ICSE Class 10 Biology: Genetics — Heredity & Variation Notes 2026

Introduction to Genetics

• Genetics: study of heredity (transmission of traits) and variation
• Gene: unit of heredity on a chromosome; controls a specific trait
• Alleles: alternate forms of a gene at the same locus

Mendel's Experiment and Laws

• Mendel worked with Pisum sativum (garden pea); 7 pairs of contrasting traits
• Law of Segregation: allele pairs separate during gamete formation; each gamete has one allele
• Law of Independent Assortment: genes for different traits assort independently (applies to unlinked genes)

Monohybrid Cross

• Dominant allele (T, tall) vs. recessive allele (t, dwarf); genotype TT, Tt, tt
• F1: all Tt (tall); F2: TT:Tt:tt = 1:2:1 genotypic; 3:1 phenotypic ratio
• Punnet square method: ICSE expects proper diagram with parental gametes

Dihybrid Cross

• Two traits simultaneously: e.g., seed shape (round R, wrinkled r) and seed colour (yellow Y, green y)
• F2 phenotypic ratio: 9:3:3:1 (round yellow : round green : wrinkled yellow : wrinkled green)
• Demonstrates independent assortment; 16-cell Punnet square required in ICSE

Sex Determination in Humans

• Humans: 23 pairs of chromosomes — 22 autosomes + 1 pair sex chromosomes
• Female: XX; male: XY; sex determined by which sex chromosome the sperm carries (X or Y)
• X-linked inheritance: colour blindness and haemophilia carried on X chromosome

Mutations

• Mutation: change in DNA sequence; can be gene mutation or chromosomal mutation
• Down syndrome: trisomy 21 (47 chromosomes); caused by non-disjunction during meiosis
• Mutations caused by: radiation (X-rays, UV), chemicals (mutagens); most are harmful, some neutral

ICSE Exam — Genetics

• Solve a cross: if both parents are Tt, find the probability of a dwarf offspring
• Explain why haemophilia is more common in males than females
• State one similarity and one difference between mitosis and meiosis