ICSE Class 10 Biology: Genetics — Important Questions with Answers 2026

Short Answer Questions (2-3 Marks)

• Q: Define: (i) Gene, (ii) Allele, (iii) Genotype, (iv) Phenotype.
  Ans: (i) Gene: Unit of heredity; a segment of DNA that codes for a specific protein/trait. Located on chromosomes. (ii) Allele: Alternative forms of a gene occupying the same locus on homologous chromosomes. Example: T (tall) and t (dwarf). (iii) Genotype: Genetic makeup of an organism. Example: TT, Tt, tt. (iv) Phenotype: Physical appearance/expression of a trait. Example: Tall, Dwarf.
• Q: State Mendel's Law of Segregation with an example.
  Ans: Law of Segregation (First Law): During gamete formation, the two alleles of a gene separate from each other so that each gamete carries only one allele. Example: Cross Tt × Tt. Parent Tt produces gametes T and t. F2 ratio: TT:Tt:tt = 1:2:1 (genotypic). Phenotypic ratio: Tall:Dwarf = 3:1.
• Q: In a monohybrid cross between tall (TT) and dwarf (tt) pea plants, show the F1 and F2 generations.
  Ans: P: TT (tall) × tt (dwarf). F1: All Tt (tall) — tall is dominant. F1 × F1: Tt × Tt. Gametes: T, t × T, t. F2 Punnett square: TT, Tt, Tt, tt. Genotypic ratio: 1 TT : 2 Tt : 1 tt. Phenotypic ratio: 3 Tall : 1 Dwarf.

Long Answer / Application Questions (4-6 Marks)

• Q: What is sex determination in humans? How is the sex of a child determined?
  Ans: In humans, sex is determined by sex chromosomes. Females have XX chromosomes, males have XY. Mother always contributes X chromosome. Father contributes either X or Y. If sperm carrying X fertilises egg (XX) → female child. If sperm carrying Y fertilises egg (XY) → male child. Therefore, the father's chromosome determines the sex of the child. Probability: 50% male, 50% female.
• Q: Differentiate between dominant and recessive traits.
  Ans: Dominant trait: Expresses itself in both homozygous (TT) and heterozygous (Tt) condition. Represented by capital letter. Example: Tallness in pea plants. Recessive trait: Expresses itself only in homozygous condition (tt). Masked by dominant allele in heterozygous state. Represented by small letter. Example: Dwarfness in pea plants.

Exam Tips for This Chapter

• Revise all definitions and laws from Genetics — they are commonly asked as 1-2 mark questions
• Practice diagrams related to Genetics — neat labelled diagrams carry 2-3 marks
• For numericals, always show formula → substitution → answer with correct units
• Previous year analysis shows Genetics carries 8-12 marks in the board exam