MCQs on Principles of Inheritance and Variation – Class 12 Biology

Course: CBSE Class 12 Biology
Unit II: Genetics and Evolution
Topic: Principles of Inheritance and Variation
Based strictly on NCERT | Exam-oriented | Concept-clearing explanations

Section A: Mendel’s Experiments and Basic Genetic Concepts (Q1–Q15)

Q1. Mendel selected pea plants for his experiments mainly because

A. they show a long life cycle
B. they are wind pollinated
C. they have many contrasting characters
D. they reproduce only asexually

Correct Answer: C
Explanation: Pea plants possess several easily distinguishable contrasting traits (like tall/dwarf, round/wrinkled seeds), making them ideal for studying inheritance patterns.

Q2. The principle of dominance states that

A. both alleles express equally
B. recessive allele suppresses dominant allele
C. one allele expresses while the other is masked
D. traits blend in offspring

Correct Answer: C
Explanation: In a heterozygous condition, the dominant allele expresses itself while the recessive allele remains masked.

Q3. In a monohybrid cross, the phenotypic ratio in F₂ generation is

A. 1:2:1
B. 3:1
C. 9:3:3:1
D. 1:1

Correct Answer: B
Explanation: A monohybrid cross involving one character produces a 3:1 phenotypic ratio in the F₂ generation.

Q4. The genotypic ratio in a monohybrid cross is

A. 3:1
B. 9:3:3:1
C. 1:2:1
D. 1:1

Correct Answer: C
Explanation: Genotypically, F₂ offspring show one homozygous dominant, two heterozygous, and one homozygous recessive individual.

Q5. The law of segregation explains

A. independent assortment of genes
B. separation of alleles during gamete formation
C. blending inheritance
D. mutation

Correct Answer: B
Explanation: The law of segregation states that allele pairs separate during gamete formation, so each gamete carries only one allele.

Q6. A test cross involves crossing an individual with

A. homozygous dominant
B. homozygous recessive
C. heterozygous dominant
D. F₁ hybrid

Correct Answer: B
Explanation: Test cross is done to determine the genotype of an individual by crossing it with a homozygous recessive organism.

Q7. Mendel proposed the concept of genes as

A. blending units
B. continuous factors
C. discrete units of inheritance
D. proteins

Correct Answer: C
Explanation: Mendel proposed that inheritance is governed by discrete units (now called genes), not by blending of traits.

Q8. The F₁ generation of a monohybrid cross is always

A. phenotypically recessive
B. genotypically pure
C. phenotypically uniform
D. phenotypically variable

Correct Answer: C
Explanation: All F₁ offspring express the dominant trait, showing phenotypic uniformity.

Q9. Mendel’s work remained unnoticed initially because

A. it contradicted Darwin
B. it lacked statistical analysis
C. it was ahead of its time
D. journals rejected it

Correct Answer: C
Explanation: Mendel’s findings were not appreciated because the scientific community was not ready to accept particulate inheritance.

Q10. Which trait is recessive in pea plants?

A. Yellow seed
B. Tall stem
C. Round seed
D. Wrinkled seed

Correct Answer: D
Explanation: Wrinkled seed shape is a recessive trait compared to round seed shape.

Q11. The F₂ ratio deviates from Mendelian ratio due to

A. independent assortment
B. linkage
C. dominance
D. segregation

Correct Answer: B
Explanation: Linked genes do not assort independently, leading to deviation from expected Mendelian ratios.

Q12. Mendel studied inheritance using how many traits in pea plants?

A. 5
B. 6
C. 7
D. 8

Correct Answer: C
Explanation: Mendel studied seven pairs of contrasting characters in pea plants.

Q13. A pure line means

A. heterozygous condition
B. genetically identical individuals
C. hybrid population
D. mixed genotype

Correct Answer: B
Explanation: A pure line consists of individuals homozygous for a trait, producing identical offspring.

Q14. Which cross gives a 1:1 phenotypic ratio?

A. Monohybrid cross
B. Dihybrid cross
C. Test cross
D. Selfing of F₁

Correct Answer: C
Explanation: Test cross between heterozygote and homozygous recessive gives a 1:1 ratio.

Q15. Alleles are

A. identical genes on different chromosomes
B. alternative forms of a gene
C. non-functional DNA
D. linked genes

Correct Answer: B
Explanation: Alleles are different forms of the same gene occupying the same locus on homologous chromosomes.

Section B: Dihybrid Cross, Linkage and Chromosomal Theory (Q16–Q30)

Q16. The phenotypic ratio in a Mendelian dihybrid cross is

A. 3:1
B. 1:2:1
C. 9:3:3:1
D. 1:1:1:1

Correct Answer: C
Explanation: Independent assortment of two traits produces a 9:3:3:1 ratio in the F₂ generation.

Q17. The law of independent assortment applies when

A. genes are linked
B. genes are on same chromosome
C. genes are on different chromosomes
D. genes are dominant

Correct Answer: C
Explanation: Independent assortment occurs only when genes are located on different chromosomes or far apart.

Q18. Linkage was discovered by

A. Mendel
B. Morgan
C. Watson
D. Darwin

Correct Answer: B
Explanation: Thomas Hunt Morgan discovered linkage while working on fruit flies.

Q19. Genes present on the same chromosome are called

A. alleles
B. linked genes
C. homologues
D. polygenes

Correct Answer: B
Explanation: Genes located on the same chromosome tend to be inherited together and are called linked genes.

Q20. Crossing over occurs during

A. metaphase I
B. anaphase I
C. pachytene of prophase I
D. telophase II

Correct Answer: C
Explanation: Exchange of genetic material occurs between non-sister chromatids during pachytene stage.

Q21. Recombination frequency is used to

A. determine mutation rate
B. map genes on chromosome
C. calculate dominance
D. identify alleles

Correct Answer: B
Explanation: Recombination frequency helps determine the distance between genes on a chromosome.

Q22. Chromosomal theory of inheritance was proposed by

A. Mendel
B. Sutton and Boveri
C. Morgan
D. Darwin

Correct Answer: B
Explanation: Sutton and Boveri correlated Mendel’s laws with chromosome behavior.

Q23. The unit of linkage is

A. chromosome
B. gene
C. allele
D. genome

Correct Answer: A
Explanation: A chromosome is considered the unit of linkage as linked genes are inherited together.

Q24. Which organism was used by Morgan for genetic studies?

A. Pea plant
B. Maize
C. Drosophila melanogaster
D. Frog

Correct Answer: C
Explanation: Drosophila has short life cycle and easily observable traits, ideal for genetics.

Q25. Linkage reduces

A. variation
B. recombination
C. dominance
D. segregation

Correct Answer: B
Explanation: Linked genes show reduced recombination as they are inherited together.

Q26. Genetic maps are based on

A. number of genes
B. chromosome length
C. recombination frequency
D. dominance

Correct Answer: C
Explanation: Gene distances are calculated using recombination frequencies.

Q27. Complete linkage occurs when

A. crossing over is maximum
B. genes are far apart
C. genes never separate
D. mutation occurs

Correct Answer: C
Explanation: In complete linkage, genes are inherited together without crossing over.

Q28. Sex-linked genes are located on

A. autosomes
B. X or Y chromosomes
C. mitochondria
D. plastids

Correct Answer: B
Explanation: Sex-linked genes are present on sex chromosomes.

Q29. The white-eye mutation in Drosophila is

A. autosomal recessive
B. autosomal dominant
C. X-linked recessive
D. Y-linked

Correct Answer: C
Explanation: White-eye color in fruit fly is an X-linked recessive trait.

Q30. Recombination frequency can never exceed

A. 25%
B. 50%
C. 75%
D. 100%

Correct Answer: B
Explanation: Maximum recombination frequency is 50%, indicating independent assortment.

Section C: Sex Determination, Pedigree Analysis and Variation (Q31–Q50)

Q31. In humans, sex determination is

A. ZW type
B. XO type
C. XY type
D. haplodiploid

Correct Answer: C
Explanation: Males are heterogametic (XY) and females are homogametic (XX).

Q32. Colour blindness is inherited as

A. autosomal dominant
B. autosomal recessive
C. X-linked recessive
D. Y-linked

Correct Answer: C
Explanation: Colour blindness is an X-linked recessive disorder, more common in males.

Q33. Pedigree analysis is used to

A. study evolution
B. track inheritance of traits
C. detect mutations
D. classify organisms

Correct Answer: B
Explanation: Pedigree charts help trace inheritance patterns across generations.

Q34. Autosomal recessive disorders appear

A. in every generation
B. only in males
C. when both alleles are recessive
D. only in females

Correct Answer: C
Explanation: Recessive disorders manifest only when both alleles are recessive.

Q35. Haemophilia is also called

A. blue baby syndrome
B. night blindness
C. bleeder’s disease
D. sickle cell disease

Correct Answer: C
Explanation: Haemophilia causes excessive bleeding due to clotting factor deficiency.

Q36. Mutation is defined as

A. gene flow
B. genetic drift
C. sudden heritable change
D. recombination

Correct Answer: C
Explanation: Mutation is a sudden, permanent, heritable change in genetic material.

Q37. Point mutation affects

A. whole chromosome
B. a single base pair
C. entire genome
D. cell organelle

Correct Answer: B
Explanation: Point mutation involves change in a single nucleotide base.

Q38. Sickle cell anaemia is caused due to

A. deletion
B. insertion
C. substitution
D. duplication

Correct Answer: C
Explanation: It results from substitution of valine for glutamic acid in haemoglobin.

Q39. Variation is important because it

A. causes diseases
B. reduces population
C. helps in evolution
D. stops inheritance

Correct Answer: C
Explanation: Variations provide raw material for natural selection and evolution.

Q40. Polygenic inheritance shows

A. discontinuous variation
B. 3:1 ratio
C. continuous variation
D. linkage

Correct Answer: C
Explanation: Traits controlled by many genes show continuous variation, like height.

Q41. ABO blood group inheritance is an example of

A. polygenic inheritance
B. incomplete dominance
C. codominance
D. linkage

Correct Answer: C
Explanation: Both A and B alleles express equally in AB blood group.

Q42. Multiple alleles are

A. more than two alleles of a gene
B. two alleles in heterozygote
C. linked genes
D. recessive alleles

Correct Answer: A
Explanation: A gene having more than two alleles in a population shows multiple allelism.

Q43. Turner’s syndrome individuals have

A. XX
B. XY
C. XO
D. XXY

Correct Answer: C
Explanation: Turner’s syndrome is caused by monosomy of X chromosome.

Q44. Klinefelter’s syndrome genotype is

A. XO
B. XY
C. XXY
D. XYY

Correct Answer: C
Explanation: Presence of an extra X chromosome in males causes Klinefelter’s syndrome.

Q45. Crossing over increases

A. linkage
B. homozygosity
C. variation
D. dominance

Correct Answer: C
Explanation: Crossing over produces new gene combinations, increasing variation.

Q46. Which disorder is autosomal dominant?

A. Sickle cell anaemia
B. Haemophilia
C. Huntington’s disease
D. Colour blindness

Correct Answer: C
Explanation: Huntington’s disease is an autosomal dominant disorder.

Q47. Genetic drift is more significant in

A. large populations
B. small populations
C. stable populations
D. evolving species

Correct Answer: B
Explanation: Random changes in allele frequency are more pronounced in small populations.

Q48. Which mechanism creates new gene combinations?

A. Mutation
B. Recombination
C. Segregation
D. Dominance

Correct Answer: B
Explanation: Recombination during meiosis produces new allele combinations.

Q49. Sex-linked inheritance shows

A. equal occurrence in both sexes
B. only maternal inheritance
C. criss-cross inheritance
D. polygenic pattern

Correct Answer: C
Explanation: X-linked traits often pass from mother to son, showing criss-cross inheritance.

Q50. The ultimate source of genetic variation is

A. recombination
B. mutation
C. segregation
D. dominance

Correct Answer: B
Explanation: Mutations create new alleles and are the primary source of genetic variation.