HEREDITY AND EVOLUTION — Class 10 CBSE
Notes
1. Introduction
Heredity: Passing of traits (characters) from parents to offspring.
Evolution: Gradual change in living organisms over generations
leading to formation of new species.
Both are connected — heredity provides the basis for evolution
through variations.
🧠 PART I – HEREDITY
2. Key Terms
Term Meaning
Features or characteristics (e.g., height, eye
Traits
colour)
Genes Units of heredity; control traits
Alleles Alternative forms of a gene
Genotype Genetic makeup (TT, Tt, tt)
Phenotype Physical appearance (Tall or Dwarf)
Dominant
Expressed in heterozygous condition (T)
Trait
Recessive
Hidden in heterozygous condition (t)
Trait
Chromosom Thread-like structures in nucleus carrying
es genes
3. Mendel’s Experiments
Gregor Mendel → “Father of Genetics”
Studied inheritance in pea plants (Pisum sativum).
, Observed traits like height, colour, shape, etc.
(a) Monohybrid Cross (One trait)
Cross between tall (TT) and dwarf (tt).
F₁ Generation: All tall (Tt) — dominant trait appears.
F₂ Generation: 3 tall : 1 dwarf → Ratio = 3:1
(b) Dihybrid Cross (Two traits)
Cross between plants differing in 2 traits (e.g., seed shape & colour).
F₂ Ratio: 9:3:3:1 → Independent assortment of genes.
4. Mendel’s Laws of Inheritance
1. Law of Dominance:
One factor (gene) masks the other (T dominates t).
2. Law of Segregation:
Gene pairs separate during gamete formation.
3. Law of Independent Assortment:
Different gene pairs assort independently.
5. Sex Determination
(a) In Humans:
Chromosomes: 23 pairs → 22 autosomes + 1 sex pair
o Female: XX
o Male: XY
Male decides child’s gender:
o X sperm + X egg → Girl (XX)
o Y sperm + X egg → Boy (XY)
(b) In Birds:
Male: ZZ, Female: ZW (female decides sex).
6. Variation
Differences between individuals of same species.
Notes
1. Introduction
Heredity: Passing of traits (characters) from parents to offspring.
Evolution: Gradual change in living organisms over generations
leading to formation of new species.
Both are connected — heredity provides the basis for evolution
through variations.
🧠 PART I – HEREDITY
2. Key Terms
Term Meaning
Features or characteristics (e.g., height, eye
Traits
colour)
Genes Units of heredity; control traits
Alleles Alternative forms of a gene
Genotype Genetic makeup (TT, Tt, tt)
Phenotype Physical appearance (Tall or Dwarf)
Dominant
Expressed in heterozygous condition (T)
Trait
Recessive
Hidden in heterozygous condition (t)
Trait
Chromosom Thread-like structures in nucleus carrying
es genes
3. Mendel’s Experiments
Gregor Mendel → “Father of Genetics”
Studied inheritance in pea plants (Pisum sativum).
, Observed traits like height, colour, shape, etc.
(a) Monohybrid Cross (One trait)
Cross between tall (TT) and dwarf (tt).
F₁ Generation: All tall (Tt) — dominant trait appears.
F₂ Generation: 3 tall : 1 dwarf → Ratio = 3:1
(b) Dihybrid Cross (Two traits)
Cross between plants differing in 2 traits (e.g., seed shape & colour).
F₂ Ratio: 9:3:3:1 → Independent assortment of genes.
4. Mendel’s Laws of Inheritance
1. Law of Dominance:
One factor (gene) masks the other (T dominates t).
2. Law of Segregation:
Gene pairs separate during gamete formation.
3. Law of Independent Assortment:
Different gene pairs assort independently.
5. Sex Determination
(a) In Humans:
Chromosomes: 23 pairs → 22 autosomes + 1 sex pair
o Female: XX
o Male: XY
Male decides child’s gender:
o X sperm + X egg → Girl (XX)
o Y sperm + X egg → Boy (XY)
(b) In Birds:
Male: ZZ, Female: ZW (female decides sex).
6. Variation
Differences between individuals of same species.
