Principles of Inheritance and Variation (NCERT Practice Paper)

Q1. Regarding 'Phenylketonuria':
(i) Autosomal recessive trait.
(ii) Lack of enzyme phenyl alanine hydroxylase.
(iii) Accumulation of phenylalanine in brain.

Correct Answer: (a)
PKU is an autosomal recessive disorder caused by deficiency of phenylalanine hydroxylase, leading to phenylalanine accumulation in the brain. All three statements are correct.

Q2. Match the following for Drosophila:

Feature	Description
A. Life cycle	(I) 2 weeks
B. Media	(II) Simple synthetic
C. Sexes	(III) Easily distinguishable

Correct Answer: (a)
Drosophila has a 2‑week life cycle (I), can be grown on simple synthetic medium (II), and sexes are easily distinguishable (III). The correct mapping is A–I, B–II, C–III.

Q3. In Snapdragon (Antirrhinum sp.), the F1 generation of a cross between red and white flowers is:

Correct Answer: (c)
Snapdragon exhibits incomplete dominance, where the F1 hybrid (Rr) produces an intermediate pink phenotype. Neither allele is completely dominant over the other. The pink colour results from the production of a reduced amount of red pigment.

Q4. Match the Following Mendelian Disorders with their nature:

Disorder	Nature
A. Haemophilia	(I) Autosomal recessive
B. Myotonic dystrophy	(II) X-linked recessive
C. Sickle-cell anaemia	(III) Autosomal dominant

Correct Answer: (a)
Haemophilia is X‑linked recessive (II), myotonic dystrophy is autosomal dominant (III), and sickle‑cell anaemia is autosomal recessive (I). The correct mapping is A–II, B–III, C–I.

Q5. Which of the following is correct regarding 'Sex determination in Honey bees'?
(i) Males are haploid (16 chromosomes).
(ii) Females are diploid (32 chromosomes).
(iii) Males produce sperms by meiosis.

Correct Answer: (a)
In honey bees, males (drones) are haploid (n=16) and females (queens and workers) are diploid (2n=32). Males produce sperms by mitosis, not meiosis, so (iii) is false. Thus, (i) and (ii) are correct.

Q6. Assertion: Mendel’s true-breeding lines showed stable trait inheritance. Reason: They underwent continuous self-pollination for several generations.

Correct Answer: (a)
Repeated self‑pollination leads to homozygosity at all loci, so the resulting lines breed true for the traits they carry. This stability is the reason Mendel could rely on these lines as consistent parents in his crosses. Continuous selfing eliminates heterozygous conditions and ensures that offspring are identical to the parent.

Q7. Match the symbols used in pedigree analysis:

Symbol	Meaning
A. Empty Circle	(I) Affected male
B. Solid Square	(II) Mating
C. Horizontal line between square and circle	(III) Female

Correct Answer: (a)
In pedigree charts: empty circle = female (III), solid square = affected male (I), and a horizontal line connecting them = mating (II). The correct mapping is A–III, B–I, C–II.

Q8. Identify true statements for 'Chromatin':
(i) nucleoprotein fibres.
(ii) Observed in the interphase nucleus.
(iii) Stained by basic dyes.

Correct Answer: (a)
Chromatin is a complex of DNA and proteins (nucleoprotein), visible in the interphase nucleus, and stains readily with basic dyes due to its acidic nature. All three statements are correct.

Q9. Match the Following for Phenylketonuria:

A. Enzyme missing	Phenyl alanine hydroxylase
B. Accumulation	Phenylpyruvic acid

Correct Answer: (a)
In PKU, the enzyme phenylalanine hydroxylase is deficient, leading to accumulation of phenylpyruvic acid. Both statements are correct.

Q10. Which of the following is correct?
(i) James Watson and Francis Crick proposed the double helix DNA structure in 1953.
(ii) They received Nobel Prize in 1962.
(iii) Sutton and Boveri proposed the chromosomal theory.

Correct Answer: (a)
Watson and Crick published the double helix model of DNA in 1953 and shared the Nobel Prize in 1962. Sutton and Boveri's chromosomal theory preceded them. All three statements are correct.

Q11. Which of the following is being described by the given statements?
(i) They are the units of inheritance.
(ii) They contain information required to express a particular trait.
(iii) Mendel called them 'factors'.

Correct Answer: (b)
Genes are the fundamental units of inheritance that carry the information for traits. Mendel referred to them as 'factors'. They are segments of DNA that code for proteins or functional RNA. Alleles are alternative forms of a gene, not the gene itself.

Q12. The process by which characters are passed from parent to progeny is:

Correct Answer: (c)
Inheritance is the transmission of genetic information from parents to offspring. It is the fundamental process that ensures continuity of traits across generations. Heredity and variation together form the basis of genetics.

Q13. Assertion: Turner's syndrome occurs in females. Reason: It is caused by the absence of one of the X chromosomes (45, XO).

Correct Answer: (a)
Turner syndrome is a chromosomal condition that affects females, characterised by a single X chromosome (45,XO) instead of the usual pair. The absence of one X leads to a spectrum of developmental features including short stature and infertility. The karyotype directly explains the phenotype, so the reason is the correct explanation.

Q14. Assertion: A dihybrid cross involves two pairs of contrasting traits. Reason: Mendel crossed round yellow seeds with wrinkled green seeds.

Correct Answer: (a)
A dihybrid cross is defined as a cross that considers two different traits simultaneously. Mendel's cross between round‑yellow and wrinkled‑green pea plants is a classic example of a dihybrid cross, where he followed the inheritance of seed shape and seed colour. The example perfectly illustrates the definition, so the reason correctly explains the assertion.

Q15. Assertion: Alleles are slightly different forms of the same gene. Reason: They occupy different sites on different chromosomes.

Correct Answer: (a)
Alleles are alternative versions of a gene that reside at the same locus on homologous chromosomes. The reason states that they occupy different sites on different chromosomes, which is incorrect because they are found at corresponding positions on homologous chromosomes. Therefore, the assertion is true but the reason is false.

Q16. Assertion: Sickle cell anemia is a Mendelian disorder. Reason: It is caused by the substitution of Glutamic acid by Valine.

Correct Answer: (a)
Sickle cell anemia follows a single‑gene inheritance pattern (autosomal recessive), making it a Mendelian disorder. The reason gives the molecular defect (point mutation leading to amino acid substitution), which is true but does not explain why it is Mendelian—that requires understanding of its inheritance pattern. The disorder is caused by a mutation in the beta‑globin gene, but the Mendelian nature arises from the segregation of alleles.

Q17. Regarding ABO blood grouping, which statement is true?
(i) It is controlled by three alleles IA, IB and i.
(ii) IA and IB are completely dominant over i.
(iii) IA and IB are co-dominant to each other.
(iv) Allele i produces a different form of sugar than IA.

Correct Answer: (a)
The ABO system has three alleles: IA, IB, and i. IA and IB are dominant over i, and co‑dominant with each other. Allele i does not produce any sugar, so statement (iv) is false. Thus, (i), (ii), and (iii) are correct.

Q18. Assertion: Aneuploidy results in the gain or loss of a chromosome. Reason: It is caused by the failure of segregation of chromatids during cell division.

Correct Answer: (a)
Aneuploidy refers to a change in chromosome number involving one or a few chromosomes. It is usually caused by non‑disjunction—the failure of homologous chromosomes or sister chromatids to separate properly during meiosis or mitosis. This error results in gametes with extra or missing chromosomes, leading to aneuploid conditions such as trisomy or monosomy.

Q19. Match the Following scientists with their contribution:

Column-I	Column-II
A. Henking	(I) X-ray Diffraction of proteins
B. Alfred Sturtevant	(II) Chromosome mapping
C. Francis Crick	(III) Discovery of X-body

Correct Answer: (a)
Henking discovered the X‑body (III), Sturtevant developed chromosome mapping (II), and Crick contributed to understanding protein structure using X‑ray diffraction (I). The correct mapping is A–III, B–II, C–I.

Q20. Assertion: Mendel's laws were based on statistical analysis. Reason: He used a large sampling size which gave greater credibility to his data.

Correct Answer: (a)
Mendel applied statistical methods to his experimental results, which was revolutionary for biological research at that time. By using large numbers of progeny, he ensured that his observed ratios were not due to chance. The large sample size provided strong support for his laws and allowed him to detect consistent patterns.

Q21. Assertion: Haemophilia is more common in males. Reason: It is an X-linked recessive disorder and males have only one X chromosome.

Correct Answer: (a)
Haemophilia is caused by a recessive allele on the X chromosome. Males are hemizygous (have only one X), so a single copy of the mutant allele results in the disease. Females need two copies to be affected, making the disorder much rarer in females. This X‑linked recessive inheritance explains the higher incidence in males.

Q22. Which of the following statements about 'Incomplete Dominance' is correct?
(i) The F1 phenotype is in between the two parents.
(ii) The genotypic ratio is 1:2:1.
(iii) The phenotypic ratio is 3:1.

Correct Answer: (a)
In incomplete dominance, the F1 shows an intermediate phenotype (i), and the F2 genotypic ratio is 1:2:1 (ii). However, the phenotypic ratio is also 1:2:1, not 3:1 (iii is false). Thus, (i) and (ii) are correct.

Q23. Assertion: Klinefelter's syndrome individuals are sterile. Reason: They have a karyotype of 47, XXY.

Correct Answer: (b)
Klinefelter syndrome (47, XXY) occurs in males who have an extra X chromosome. The extra X interferes with testicular development, leading to reduced testosterone production and impaired spermatogenesis, which results in sterility. The karyotype directly causes the reproductive dysfunction, so the reason is the correct explanation.

Q24. Match the blood genotypes with their phenotypes:

Column-I (Genotype)	Column-II (Phenotype)
A. IA i	(I) AB
B. IA IB	(II) A
C. ii	(III) B
D. IB i	(IV) O

Correct Answer: (a)
The ABO genotypes and their phenotypes: IA i gives A (II), IA IB gives AB (I), ii gives O (IV), and IB i gives B (III). The correct matching is A–II, B–I, C–IV, D–III.

Q25. Regarding 'Sickle-cell Anaemia', which are true?
(i) Controlled by a single pair of alleles, HbA and HbS.
(ii) homozygous HbS individuals show the disease.
(iii) It is an X-linked recessive disorder.

Correct Answer: (a)
Sickle‑cell anaemia is caused by a single pair of alleles (HbA and HbS) and is autosomal recessive. Homozygous HbS individuals have the disease. It is not X‑linked, so (iii) is false. Therefore, (i) and (ii) are correct.

Q26. Which statement is true for 'Test Cross'?
(i) Crossing dominant phenotype with recessive parent.
(ii) Used to find the genotype of the organism.
(iii) Ratio is always 3:1.

Correct Answer: (a)
A test cross involves crossing an individual of dominant phenotype with a homozygous recessive to determine its genotype. The resulting ratio depends on whether the dominant individual is homozygous (all dominant) or heterozygous (1:1). The ratio is not always 3:1, so (iii) is false. Thus, (i) and (ii) are correct.

Q27. Gregor Mendel conducted hybridisation experiments on garden peas for how many years?

Correct Answer: (c)
Mendel performed his famous pea plant experiments from 1856 to 1863, spanning seven years. During this period he meticulously cross‑pollinated thousands of plants and analysed their traits. His extensive work laid the cornerstone of classical genetics.

Q28. Match the Following for Thalassemia:

Type	Affected Chromosome
A. Alpha-thalassemia	(I) Chromosome 11
B. Beta-thalassemia	(II) Chromosome 16

Correct Answer: (a)
Alpha‑thalassemia results from mutations on chromosome 16 (II), while beta‑thalassemia involves chromosome 11 (I). The correct mapping is A–II, B–I.

Q29. Assertion: The law of segregation is also known as the law of purity of gametes. Reason: Alleles do not show any blending and are recovered as such in F2.

Correct Answer: (a)
The law of segregation states that the two alleles for a trait separate during gamete formation, so each gamete receives only one allele. Because alleles remain distinct and do not blend, they reappear unchanged in the F2 generation, hence the term 'purity of gametes'. This principle was confirmed by the reappearance of recessive traits in the F2.

Q30. Assertion: Drosophila is an ideal material for genetic studies. Reason: It can be grown on simple synthetic medium and has a short life cycle.

Correct Answer: (a)
Drosophila's short generation time (about two weeks) allows rapid observation of multiple generations. It can be reared easily on inexpensive media in the laboratory, and its large number of offspring facilitates statistical analysis. These practical advantages made it the model organism of choice for Morgan and subsequent geneticists.

Q31. Match the Following terms:

Term	Meaning
A. Genotype	(I) Observable characteristic
B. Phenotype	(II) Genetic makeup
C. Alleles	(III) Contrasting traits of a gene

Correct Answer: (a)
Genotype is the genetic makeup (II), phenotype is the observable characteristic (I), and alleles are contrasting forms of a gene (III). The correct mapping is A–II, B–I, C–III.

Q32. Match the organism with its sex determination type:

Column-I	Column-II
A. Humans	(I) ZW type
B. Grasshopper	(II) XY type
C. Birds	(III) XO type

Correct Answer: (a)
Humans have XY sex determination (II), grasshoppers have XO (III), and birds have ZW (I). The correct mapping is A–II, B–III, C–I.

Q33. Regarding 'Chromosomal Theory of Inheritance':
(i) Behaviour of chromosomes is parallel to genes.
(ii) Chromosomes as well as genes occur in pairs.
(iii) Two alleles are located on homologous sites.

Correct Answer: (a)
The Chromosomal Theory states that genes are located on chromosomes and that the behaviour of chromosomes during meiosis explains Mendelian inheritance. Both chromosomes and genes exist in pairs, and alleles occupy corresponding loci on homologous chromosomes. All three statements are correct.

Q34. Which cross is used to determine the genotype of a tall plant at F2?

Correct Answer: (b)
A test cross involves crossing an individual showing the dominant phenotype (but unknown genotype) with a homozygous recessive parent. If the offspring show a 1:1 ratio of dominant to recessive, the unknown parent is heterozygous; if all offspring show the dominant trait, it is homozygous dominant. This cross was devised by Mendel to verify the genotype of F2 tall plants.

Q35. Match the following for Mendel's traits:

Character	Dominant Trait
A. Seed shape	(I) Full
B. Pod shape	(II) Round
C. Seed colour	(III) Yellow

Correct Answer: (a)
In peas, the dominant traits are: seed shape – Round (II), pod shape – Full (I), and seed colour – Yellow (III). The correct mapping is A–II, B–I, C–III.

Q36. How many genotypes are possible for the human ABO blood types?

Correct Answer: (c)
With three alleles (IA, IB, i) at a single locus, the number of possible genotypes is 3×4/2 = 6. These are: IAIA, IAi, IBIB, IBi, IAIB, and ii. The six genotypes correspond to four phenotypes: A, B, AB, and O.

Q37. Identify the correct statements regarding Drosophila melanogaster as an experimental tool:
(i) They complete their life cycle in about two weeks.
(ii) A single mating could produce a large number of progeny.
(iii) The male and female flies are easily distinguishable.
(iv) It cannot be grown on simple synthetic medium.

Correct Answer: (a)
Drosophila has a short life cycle, high fecundity, and sexually dimorphic adults, making it an excellent model. It can be grown easily on simple synthetic medium, so statement (iv) is false. Therefore, (i), (ii), and (iii) are correct.

Q38. Phenylketonuria is an example of:

Correct Answer: (b)
Phenylketonuria (PKU) is caused by a single gene mutation but has multiple phenotypic effects, including mental retardation, reduced pigmentation, and eczema. This phenomenon, where one gene influences several traits, is called pleiotropy. The defect in the enzyme phenylalanine hydroxylase leads to accumulation of phenylalanine and its derivatives.

Q39. Starch grain size in pea seeds is an example of:

Correct Answer: (b)
In pea seeds, the gene controlling seed shape (round vs. wrinkled) also influences starch grain size. Heterozygotes produce starch grains of intermediate size, demonstrating incomplete dominance at the phenotypic level. The round allele produces large grains, the wrinkled allele produces small ones, and the heterozygote yields intermediate‑sized grains.

Q40. Assertion: Thalassemia is a quantitative problem of globin synthesis. Reason: It involves the synthesis of incorrectly functioning globin.

Correct Answer: (a)
Thalassemia is characterised by reduced or absent production of globin chains, a quantitative defect. In contrast, sickle cell anemia involves a qualitative defect—the synthesis of a structurally abnormal globin. The reason incorrectly describes thalassemia as involving incorrectly functioning globin, which is true for sickle cell but not for thalassemia.

Q41. Thomas Hunt Morgan worked with which organism?

Correct Answer: (c)
Thomas Hunt Morgan chose the fruit fly Drosophila melanogaster for his genetic experiments because of its short generation time, high fecundity, and ease of culture. His work with Drosophila established the chromosomal basis of inheritance and led to the discovery of sex‑linked traits.

Q42. Who proposed the Chromosomal Theory of Inheritance?

Correct Answer: (c)
Walter Sutton and Theodor Boveri independently proposed the Chromosomal Theory of Inheritance around 1902. They noted that the behaviour of chromosomes during meiosis parallels the behaviour of Mendelian factors (genes). Their work provided a physical basis for Mendel's laws.

Q43. A true-breeding line is one that has undergone continuous ______.

Correct Answer: (b)
True-breeding lines are obtained by repeated self‑pollination over many generations, which ensures homozygosity for all traits. Such lines produce offspring identical to themselves when selfed. Mendel used true‑breeding varieties to ensure consistent inheritance patterns in his crosses.

Q44. Which are correct for 'Mendel's Law of Independent Assortment'?
(i) Applies to traits combined in a hybrid.
(ii) Segregation of one pair of characters is independent of the other.
(iii) Dihybrid ratio is 9:3:3:1.

Correct Answer: (a)
Independent assortment states that alleles of different genes segregate independently during gamete formation. It applies to traits studied in dihybrid crosses, yielding a 9:3:3:1 phenotypic ratio. All three statements are correct.

Q45. The ABO blood groups in humans are controlled by the gene:

Correct Answer: (d)
The ABO blood group system is governed by a single gene designated 'I' (for isoagglutinin). This gene has three major alleles: IA, IB, and i. The IA and IB alleles are co‑dominant, while i is recessive to both.

Q46. Assertion: Down's syndrome is a chromosomal disorder. Reason: It is caused by the trisomy of chromosome 21.

Correct Answer: (a)
Down's syndrome results from an extra copy of chromosome 21 (trisomy 21), which is a chromosomal abnormality. The presence of the extra chromosome disrupts normal development and leads to the characteristic features. The reason correctly identifies the causative chromosomal event, thereby explaining the assertion.

Q47. Identify the features of 'Down's Syndrome':
(i) Short statured with small round head.
(ii) Furrowed tongue and partially open mouth.
(iii) Palm is broad with characteristic palm crease.
(iv) Caused by monosomy of chromosome 21.

Correct Answer: (a)
Down's syndrome features include short stature, small round head, furrowed tongue, broad palm with a single crease, and other characteristics. However, it is caused by trisomy of chromosome 21, not monosomy. Thus, (i), (ii), and (iii) are correct.

Q48. Assertion: Point mutation involves a change in a single base pair of DNA. Reason: Sickle cell anemia is caused by a point mutation in the beta-globin gene.

Correct Answer: (a)
A point mutation is defined as a change in a single nucleotide pair of DNA. Sickle cell anemia is a well‑known example of such a mutation, but the reason provides an example rather than an explanation of what a point mutation is. The assertion is a definition, and the reason supports it but does not define the term.

Q49. In polygenic inheritance, the phenotype reflects the contribution of each allele, meaning the effect is:

Correct Answer: (c)
Polygenic traits are controlled by multiple genes, each contributing a small, additive effect to the phenotype. The cumulative effect of these alleles produces a continuous distribution of phenotypes (e.g., human skin colour). This contrasts with Mendelian traits that show discrete categories.

Q50. Correct statements for 'Haemophilia' include:
(i) Sex-linked recessive disease.
(ii) Single protein of the blood clotting cascade is affected.
(iii) Heterozygous female is a carrier.

Correct Answer: (a)
Haemophilia is an X‑linked recessive disorder where a single clotting factor protein is deficient. Females heterozygous for the mutant allele are carriers. All three statements are correct.

Q51. Match the Following mutations:

Type	Cause
A. Point mutation	(I) Loss or gain of a segment of DNA
B. Chromosomal aberration	(II) Change in a single base pair

Correct Answer: (a)
Point mutation is a change in a single base pair (II), while chromosomal aberrations involve larger structural changes like loss or gain of DNA segments (I). The correct mapping is A–II, B–I.

Q52. Assertion: The symbol of a solid square in a pedigree chart represents an affected male. Reason: In pedigree analysis, females are represented by circles.

Correct Answer: (a)
Pedigree charts use standardised symbols: squares represent males, circles represent females, and filled symbols indicate affected individuals. The fact that females are represented by circles does not explain why a solid square is an affected male—that is simply a convention. Both statements are true, but the reason does not provide the explanation for the assertion.

Q53. Assertion: Male honey bees have 16 chromosomes. Reason: They are produced by parthenogenesis and are haploid.

Correct Answer: (a)
In honey bees, males (drones) develop from unfertilised eggs via parthenogenesis, so they are haploid with 16 chromosomes (n=16). The haploid state directly explains the chromosome number, as they receive only the maternal set. Thus, the reason correctly explains why male honey bees have 16 chromosomes.

Q54. Identify true statements for 'Turner's syndrome':
(i) Absence of one X chromosome.
(ii) Karyotype is 45 with X0.
(iii) Ovaries are rudimentary.

Correct Answer: (a)
Turner syndrome occurs in females with a single X chromosome (45, XO), resulting in rudimentary ovaries and other developmental features. All three statements are correct.

Q55. Match the Following types of dominance:

Column-I	Column-II
A. Incomplete Dominance	(I) ABO Blood groups
B. Co-dominance	(II) Pea plant height
C. Law of Dominance	(III) Snapdragon flower color

Correct Answer: (a)
Incomplete dominance is seen in Snapdragon flower colour (III), co‑dominance in ABO blood groups (I), and the law of dominance in pea plant height (II). The correct mapping is A–III, B–I, C–II.

Q56. Assertion: Variation is the degree by which progeny differ from their parents. Reason: Inheritance is the basis of heredity.

Correct Answer: (a)
Variation refers to the differences between offspring and parents, while inheritance is the process of transmitting traits from one generation to the next. Both are fundamental concepts in genetics, but the reason does not explain the assertion—they are independent definitions. Variation arises from mutations, recombination, and environmental influences.

Q57. Mendel selected how many true-breeding pea plant varieties?

Correct Answer: (b)
Mendel chose 14 true‑breeding pea varieties, which represented seven pairs of contrasting traits. For each trait he had one variety expressing the dominant form and another expressing the recessive form. This allowed him to study the inheritance of each character independently.

Q58. Mendel's work remained unrecognised until the year:

Correct Answer: (b)
Mendel published his findings in 1865, but they were largely ignored until 1900, when three botanists—Hugo de Vries, Carl Correns, and Erich von Tschermak—independently rediscovered his laws. This rediscovery marked the birth of modern genetics.

Q59. In Honey bees, the unfertilised egg develops as a male (drone) by:

Correct Answer: (c)
In honey bees, males (drones) develop from unfertilised eggs through a process called parthenogenesis, which does not involve fusion of gametes. Thus, drones are haploid, containing only one set of chromosomes. Females (queens and workers) are diploid and develop from fertilised eggs.

Q60. Assertion: Sex-linked genes do not show independent assortment. Reason: They are located on the same chromosome (X).

Correct Answer: (a)
Genes on the same chromosome are physically linked and tend to be inherited together, violating independent assortment. Sex‑linked genes are those located on the X chromosome, and their inheritance pattern differs in males and females because males have only one X. This linkage explains why they do not assort independently of each other.

Q61. Regarding 'Pleiotropy', which are correct?
(i) Single gene exhibits multiple phenotypic expressions.
(ii) Effect of gene on metabolic pathways.
(iii) Example is Phenylketonuria.

Correct Answer: (a)
Pleiotropy occurs when one gene influences multiple traits, often through its role in a metabolic pathway. Phenylketonuria is a classic example. All three statements are correct.

Q62. Identify the correct statements for 'Polygenic Inheritance':
(i) Controlled by three or more genes.
(ii) Influence of environment is taken into account.
(iii) Phenotype reflects the additive effect of each allele.

Correct Answer: (a)
Polygenic traits are governed by multiple genes, show additive effects, and are often influenced by environmental factors. All three statements are accurate descriptions of polygenic inheritance.

Q63. Which of the following is correct for 'Linkage' and 'Recombination'?
(i) Recombination describes the generation of non-parental gene combinations.
(ii) Recombination is higher for loosely linked genes.
(iii) Linkage describes the physical association of genes on a chromosome.

Correct Answer: (a)
Linkage refers to genes on the same chromosome being inherited together. Recombination produces new combinations of alleles, and the frequency of recombination is proportional to the distance between genes—higher for loosely linked (far apart) genes. All three statements are correct.

Q64. Which of the following is correct for 'Klinefelter's syndrome'?
(i) Presence of an additional copy of X-chromosome.
(ii) Karyotype is 47, XXY.
(iii) Development of breast (Gynaecomastia) occurs.

Correct Answer: (a)
Klinefelter syndrome is characterised by an extra X chromosome (47, XXY) in males, leading to features such as gynaecomastia (breast development) and sterility. All three statements are correct.

Q65. Which of the following is a dominant trait in the pea plants studied by Mendel?

Correct Answer: (c)
Among the seven contrasting traits Mendel studied, violet flower colour is dominant over white. The dominant allele masks the expression of the recessive allele in the heterozygous condition. This was consistently observed in all his monohybrid crosses.

Q66. Humans knew as early as 8000-1000 B.C. that variation was hidden in:

Correct Answer: (b)
Early human civilisations observed that offspring from sexual reproduction often differ from parents, while asexual reproduction produces identical copies. This realisation laid the foundation for understanding variation. Sexual reproduction combines genetic material from two parents, generating new combinations.

Q67. Assertion: Human skin colour is a polygenic trait. Reason: It is controlled by three genes (A, B, C) and shows additive effects.

Correct Answer: (a)
Human skin colour is a classic example of polygenic inheritance, influenced by multiple genes each contributing additively to pigment production. The number of dominant alleles determines the darkness of the skin, producing a continuous range of phenotypes. This additive effect is the basis of the polygenic nature, so the reason correctly explains the assertion.

Q68. Select correct statements about 'Mutation':
(i) Alteration of DNA sequences.
(ii) Results in changes in genotype and phenotype.
(iii) UV radiation is a mutagen.

Correct Answer: (a)
Mutations are changes in DNA sequence that can lead to altered genotypes and phenotypes. Ultraviolet radiation is a well‑known mutagen that can cause DNA damage. All three statements are correct.

Q69. Match the Following chromosome positions:

Type	Feature
A. Metacentric	(I) Two equal arms
B. Acrocentric	(II) One extremely short and one very long arm

Correct Answer: (a)
Metacentric chromosomes have arms of equal length (I), while acrocentric chromosomes have one very short and one very long arm (II). The correct mapping is A–I, B–II.

Q70. Assertion: The genetic makeup of the sperm determines the sex of the child in humans. Reason: Males produce two types of gametes (50% X and 50% Y).

Correct Answer: (a)
Human males produce sperm carrying either an X or a Y chromosome, while females produce only X‑bearing eggs. The sex of the offspring is determined by which type of sperm fertilises the egg—XX gives female, XY gives male. Thus, the genetic composition of the sperm is the deciding factor, and the reason explains this mechanism.

Q71. Mendel’s Law of Independent Assortment is based on which type of cross?

Correct Answer: (b)
The Law of Independent Assortment was derived from Mendel's dihybrid crosses, where he followed two characters simultaneously. He observed that alleles for different traits segregate independently of each other during gamete formation. The characteristic 9:3:3:1 phenotypic ratio in the F2 generation of a dihybrid cross supports this law.

Q72. Genes which code for a pair of contrasting traits are known as:

Correct Answer: (b)
Alleles are alternative forms of the same gene that occupy corresponding loci on homologous chromosomes. They determine contrasting traits, such as tall vs. dwarf or round vs. wrinkled. Mendel referred to them as 'factors', but today they are called alleles.

Q73. Match the Mendelian ratios:

Column-I	Column-II
A. F2 Monohybrid Phenotypic	(I) 1:2:1
B. F2 Monohybrid Genotypic	(II) 9:3:3:1
C. F2 Dihybrid Phenotypic	(III) 3:1

Correct Answer: (a)
The F2 monohybrid phenotypic ratio is 3:1 (III), the genotypic ratio is 1:2:1 (I), and the F2 dihybrid phenotypic ratio is 9:3:3:1 (II). The correct mapping is A–III, B–I, C–II.

Q74. Henking (1891) observed a specific nuclear structure during spermatogenesis and called it:

Correct Answer: (b)
Henking discovered a peculiar nuclear structure in the sperm cells of certain insects and named it the 'X‑body'. Later, it was identified as the X chromosome. This observation was a key step towards understanding sex chromosomes and sex determination.

Q75. Assertion: Pedigree analysis is useful in human genetics. Reason: Control crosses cannot be performed in human beings.

Correct Answer: (a)
In humans, experimental crosses are impossible for ethical and practical reasons. Pedigree analysis—the study of family histories—is therefore the primary tool for studying inheritance patterns in humans. It allows geneticists to infer modes of inheritance and predict the risk of genetic disorders. Thus, the reason justifies the use of pedigree analysis.

Q76. Identify correct features of 'Aneuploidy':
(i) Gain or loss of a chromosome.
(ii) Caused by failure of chromatid segregation.
(iii) Example is Down's Syndrome.

Correct Answer: (a)
Aneuploidy involves an abnormal number of chromosomes due to non‑disjunction. Down's syndrome (trisomy 21) is a classic example. All three statements are correct.

Q77. In birds, the sex determination mechanism is:

Correct Answer: (b)
Birds exhibit female heterogamety, meaning the female has two different sex chromosomes (ZW) while the male has two alike (ZZ). This is the reverse of the XY system seen in mammals. The sex of the offspring is determined by the female gamete, which can carry either Z or W.

Q78. Match theFollowing linkage results from Morgan's study:

Cross	Recombination %
A. White and Yellow	(I) 37.2%
B. White and Miniature wing	(II) 1.3%

Correct Answer: (a)
Morgan's experiments in Drosophila showed that genes close together on the chromosome have low recombination (e.g., white and yellow genes – 1.3%), while farther apart genes have higher recombination (white and miniature wing – 37.2%). The correct mapping is A–II, B–I.

Q79. Match the Following chromosomal abnormalities:

A. 2n + 1	Trisomy
B. 2n - 1	Monosomy
C. 3n or 4n	Polyploidy

Correct Answer: (a)
Aneuploidy includes trisomy (2n+1) and monosomy (2n‑1); polyploidy refers to 3n or 4n chromosome sets. All three are correctly matched.

Q80. Correct statements about 'Thalassemia' include:
(i) Autosome-linked recessive blood disease.
(ii) Results in reduced rate of globin synthesis.
(iii) It is a qualitative problem.

Correct Answer: (a)
Thalassemia is an autosomal recessive disorder characterised by reduced synthesis of globin chains (quantitative defect). It is not a qualitative problem (that would be sickle‑cell anaemia). Hence, (i) and (ii) are correct, while (iii) is false.

Q81. Assertion: In ABO blood groups, alleles IA and IB are co-dominant. Reason: Both IA and IB express their own types of sugars when present together.

Correct Answer: (a)
Co‑dominance occurs when both alleles in a heterozygote are fully expressed. In the ABO system, individuals with IAIB produce both A‑type and B‑type antigens on their red blood cells, resulting in the AB blood type. This simultaneous expression is the defining feature of co‑dominance and is correctly explained by the production of both sugars.

Q82. Match the characters with their recessive traits in Pea plant:

Column-I (Character)	Column-II (Recessive Trait)
A. Stem height	(I) Terminal
B. Flower position	(II) Dwarf
C. Pod shape	(III) Yellow
D. Pod colour	(IV) Constricted

Correct Answer: (a)
In pea plants, the recessive traits are: dwarf stem height (II), terminal flower position (I), constricted pod shape (IV), and yellow pod colour (III). The correct mapping is A–II, B–I, C–IV, D–III.

Q83. The term used to describe the physical association of genes on a chromosome is:

Correct Answer: (b)
Linkage refers to the tendency of genes located on the same chromosome to be inherited together. Morgan coined this term after observing that certain traits in Drosophila did not assort independently. The strength of linkage depends on the physical distance between genes.

Q84. Match the syndrome with its chromosomal composition:

Column-I (Syndrome)	Column-II (Chromosomes)
A. Down’s syndrome	(I) 47, XXY
B. Klinefelter’s syndrome	(II) 45, XO
C. Turner’s syndrome	(III) 47, Trisomy 21

Correct Answer: (a)
Down’s syndrome is trisomy of chromosome 21 (III), Klinefelter’s is 47, XXY (I), and Turner’s is 45, XO (II). The correct mapping is A–III, B–I, C–II.

Q85. Which are correct for 'Multiple Alleles'?
(i) More than two alleles governing the same character.
(ii) Found only when population studies are made.
(iii) Example is ABO blood groups.

Correct Answer: (a)
Multiple alleles occur when more than two alleles exist for a gene in a population; they are detected through population studies. The ABO blood group system is a prime example. All three statements are correct.

Q86. Alfred Sturtevant used the frequency of ______ to map the position of genes.

Correct Answer: (c)
Sturtevant, a student of Morgan, realised that the frequency of recombination between two genes is proportional to the distance separating them on the chromosome. He used these frequencies to construct the first genetic linkage maps. Recombination occurs during crossing over in prophase I of meiosis.

Q87. Assertion: Phenylketonuria results in mental retardation. Reason: It is caused by the accumulation of phenylalanine in the brain.

Correct Answer: (a)
PKU is caused by a defect in the enzyme phenylalanine hydroxylase, leading to accumulation of phenylalanine and its toxic metabolites in the brain. This accumulation impairs brain development and causes irreversible mental retardation if untreated. The reason accurately describes the causative mechanism, thus explaining the assertion.

Q88. Assertion: Colour blindness is a sex-linked recessive disorder. Reason: It is more common in males (8%) than females (0.4%).

Correct Answer: (a)
Colour blindness is caused by a recessive allele on the X chromosome. Because males have only one X, they express the disorder if they inherit the mutant allele, while females need two copies. The higher frequency in males is a consequence of this inheritance pattern, but it does not define the disorder as sex‑linked recessive—that is the underlying genetics. The reason provides supporting evidence but is not the defining explanation.

Q89. Identify the correct phylum/group being described:
(i) They are male heterogametic (XY).
(ii) Female has a pair of XX chromosomes.
(iii) Determinant of male characteristic is the Y chromosome.

Correct Answer: (a)
Humans have male heterogamety (XY), females XX, and the Y chromosome is responsible for maleness. This description fits humans exactly. Birds have female heterogamety (ZW), grasshoppers have XO, and honey bees have haplodiploidy.

Q90. Match the Following inheritance patterns:

Pattern	Example
A. Multiple Alleles	(I) Human skin colour
B. Polygenic inheritance	(II) ABO blood groups
C. Pleiotropy	(III) Phenylketonuria

Correct Answer: (a)
Multiple alleles are exemplified by ABO blood groups (II), polygenic inheritance by human skin colour (I), and pleiotropy by phenylketonuria (III). The correct mapping is A–II, B–I, C–III.

Q91. Assertion: Polyploidy is often seen in plants. Reason: It results from the failure of cytokinesis after telophase.

Correct Answer: (a)
Polyploidy, the condition of having more than two complete sets of chromosomes, is common in plants. It can arise from the failure of cytokinesis during cell division, leading to a doubling of chromosome number. This mechanism explains how polyploid individuals are formed, so the reason correctly explains the assertion.

Q92. The phenotypic ratio of a typical Mendelian monohybrid cross in F2 generation is:

Correct Answer: (c)
In a monohybrid cross, the F2 generation exhibits a 3:1 phenotypic ratio – three individuals showing the dominant trait for every one showing the recessive trait. This ratio arises from the segregation of alleles during gamete formation and their random fusion. It is a hallmark of complete dominance.

Q93. Match the following for Honey bees:

Bee Type	Chromosome No.
A. Queen (Female)	(I) 16
B. Drone (Male)	(II) 32

Correct Answer: (a)
In honey bees, females are diploid (2n=32) and males are haploid (n=16). Thus, queen (female) has 32 (II) and drone (male) has 16 (I). The correct mapping is A–II, B–I.

Q94. Grasshopper is an example of which type of sex determination?

Correct Answer: (c)
In grasshoppers, sex is determined by the XO system, where males have only one X chromosome (XO) and females have two (XX). This was one of the early systems used to illustrate the chromosomal basis of sex determination. The absence of a Y chromosome in males makes them hemizygous for X‑linked genes.

Q95. Select the correct statements for 'Mendel's Law of Segregation':
(i) Alleles do not show any blending.
(ii) A gamete receives only one of the two factors.
(iii) Both characters are recovered as such in F2.

Correct Answer: (a)
The law of segregation states that alleles remain discrete and do not blend, each gamete receives one allele, and the original traits reappear in the F2 generation. All three statements correctly describe this law.