromosome-independent. Option D wrongly places the gene on the Y chromosome; the classical result is X-linkage, with the Y chromosome lacking the corresponding locus.

   Common mistake: Students confuse X-linkage with Y-linkage or with cytoplasmic (maternal) inheritance, which have very different transmission patterns.

   Key point: The X-linked white-eye trait in Drosophila was the first direct experimental evidence linking a specific gene to a specific chromosome.

5. Question 5 · easy · Chromosomal Theory and Linkage

   What is meant by 'linkage' between two genes, as first recognised through Morgan's work on Drosophila?

   • A.Two genes located on the same chromosome tend to be inherited together, rather than assorting independently as Mendel's law would predict for genes on different chromosomes.
   • B.Two genes located on different chromosomes always segregate together into the same gamete.
   • C.Two genes that produce the same phenotype are said to be linked, regardless of their chromosomal location.
   • D.Two genes are linked only when they lie on homologous chromosomes inherited from the same parent.
   Show answer and explanation

   Answer: A. Two genes located on the same chromosome tend to be inherited together, rather than assorting independently as Mendel's law would predict for genes on different chromosomes.

   Linkage refers to genes on the same chromosome being inherited together more often than expected, deviating from Mendel's law of independent assortment, which applies to genes on different chromosomes.

   Linkage is a physical, positional phenomenon: when two genes lie on the same chromosome, they tend to travel together into the same gamete because the chromosome (not the gene) is the unit that segregates during meiosis, except when separated by crossing over. Option B is wrong because genes on different chromosomes assort independently by definition — that is not linkage. Option C wrongly defines linkage by phenotype rather than physical chromosome location. Option D introduces an irrelevant condition about parental origin that is not part of the linkage definition.

   Common mistake: Students think linkage applies to genes with similar phenotypic effects rather than genes physically located together on a chromosome.

   Key point: Linkage = genes on the SAME chromosome tending to be inherited together, deviating from independent assortment.

6. Question 6 · medium · Chromosomal Theory and Linkage

   Two genes, P and Q, are located very close together on the same chromosome, while genes R and S are located far apart on another chromosome. Based on the relationship between physical distance and linkage, which statement correctly compares their behaviour?

   • A.P and Q will show a high recombination frequency because their proximity increases the chance of crossing over occurring between them.
   • B.P and Q will show tight linkage with a low recombination frequency, while R and S will show weaker linkage with a comparatively higher recombination frequency.
   • C.Both gene pairs will show exactly 50% recombination frequency regardless of distance, since recombination frequency is independent of physical distance.
   • D.R and S will show tighter linkage than P and Q because genes on different chromosomes are more strongly linked than genes on the same chromosome.
   Show answer and explanation

   Answer: B. P and Q will show tight linkage with a low recombination frequency, while R and S will show weaker linkage with a comparatively higher recombination frequency.

   Genes located close together (P and Q) show tight linkage with low recombination frequency because crossing over rarely occurs between them, whereas genes far apart show weaker linkage with a higher recombination frequency.

   Recombination frequency is directly proportional to the physical distance between two genes on a chromosome: closer genes are rarely separated by a crossover event, giving low recombination frequency and strong (tight) linkage, whereas genes far apart on the same chromosome are more often separated by crossovers, giving higher recombination frequency and weaker linkage. Option A reverses this relationship. Option C is wrong because recombination frequency does depend on distance (it approaches but does not always equal 50%). Option D is a category error — R and S are stated to be on 'another chromosome' from P/Q in the same organism, but the comparison intended is between two pairs both on their own single chromosomes; regardless, genes on different chromosomes are not 'linked' at all, they assort independently, so calling them 'tighter linked' is incorrect by definition.

   Common mistake: Students think proximity causes MORE crossing over and hence higher recombination, when in fact proximity reduces the chance of an intervening crossover.

   Key point: Closer genes = lower recombination frequency = stronger linkage; farther genes = higher recombination frequency = weaker linkage.

7. Question 7 · medium · Chromosomal Theory and Linkage

   Alfred Sturtevant, a student of Morgan, used recombination frequency data between multiple gene pairs to construct the first genetic (linkage) map. What is the fundamental principle underlying this gene mapping technique?

   • A.Recombination frequency is used as a measure of the relative physical distance between genes on a chromosome, allowing their linear order and spacing to be inferred.
   • B.The absolute number of nucleotide base pairs between two genes is measured directly under a light microscope to construct the map.
   • C.Genes are mapped strictly according to the order in which their phenotypes appear during embryonic development.
   • D.Genetic maps are constructed by counting the total number of chromosomes present in the somatic cells of the organism.
   Show answer and explanation

   Answer: A. Recombination frequency is used as a measure of the relative physical distance between genes on a chromosome, allowing their linear order and spacing to be inferred.

   Sturtevant's gene mapping used recombination frequency as a proxy for physical distance: genes with higher recombination frequency are assumed to be farther apart, allowing their linear arrangement to be plotted.

   Sturtevant's landmark contribution was recognising that recombination frequency between two genes correlates with the physical distance separating them on a chromosome, expressed in map units (centimorgans). By comparing recombination frequencies across many gene pairs, he could deduce their relative linear order and distances, producing the first genetic linkage map — all without any molecular sequencing tools, since DNA structure was unknown at the time. Option B is anachronistic (physical DNA measurement was not possible then). Option C invents an unrelated developmental criterion. Option D confuses gene mapping with simple chromosome counting/karyotyping.

   Common mistake: Students assume genetic maps are built from direct physical/molecular distance measurements rather than from recombination frequency data.

   Key point: Recombination frequency is used as an indirect measure of physical gene distance to build linear genetic (linkage) maps — pioneered by Sturtevant.

8. Question 8 · easy · Chromosomal Theory and Linkage

   Statement I: Walter Sutton and Theodor Boveri independently proposed the chromosomal theory of inheritance in the early 1900s. Statement II: This theory was based purely on theoretical mathematical modelling of allele frequencies, without any cytological (microscope-based) observation of chromosome behaviour. Which one of the following is correct regarding the above statements?

   • A.Both Statement I and Statement II are correct.
   • B.Statement I is correct, but Statement II is incorrect.
   • C.Statement I is incorrect, but Statement II is correct.
   • D.Both Statement I and Statement II are incorrect.
   Show answer and explanation

   Answer: B. Statement I is correct, but Statement II is incorrect.

   Sutton and Boveri did independently propose the theory (Statement I is true), but it was grounded in direct microscopic/cytological observation of chromosome behaviour during meiosis, not mathematical modelling (Statement II is false).

   Statement I correctly reflects the well-documented history: Sutton (studying grasshopper chromosomes) and Boveri (studying sea urchins) independently arrived at similar conclusions around 1902. Statement II is false because their theory arose from direct cytological observation under the microscope of how chromosomes pair, segregate, and maintain individuality during meiosis — not from abstract mathematical/statistical modelling, which was not the method used to establish chromosomal parallelism.

   Common mistake: Students may assume all foundational genetics theories, including this one, were derived from statistical/mathematical analysis similar to Mendel's approach.

   Key point: The chromosomal theory was established through direct cytological (microscopic) observation, not mathematical or statistical modelling.

Questions about Principles of Inheritance and Variation for NEET

More Botany chapters

← Biodiversity and ConservationAll Botany chaptersMolecular Basis of Inheritance →

Questions are original NEET720 compositions reviewed for correctness, syllabus fit and option quality. Counts update as the bank grows (1,250 active practice questions in this chapter today).