Heredity and Evolution – Class 10 Biology Chapter 8

Heredity and Evolution

Heredity is the transmission of characters (traits) from parents to offspring. Evolution is the gradual change in inherited traits over successive generations.

Mendel's Contribution

Gregor Johann Mendel (Father of Genetics) studied inheritance using garden pea plants (Pisum sativum). He chose 7 contrasting pairs of characters (e.g., tall vs dwarf, round vs wrinkled seeds).

Key Terms

• Gene: Unit of inheritance; a segment of DNA on a chromosome.
• Allele: Alternative forms of a gene (e.g., T for tall, t for dwarf).
• Dominant: The trait that expresses itself even in heterozygous condition (Tt → Tall).
• Recessive: The trait that is masked by the dominant allele (only expressed in tt → Dwarf).
• Genotype: Genetic makeup (TT, Tt, tt).
• Phenotype: Physical appearance (Tall or Dwarf).

Mendel's Laws

1. Law of Dominance: In a cross between two pure organisms differing in a pair of characters, only one character appears in F₁ (dominant).
2. Law of Segregation: The two alleles of a gene separate during gamete formation so each gamete receives only one allele.
3. Law of Independent Assortment: Genes for different traits are inherited independently of each other (applies to dihybrid crosses).

Monohybrid Cross

Parents: TT (Tall) × tt (Dwarf)

F₁: All Tt (Tall)

F₂: TT : Tt : Tt : tt → 3 Tall : 1 Dwarf

Genotypic ratio = 1:2:1 | Phenotypic ratio = 3:1

Sex Determination in Humans

Humans have 23 pairs of chromosomes (22 autosomes + 1 pair of sex chromosomes).

• Females: XX
• Males: XY

The sex of the child is determined by the father's gamete: X sperm → girl, Y sperm → boy.

Evolution

• Homologous organs: Same origin, different functions (e.g., forelimbs of whale, bat, human) — evidence of divergent evolution.
• Analogous organs: Different origin, same function (e.g., wings of butterfly and bird) — evidence of convergent evolution.
• Fossils: Preserved remains of ancient organisms that provide evidence of evolution.

Natural Selection (Darwin)

Organisms with favourable variations survive and reproduce more successfully in a given environment ("survival of the fittest"). Over generations, this leads to evolution and speciation.

Reference: NCERT Science Textbook for Class 10 – Chapter 8; Lakhmir Singh Biology; Pradeep's Biology.