Principles of Inheritance and Variation — Class 12 Biology NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 12 Biology chapter "Principles of Inheritance and Variation" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 12 Biology chapter "Principles of Inheritance and Variation" — 8 important questions with detailed answers…
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NCERT Solutions for Class 12 Biology — Principles of Inheritance and Variation. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Principles of Inheritance and Variation — All 8 Questions Solved
Q1. Define gene, allele, phenotype, and genotype.
Key genetic terms:
Gene: Segment of DNA coding for a specific trait or protein. Basic unit of heredity. Located on chromosomes at fixed positions (loci). Genes control traits through protein synthesis.
Allele: Alternative forms of a gene at the same locus on homologous chromosomes. Example: gene for flower color has alleles for red, white, pink. Organisms are diploid, carrying two alleles for each gene (one from each parent).
Genotype: Genetic composition of an organism; allele combination at loci for specific traits. Example: AA, Aa, or aa for a trait. Genotype is inherited and determines potential for trait expression.
Phenotype: Observable physical characteristics of organism resulting from genotype interaction with environment. Example: red flowers, blue eyes, height. Phenotype is visible expression of genotype; same genotype may show different phenotypes in different environments. Example: tall plant genotype shows shorter height in poor soil.
Relationship: Genotype + Environment = Phenotype
Genotype is constant (inherited), phenotype can vary with environmental factors (temperature, nutrition, light). Phenotype determines what we observe; genotype determines hereditary potential.
Q2. State Mendel's law of segregation with explanation and example.
Law of Segregation (First Law of Inheritance):
Statement: When gametes are formed during meiosis, allele pairs separate so that each gamete receives only one allele of each gene. During fertilization, alleles from both parents combine to restore the pair in offspring.
Explanation:
- Diploid organisms carry two alleles for each trait (one from each parent)
- During meiosis, homologous chromosomes separate, carrying different alleles to different gametes
- Each gamete receives only ONE allele
- At fertilization, gametes fuse restoring diploid condition
- Offspring receives one allele from each parent
Mendel's monohybrid cross example (Pea plant, seed color):
P: Yellow (YY) × Green (yy)
F1: All Yy (yellow) - shows dominant trait
F1 × F1: Yy × Yy
F2: 1 YY (yellow) : 2 Yy (yellow) : 1 yy (green) = 3:1 phenotypic ratio
Genetic explanation:
- Yy produces two types of gametes: Y and y (equal proportion)
- Fertilization combinations: YY, Yy, Yy, yy
- Dominant allele (Y) masks recessive (y) in heterozygotes
- Recessive trait appears only in homozygous recessive (yy)
This law explains segregation of alleles and inheritance of single traits.
Q3. State and explain Mendel's law of independent assortment with an example dihybrid cross.
Law of Independent Assortment (Second Law of Inheritance):
Statement: Allele pairs of different genes assort independently during gamete formation. The inheritance of one trait is independent of inheritance of another trait (for genes on different chromosomes).
Explanation:
- Applies to genes on non-homologous chromosomes
- During meiosis, distribution of alleles for one gene does not affect distribution of alleles for another gene
- Each gene segregates independently
- Results in new combinations of alleles in offspring not present in parents
Dihybrid cross example (Pea plant, seed color and seed shape):
P: Yellow-round (YYRR) × Green-wrinkled (yyrr)
F1: All YyRr (yellow-round)
F1 × F1: YyRr × YyRr
Gamete types from YyRr: YR, Yr, yR, yr (4 types, equal proportion)
F2 phenotypic ratio: 9:3:3:1
- 9 yellow-round (Y_R_)
- 3 yellow-wrinkled (Y_rr)
- 3 green-round (yyR_)
- 1 green-wrinkled (yyrr)
Genetic explanation:
- Y and y segregate independently from R and r
- All combinations of gametes possible
- 16 possible genotypic combinations in F2
- 9:3:3:1 ratio result of 3:1 (color) × 3:1 (shape) ratios
This law explains inheritance of multiple traits and formation of new phenotypic combinations in offspring.
Q4. Explain test cross and back cross. Distinguish between them.
Test cross and back cross are controlled matings used in genetics to determine genotypes and study inheritance:
Test cross:
- Definition: Cross between individual with dominant phenotype (unknown genotype) and homozygous recessive individual
- Purpose: Determine whether dominant individual is homozygous (AA) or heterozygous (Aa)
- Parental cross: Aa (or AA) × aa
- Results:
* If dominant parent is AA: All offspring show dominant phenotype (Aa)
* If dominant parent is Aa: 1:1 ratio of dominant to recessive phenotype (Aa and aa)
- Example: Yellow pea (Y_) × Green pea (yy)
* If Y_ is YY: all F1 yellow
* If Y_ is Yy: F1 shows 1 yellow : 1 green ratio
- Significance: Reveals recessive alleles masked in phenotype
Back cross:
- Definition: Cross between hybrid individual (Aa) and one of its homozygous parental types (AA or aa)
- Purpose: To introduce or remove specific traits; used in breeding programs
- Parental cross: Aa × AA or Aa × aa
- Results with dominant parent (AA): All offspring show dominant phenotype (AA and Aa)
- Results with recessive parent (aa): 1:1 ratio of phenotypes
- Example: F1 wheat × Parent wheat for improved variety
- Significance: Transfers desirable traits to inbred line
Key differences:
- Test cross uses homozygous recessive; back cross uses homozygous of either type
- Test cross reveals genotype of unknown dominant individual; back cross transfers traits
- Test cross produces 1:1 ratio if dominant is heterozygous; back cross varies depending on parent type
Q5. Explain the concept of linkage and crossing over. How do they affect genetic variation?
Linkage and crossing over are phenomena affecting inheritance of genes on same chromosome:
Linkage:
- Definition: Genes located close together on same chromosome tend to be inherited together as a unit
- Concept: Linked genes do not assort independently; violates Mendel's law of independent assortment
- Reason: Physical proximity on chromosome; separated only by crossing over
- Strength: Linkage strength depends on distance between genes (map distance in centimorgans)
- Example: In Drosophila, white eye and miniature wing genes are linked on X chromosome
- Test cross reveals linkage: parental combinations appear more frequently than recombinant types
Crossing over (Recombination):
- Definition: Physical exchange of genetic material between homologous chromatids during prophase I of meiosis
- Mechanism: Homologous chromosomes pair (synapsis); non-sister chromatids exchange segments at chiasmata
- Produces: Recombinant chromatids with new combinations of alleles
- Frequency: Proportional to distance between genes; closer genes recombine less frequently
- Significance: Creates genetic variation within linked genes
Effect on genetic variation:
1. Linkage reduces variation: Linked alleles remain together, limiting new combinations; reduces genetic diversity
2. Crossing over increases variation: Breaks linkage by creating recombinant gametes with new allele combinations; increases genetic diversity
3. Overall effect: Balance between linkage (maintains parental combinations) and crossing over (creates new combinations) generates variation within limits of linked genes
Map distance calculation: Recombination frequency (%) between two genes equals map distance in centimorgans (cM). Example: 10% recombination = 10 cM distance
Linkage analysis: Used to determine gene order and distances on chromosomes; basis of linkage mapping.
Q6. Define mutation and explain different types of mutations with examples.
Mutation: Sudden, random change in DNA sequence causing alteration in genetic information. Source of genetic variation and evolution.
Types of mutations:
1. Gene (Point) mutations: Change in single nucleotide
- Substitution: One base replaced by another
* Silent mutation: no change in protein (codon redundancy); example: CCU→CCC (both code leucine)
* Missense mutation: changes amino acid; example: sickle cell anemia (glutamic acid→valine in β-globin)
* Nonsense mutation: creates stop codon; premature protein termination; example: some forms of cystic fibrosis
- Insertion: Extra nucleotide(s) added; shifts reading frame
- Deletion: Loss of nucleotide(s); shifts reading frame
2. Chromosomal mutations: Changes in chromosome structure or number
- Structural alterations:
* Deletion: loss of segment; example: Cri-du-chat syndrome
* Duplication: repetition of segment; gene dosage imbalance
* Inversion: segment reverses orientation; crossing over produces abnormal gametes
* Translocation: segment transfers to non-homologous chromosome; example: Philadelphia chromosome in leukemia
- Numerical alterations:
* Euploidy: change in number of chromosome sets (triploidy 3n, tetraploidy 4n)
* Aneuploidy: change in number of individual chromosomes
- Monosomy: loss of one chromosome (2n-1); example: Turner syndrome (45,X)
- Trisomy: gain of one chromosome (2n+1); example: Down syndrome (trisomy 21), Edwards syndrome (trisomy 18)
3. Causes: Spontaneous (DNA replication errors, spontaneous deamination) or induced (radiation, chemicals)
Effects: Mostly deleterious; rare beneficial mutations provide variation for natural selection.
Q7. Explain the concept of dominance, recessiveness, and codominance with examples.
Dominance relationships describe how alleles interact in heterozygotes to produce phenotype:
Dominance:
- Definition: Dominant allele expresses its phenotype in heterozygotes; recessive allele expression masked
- Genotype: Aa shows phenotype of AA
- Molecular basis: Dominant allele produces sufficient gene product (protein) to cause phenotype; recessive needs two copies
- Example: Pea plants, seed color: yellow (Y) dominant over green (y)
* YY = yellow, Yy = yellow (yellow dominant), yy = green
- Complete vs incomplete: Complete dominance shows no intermediate phenotype; incomplete dominance shows blending
Recessiveness:
- Definition: Recessive allele phenotype appears only in homozygous recessive individual
- Genotype: aa expresses recessive phenotype; Aa shows dominant phenotype
- Requirement: Two copies of recessive allele needed for trait expression
- Example: Human eye color, cystic fibrosis (autosomal recessive)
* CF/CF = normal, CF/cf = carrier (normal phenotype), cf/cf = affected
- Carrier state: Heterozygotes carry recessive allele without showing phenotype
Codominance:
- Definition: Both alleles in heterozygote fully express their phenotypes simultaneously; neither dominant
- Genotype: Aa shows both A and a phenotypes distinctly, not blended
- Molecular basis: Both alleles produce functional gene products simultaneously; no masking
- Example: Human ABO blood groups
* I^A I^B = AB blood group (both A and B antigens express on RBCs)
* I^A I^A = A, I^A i = A, I^B I^B = B, I^B i = B, ii = O
- Other examples:
* Roan cattle: RR (red) × WW (white) produces RW (roan, both colors hair)
* Snapdragon flowers: Red × White produces pink flowers (incomplete dominance) vs red and white together (codominance)
Key difference: Codominance shows distinct two phenotypes; incomplete dominance shows intermediate blended phenotype.
Q8. Define polygenic inheritance and explain with examples. How does it differ from monohybrid inheritance?
Polygenic inheritance:
Definition: Inheritance of traits controlled by multiple genes (polygenes), each contributing small additive effect to phenotype. Also called quantitative or multifactorial inheritance.
Characteristics:
- Multiple genes control single trait
- Each gene has small additive effect
- Environment also influences phenotype
- Produces continuous variation (bell curve distribution) rather than discrete categories
- No clear-cut phenotypic ratios
Examples in humans:
- Height: controlled by 700+ genes; shows continuous variation from short to tall
- Skin color: controlled by 4-6 major genes; shows spectrum from very light to very dark
- Eye color: more complex than simple dominant-recessive
- Intelligence: polygenic with environmental influence
- Weight: polygenic with strong environmental influence
Example with simplified model (2 genes, A and B):
- Tallness: A and B alleles contribute height
- Genotypes: AABB (tallest), AABb, AAbb, AaBB, AaBb, Aabb, aaBB, aaBb, aabb (shortest)
- 9 genotypes produce 5 phenotypic classes: very tall, tall, medium, short, very short
- Distribution approximates bell curve
Differences from monohybrid inheritance:
1. Number of genes: Monohybrid = 1 gene; polygenic = multiple genes
2. Phenotypic ratios: Monohybrid = discrete (3:1, 9:3:3:1); polygenic = continuous distribution
3. Phenotype categories: Monohybrid = distinct classes; polygenic = spectrum/gradation
4. Environmental influence: Monohybrid = minimal; polygenic = significant
5. Prediction: Monohybrid = predictable ratios; polygenic = statistical probabilities
6. Heritability: Monohybrid = 100% genetic; polygenic = partial (genetic + environmental)
Polygenic traits show transgressive segregation: F2 individuals can show phenotypes more extreme than either parent.
Principles of Inheritance and Variation — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. Who is considered the 'Father of Genetics'?
- A. Gregor Mendel
- B. Thomas Hunt Morgan
- C. Hugo de Vries
- D. Sutton and Boveri
Answer: A. Gregor Mendel — Gregor Mendel, an Austrian monk, conducted pioneering experiments on pea plants, laying the foundation for modern genetics and formulating the laws of inheritance.
MCQ 2. What is the phenotypic ratio of a monohybrid cross in the F2 generation when dealing with complete dominance?
- A. 1:2:1
- B. 3:1
- C. 9:3:3:1
- D. 1:1
Answer: B. 3:1 — In a monohybrid cross, the F2 generation exhibits a phenotypic ratio of 3 dominant to 1 recessive trait, as seen when crossing two F1 heterozygotes.
MCQ 3. Mendel's Law of Segregation states that:
- A. Alleles for different traits assort independently.
- B. Each gamete receives only one allele for each gene.
- C. Dominant alleles always mask recessive alleles.
- D. Traits are inherited as discrete units.
Answer: B. Each gamete receives only one allele for each gene. — The Law of Segregation, also known as the Law of Purity of Gametes, states that during gamete formation, the two alleles for a heritable character separate or segregate from each other so that each gamete receives only one allele.
MCQ 4. In Snapdragon (Mirabilis jalapa), a cross between true-breeding red-flowered plants and true-breeding white-flowered plants produces F1 offspring with pink flowers. This is an example of:
- A. Codominance
- B. Incomplete dominance
- C. Multiple allelism
- D. Pleiotropy
Answer: B. Incomplete dominance — In incomplete dominance, the heterozygous phenotype is intermediate between the two homozygous parental phenotypes, as seen with pink flowers resulting from red and white parents.
MCQ 5. Which of the following is an example of codominance in humans?
- A. Blood group A
- B. Blood group B
- C. Blood group AB
- D. Blood group O
Answer: C. Blood group AB — In AB blood group, both alleles (I^A and I^B) express themselves fully, producing both A and B antigens on the surface of red blood cells, which is a characteristic of codominance.
MCQ 6. A cross is made between a pea plant heterozygous for tallness and round seeds (Tt Rr) and a dwarf plant with wrinkled seeds (tt rr). What is the expected proportion of tall plants with wrinkled seeds in the offspring?
- A. 1/2
- B. 1/4
- C. 3/4
- D. 1/8
Answer: B. 1/4 — This is a dihybrid test cross. For tallness (Tt x tt), 1/2 will be Tall (Tt). For seed shape (Rr x rr), 1/2 will be wrinkled (rr). The probability of tall and wrinkled is (1/2) * (1/2) = 1/4.
MCQ 7. The Chromosomal Theory of Inheritance was proposed by:
- A. Gregor Mendel and Hugo de Vries
- B. Sutton and Boveri
- C. T.H. Morgan and A.H. Sturtevant
- D. Watson and Crick
Answer: B. Sutton and Boveri — Walter Sutton and Theodor Boveri independently proposed the Chromosomal Theory of Inheritance, which states that genes are located on chromosomes and that the behavior of chromosomes during meiosis accounts for the patterns of inheritance.
MCQ 8. The phenomenon where two or more genes located on the same chromosome tend to be inherited together is called:
- A. Crossing over
- B. Linkage
- C. Recombination
- D. Pleiotropy
Answer: B. Linkage — Linkage is the tendency of genes or alleles that are physically close to each other on a chromosome to be inherited together during meiosis, deviating from Mendel's law of independent assortment.
MCQ 9. If the frequency of recombination between two genes A and B is 10%, and between B and C is 20%, and between A and C is 30%, what is the likely order of these genes on the chromosome?
- A. A-B-C
- B. A-C-B
- C. B-A-C
- D. C-A-B
Answer: A. A-B-C — Recombination frequencies are roughly proportional to the distance between genes. The highest frequency (30%) is between A and C, suggesting they are farthest apart. The sum of A-B (10%) and B-C (20%) equals A-C (30%), indicating B is between A and C. So, the order is A-B-C.
MCQ 10. In humans, the sex of a child is determined by:
- A. The mother
- B. The father
- C. Both parents
- D. Environmental factors
Answer: B. The father — Human males have XY sex chromosomes, and females have XX. The father produces two types of sperm, X and Y. If an X sperm fertilizes the egg, the child is female (XX); if a Y sperm fertilizes the egg, the child is male (XY).
MCQ 11. An example of XX-XO type of sex determination is found in:
- A. Birds
- B. Humans
- C. Grasshoppers
- D. Drosophila
Answer: C. Grasshoppers — In grasshoppers and some insects, females are XX (homogametic) and males are XO (heterogametic), meaning males have only one X chromosome and lack a Y.
MCQ 12. A sudden heritable change in the genetic material of an organism is called:
- A. Variation
- B. Inheritance
- C. Mutation
- D. Recombination
Answer: C. Mutation — Mutation refers to an abrupt change in the DNA sequence and chromosome structure or number, leading to variation in an organism.
MCQ 13. Sickle cell anemia is an example of a:
- A. Frameshift mutation
- B. Point mutation
- C. Chromosomal aberration
- D. Aneuploidy
Answer: B. Point mutation — Sickle cell anemia results from a single nucleotide substitution (point mutation) in the beta-globin gene, where adenine is replaced by thymine at the sixth codon, leading to a change from glutamic acid to valine.
MCQ 14. A human male with XXY sex chromosomes suffers from:
- A. Turner's syndrome
- B. Down's syndrome
- C. Klinefelter's syndrome
- D. Phenylketonuria
Answer: C. Klinefelter's syndrome — Klinefelter's syndrome is a chromosomal disorder caused by the presence of an extra X chromosome in males (XXY), leading to overall masculine development but with some feminine characteristics.
MCQ 15. Which of the following is an X-linked recessive disorder?
- A. Sickle cell anemia
- B. Hemophilia
- C. Down's syndrome
- D. Phenylketonuria
Answer: B. Hemophilia — Hemophilia is an X-linked recessive disorder where a mutation in genes on the X chromosome prevents the production of certain blood clotting factors, leading to prolonged bleeding.
MCQ 16. In a human pedigree chart, a square represents:
- A. Female
- B. Male
- C. Unaffected individual
- D. Affected individual
Answer: B. Male — Standard pedigree symbols use squares to represent males and circles to represent females.
MCQ 17. If a genetic disease affects individuals in every generation, and affected individuals always have at least one affected parent, it is most likely a/an:
- A. Autosomal recessive disorder
- B. Autosomal dominant disorder
- C. X-linked recessive disorder
- D. Y-linked disorder
Answer: B. Autosomal dominant disorder — Autosomal dominant disorders typically appear in every generation, and an affected individual must have at least one affected parent. Both males and females are affected equally.
MCQ 18. The condition in which a single gene affects multiple phenotypic traits is called:
- A. Polygenic inheritance
- B. Multiple allelism
- C. Pleiotropy
- D. Epistasis
Answer: C. Pleiotropy — Pleiotropy occurs when a single gene mutation has multiple phenotypic effects, such as in phenylketonuria or sickle cell anemia.
MCQ 19. A test cross is performed to:
- A. Determine the genotype of an F1 hybrid
- B. Determine the genotypic ratio of F2 generation
- C. Determine the phenotypic ratio of F2 generation
- D. Produce new genetic combinations
Answer: A. Determine the genotype of an F1 hybrid — A test cross involves crossing an individual with an unknown genotype (showing the dominant phenotype) with a homozygous recessive individual to determine the unknown genotype.
MCQ 20. Human skin color is an example of:
- A. Multiple allelism
- B. Codominance
- C. Polygenic inheritance
- D. Incomplete dominance
Answer: C. Polygenic inheritance — Human skin color is controlled by three or more genes, with the dominant alleles contributing to the darker skin tone, demonstrating polygenic inheritance.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.