NEET BotanyUpdated 19 Oct 2026

Principles of Inheritance and Variation: NEET PYQs and weightage

Principles of Inheritance and Variation carries 7.9 of 100 Botany marks in NEET by past-paper share: 63 of the 793 tagged NEET Botany PYQs in the ViJEEta bank (NEET 2013–2026). It ranks 4 of 21 Botany chapters.

Take the 25-question Principles of Inheritance and Variation test Key facts ↓All NEET Botany chapters

7.9/100 Weight in Botany

Exam importance (5/5, by past-paper share)

63 NEET PYQs

525 Practice questions

476 With measured difficulty

How the weight is computed: this chapter's tagged NEET PYQs divided by all tagged NEET Botany PYQs in the bank (63 of 793), times 100, with repeated questions counted once. The bank does not store the year of each NEET question, so none is printed. Computed from the bank at build time. Updated 19 Oct 2026.

Principles of Inheritance and Variation: key facts for NEET

16 facts the paper keeps asking, each with how it is asked. Copy it onto one page of your notebook: that page is this chapter's handbook.

Mendel's experiments
Mendel worked on garden pea for 7 years (1856-1863), using 14 true-breeding varieties showing 7 pairs of contrasting traits.
Asked as: which trait was NOT among Mendel's seven (flower position axial/terminal and pod colour green/yellow are among them).
Monohybrid cross
Monohybrid cross ( $T T \times tt$ ): all $F_{1}$ are tall; $F_{2}$ phenotypic ratio is $3 : 1$ and genotypic ratio is $1 : 2 : 1$ . This gives the Law of Dominance and the Law of Segregation.
Asked as: the fraction of $F_{2}$ tall plants that are heterozygous ( $\frac{2}{3}$ ), or which law explains the reappearance of dwarf plants in $F_{2}$ .
Test cross
Test cross: a plant with the dominant phenotype is crossed with the recessive parent to find out whether it is $T T$ or $T t$ .
Asked as: identify the test cross among given crosses ( $T t \times tt$ ).
Incomplete dominance (snapdragon)
Incomplete dominance in snapdragon (Antirrhinum): $R R$ red $\times$ $r r$ white gives pink $R r$ in $F_{1}$ ; $F_{2}$ is $1$ red $:$ $2$ pink $:$ $1$ white, so the phenotypic and genotypic ratios are the same.
Asked as: the $F_{2}$ phenotypic ratio in snapdragon ( $1 : 2 : 1$ , not $3 : 1$ ).
ABO blood group
The ABO blood group is controlled by gene $I$ , which has three alleles ( $I^{A}$ , $I^{B}$ , $i$ ). $I^{A}$ and $I^{B}$ are co-dominant, and both are dominant over $i$ . This gives 6 genotypes and 4 phenotypes, and it is also an example of multiple alleles.
Asked as: the parents of an O-group child are A and B, so what are their genotypes ( $I^{A} i$ and $I^{B} i$ )?
Pea starch gene
Pea starch gene ( $B / b$ ): for seed shape, round is dominant over wrinkled, but $B b$ produces starch grains of intermediate size. So dominance depends on the phenotype you look at.
Asked as: the same gene shows dominance for seed shape and incomplete dominance for starch grain size.
Dihybrid cross
Dihybrid cross $R R Y Y \times r r y y$ : the $F_{1}$ $R r Y y$ makes 4 kinds of gametes ( $R Y$ , $R y$ , $r Y$ , $r y$ ), each $\frac{1}{4}$ . The $F_{2}$ phenotypic ratio is $9 : 3 : 3 : 1$ , which gives the Law of Independent Assortment.
Asked as: the number of gamete types from a plant heterozygous at $n$ loci ( $2^{n}$ ).
Rediscovery and chromosomal theory
Mendel's work was rediscovered in 1900 by de Vries, Correns and von Tschermak. Sutton and Boveri proposed the chromosomal theory of inheritance.
Asked as: who proposed the chromosomal theory of inheritance.
Linkage and recombination
Morgan worked with Drosophila melanogaster and coined the terms linkage and recombination. White and yellow genes showed $1.3%$ recombination, while white and miniature wing showed $37.2%$ . Sturtevant used recombination frequency to map genes.
Asked as: tightly linked genes show low recombination; recombination frequency is used as a measure of gene distance.
Polygenic inheritance
Polygenic inheritance, for example human skin colour, is controlled by three or more genes. The effect of each allele is additive, and the environment also has an influence. $AA B B C C$ gives the darkest skin and $aabb cc$ the lightest.
Asked as: a continuous range of phenotypes points to polygenic inheritance.
Pleiotropy
Pleiotropy means one gene has many phenotypic effects. An example is phenylketonuria, caused by a mutation in the phenylalanine hydroxylase gene, which leads to mental retardation and reduced hair and skin pigmentation.
Asked as: PKU is an example of pleiotropy and an autosomal recessive inborn error of metabolism.
Sex determination
Henking (1891) named the "X body". In the $XO$ type (grasshopper), the male has one $X$ . In the $XY$ type (humans, Drosophila), the male is heterogametic. In the $ZW$ type (birds), the female is $ZW$ and the male $ZZ$ .
Asked as: in which organism the female decides the sex of the offspring (birds).
Honey bee sex determination
Honey bee has haplodiploid sex determination. A fertilised egg gives a female with 32 chromosomes, and an unfertilised egg gives a drone with 16 chromosomes by parthenogenesis. Males make sperm by mitosis, have no father and no sons, but do have a grandfather and grandsons.
Asked as: a drone has no father but has a grandfather.
Sickle-cell anaemia and thalassemia
Sickle-cell anaemia is autosomal recessive (only $H b^{S} H b^{S}$ is diseased). Valine replaces glutamic acid at the 6th position of the $β$ -globin chain ( $GAG \to GUG$ ), which is a qualitative defect. Thalassemia is a quantitative defect where too few globin chains are made. $α$ -thalassemia involves HBA1 and HBA2 on chromosome 16, and $β$ -thalassemia involves HBB on chromosome 11.
Asked as: which codon change or amino acid substitution causes sickle-cell, or match the thalassemia type to its chromosome.
X-linked recessive disorders
Colour blindness and haemophilia are X-linked recessive and pass from a carrier mother to sons. Colour blindness affects about $8%$ of males and $0.4%$ of females. Queen Victoria was a carrier of haemophilia.
Asked as: the chance that a carrier mother's son is colour blind ( $50%$ ).
Chromosomal disorders
Down's syndrome is trisomy 21 (described by Langdon Down, 1866). Klinefelter's syndrome is $47, XXY$ (sterile male with gynaecomastia). Turner's syndrome is $45, X0$ (sterile female with rudimentary ovaries). These arise from aneuploidy caused by failure of chromatid segregation.
Asked as: match the syndrome to its karyotype.

Trap

Incomplete dominance and co-dominance are different. In incomplete dominance the F1 is in between the parents (pink snapdragon). In co-dominance the F1 shows both parental traits fully (blood group AB). In neither case do the alleles themselves blend.

How does NEET ask Principles of Inheritance and Variation?

8 real previous-year questions from this chapter, as printed in the paper, each with its worked solution. Answer the first one here with no sign-in.

Q1BotanyNEET UG PYQDeviations from Mendel's Laws

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If two persons with ‘AB’ blood group marry and have suﬃciently large number of children, these children could be classiﬁed as ‘A’ blood group: ‘B’ blood group in 1 : 2 : 1 ratio. Modern technique of protein electrophoresis reveals presence of both ‘A’ and ‘B’ type proteins in ‘AB’ blood group individuals. This is an example of

Answer key (A) & worked solution

Correct answer: Option A

When I

^{A}

and I

^{B}

antigens are present together within the RBCs, they both express their own type of sugar on the surface of RBCs. This phenomenon is called codominance.

Q2BotanyNEET UG PYQMendel's Laws of Inheritance

Match list I with list II. List IList IIA. Two or more alternative forms of a geneI. Back crossB. Cross of F

_{1}

progeny with homozygous recessive parentII. PloidyC. Cross of F

_{1}

progeny with any of the parentsIII. AlleleD. Number of chromosome sets in plantIV. Test cross Choose the correct answer from the options given below.

AA-II, B-I, C-III, D-IV

BA-III, B-IV, C-I, D-II

CA-IV, B-III, C-II, D-I

DA-I, B-II, C-III, D-IV

Answer key (B) & worked solution

Correct answer: Option B

Genes which code for a pair of contrasting traits are known as alleles, i.e., they are slightly different forms of the same gene. In a typical test cross an organism (pea plants here) showing a dominant phenotype (and whose genotype is to be determined) is crossed with the recessive parent instead of self-crossing. Cross of F

_{1}

progeny with any of the parents is called Back Cross. Ploidy is the number of sets of chromosomes in a cell or organism.

Q3BotanyNEET UG PYQLinkage and Recombination

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): Mendel’s law of Independent assortment does not hold goods for the genes that are located closely on the same chromosome. Reason (R): Closely located genes assort independently. Answer from the options given below:

ABoth (A) and (R) are correct and (R) is the correct explanation of (A)

BBoth (A) and (R) are correct but (R) is not the correct explanation of (A)

C(A) is correct but (R) is not correct

D(A) is not correct but (R) is correct

Answer key (C) & worked solution

Correct answer: Option C

Mendel’s law of Independent assortment states that the alleles of two different genes get sorted into gametes independently of one another. However, after Morgan’s experiment, it was found that if the genes are grouped on the same chromosome, some genes were very tightly linked (showed very low recombination) while others were loosely linked (showed very high recombination). Due to the process of crossover or recombination, there is a chance for two genes on the same chromosome to behave independently or as if it is linked. Two different genes situated close together on the same chromosome tend to be inherited together, hence the law of Independent assortment does not hold goods for the genes that are located closely on the same chromosome. The isolation of alleles into gametes can be under the influence of linkage. The assertion statement is correct but the reason is false.

Q4BotanyNEET UG PYQSex Determination

If a colour-blind man marries a woman who is homozygous for normal colour vision, the probability of their son being colour- blind is

A0.5

B0.75

Answer key (D) & worked solution

Correct answer: Option D

Colour-blindness is a X linked recessive disorder. The son will carry the X chromosome from the mother which is normal homozygous. Hence son will be normal. The genotype of colourblind husband is X

^{C}

Y and the genotype of normal wife is XX. Parents…. Husband × Wife Genotype…. X

^{C}

Y × XX Gametes….. X

^{C}

, Y × X, X X X X

^{C}

^{C}

^{C}

XYXYXY

Q5BotanyNEET UG PYQMendel's Laws of Inheritance

In his classic experiments on pea plants, Mendel did not use

ASeed colour

BPod length

CSeed shape

DFlower position

Answer key (B) & worked solution

Correct answer: Option B

In Mendel’s experiments on pea plants, the contrasting trait pod length was not used. Mendel used traits such as seed colour, seed shape, pod shape, pod colour, flower colour, plant height and position of flowers for his experiments.

Q6BotanyNEET UG PYQInheritance of One Gene

Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R. Assertion A : In an experiment, Mendel observed that the F1 progeny plants are all tall and none are dwarf. Reason R : Stem height is a contrasting trait, with tall being dominant and dwarf being recessive. In the light of the above statements, choose the most appropriate answer from the options given below :

ABoth A and R are correct but R is not the correct explanation of A

BA is correct but R is not correct

CA is not correct but R is correct

DBoth A and R are correct and R is the correct explanation of A

Answer key (D) & worked solution

Correct answer: Option D

Mendel crossed tall and dwarf plants and observed that all the F1 progeny plants are tall. The trait 'T' or tall is said to be dominant over the other allele 't' or dwarf trait. It is thus due to this dominance of one trait over the other that all the F1 are tall.

Q7BotanyNEET UG PYQInheritance of Two Genes

Genes R and Y follow independent assortment. If RRYY produce round yellow seeds and rryy produce wrinkled green seeds, what will be the phenotypic ratio of the F2 generation?

APhenotypic ratio - 9 : 3 : 3 : I

BPhenotypic ratio - 9 : 7

CPhenotypic ratio - 1 : 2 : 1

DPhenotypic ratio - 3 : 1

Answer key (A) & worked solution

Correct answer: Option A

Results of a dihybrid cross - Phenotypic ratio of F2 generation is 9 Round, Yellow 3 Wrinkled, Yellow 3 Round, Green 1 Wrinkled, Green

Q8BotanyNEET UG PYQMendel's Laws of Inheritance

How many true breeding pea plant varieties did mendel select as pairs, which were similar except in one character with contrasting traits?

A14

Answer key (A) & worked solution

Correct answer: Option A

A true breeding line refers to the plant that on self pollination continuously produces offspring of similar characters and are stable in the inheritance of the character. Mendel selected 14 true-breeding pea plant varieties, as pairs, which were similar except in one character with contrasting traits.

Which topics in Principles of Inheritance and Variation carry the most questions?

The 12 busiest of 14 topics, by tagged PYQs, then practice questions. Counted from the bank. Updated 19 Oct 2026.

Topic	PYQs	Practice questions
Deviations from Mendel's Laws	11	61
Mendel's Laws of Inheritance	10	51
Linkage and Recombination	8	40
Mendelian Disorders	8	36
Chromosomal Disorders	6	35
Inheritance of One Gene	5	81
Sex Determination	5	67
Pedigree Analysis	3	21
Mutations	2	14
Pleiotropy	2	6
Inheritance of Two Genes	1	49
Chromosomal Theory of Inheritance	1	32

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Chapters either side of Principles of Inheritance and Variation by weight

Cell: The Unit of Life Weight 8.1/100 Morphology of Flowering Plants Weight 7.9/100 Biological Classification Weight 7.1/100 Cell Cycle and Cell Division Weight 6.4/100

Principles of Inheritance and Variation: NEET PYQs and weightage

Principles of Inheritance and Variation: key facts for NEET

Mendel's experiments

Monohybrid cross

Test cross

Incomplete dominance (snapdragon)

ABO blood group

Pea starch gene

Dihybrid cross

Rediscovery and chromosomal theory

Linkage and recombination

Polygenic inheritance

Pleiotropy

Sex determination

Honey bee sex determination

Sickle-cell anaemia and thalassemia

X-linked recessive disorders

Chromosomal disorders

How does NEET ask Principles of Inheritance and Variation?

Which topics in Principles of Inheritance and Variation carry the most questions?

Why does Principles of Inheritance and Variation show "–" until you answer 3 questions?

What happens when you test Principles of Inheritance and Variation?

Chapters either side of Principles of Inheritance and Variation by weight