Heredity and Evolution

Why This Chapter Matters

Heredity and Evolution is one of the most conceptual chapters — 7-9 marks. Mendel's laws, inheritance patterns, and Darwin's theory of natural selection are tested every year. Understanding ratio problems is essential.

Prerequisites

Core Concepts

1. Heredity and Variation

Heredity: Transmission of characters from parents to offspring.

Variation: Differences among individuals of the same species.

Causes of variation:

Variation provides raw material for evolution.

2. Mendel's Laws

Gregor Mendel (1822-1884) — "Father of Genetics" — studied pea plants (Pisum sativum).

Why pea plants?

Mendel's First Law — Law of Segregation:

"Two alleles of a character separate during gamete formation and only one goes into each gamete."

Monohybrid Cross (one trait):

|---|---|---|

P generation: TT (Tall) × tt (dwarf)

F₁ generation: All Tt (Tall) — heterozygous

F₁ × F₁: Tt × Tt

F₂ generation: TT : Tt : tt = 1 : 2 : 1 (genotype)

Phenotype: Tall : Dwarf = 3 : 1

Mendel's Second Law — Law of Independent Assortment:

"Alleles of different traits are inherited independently."

Dihybrid Cross (two traits):

Phenotypic ratio in F₂ = 9:3:3:1 (Round Yellow : Round Green : Wrinkled Yellow : Wrinkled Green)

3. Terminology

|---|---|

4. Sex Determination in Humans

Inheritance of sex:

|---|---|---|

Result: 50% male, 50% female

Father determines the sex (Y chromosome comes from father)

This is why blaming the mother for the gender of a child is scientifically wrong.

5. Evolution

Evolution: Change in the inherited characteristics of a population over successive generations.

Darwin's Theory of Natural Selection:

Evidence for evolution:

6. Acquired vs Inherited Traits

Acquired traits: Traits gained during lifetime due to environment. NOT inherited.

Example: Muscular body from exercise, scar from injury.

Inherited traits: Traits coded in DNA, passed to offspring.

Example: Eye colour, blood group, height (partially).

Why acquired traits are NOT inherited:

Only DNA changes (mutations) can be passed. Body muscle changes don't alter DNA. (Lamarck was wrong — giraffes didn't evolve long necks by stretching!)

PYQs

2023

Q: In Mendel's monohybrid cross between tall (TT) and dwarf (tt) pea plants, what is the F₂ phenotypic ratio?

F₁ = All Tt (Tall)

F₁ × F₁: Tt × Tt → TT : Tt : tt = 1:2:1

Phenotype: Tall : Dwarf = 3:1

2022

Q: How is the sex of a child determined in humans?

Father produces two types of sperm: X-sperm and Y-sperm (50:50).

Mother produces only X-containing eggs.

If X-sperm fertilises egg → XX → Female

If Y-sperm fertilises egg → XY → Male

Sex determined by father (the Y chromosome).

2021

Q: Define evolution. What is the role of variation in evolution?

Evolution: Gradual change in the characteristics of species over many generations leading to formation of new species.

Role of variation: Provides raw material for natural selection. Individuals with favourable variations survive better and reproduce more, passing on those traits.

2020

Q: Distinguish between analogous and homologous organs with one example each.

Homologous organs: Same basic structure, different functions (human arm and whale flipper).

Analogous organs: Different structure, same function (bird wing and butterfly wing — both for flying).

MCQ Practice

Q1. In a monohybrid cross, the phenotypic ratio in F₂ is:

(A) 1:2:1 (B) 3:1 ✓ (C) 9:3:3:1 (D) 1:1

Q2. Eye colour determined by genes, not by studying hard — this is an example of:

(A) Acquired trait (B) Inherited trait ✓ (C) Evolution (D) Variation due to environment

Q3 (Hard). If father is colour blind (X^c Y) and mother is carrier (X^c X), probability of colour-blind daughter:

Mother gametes: X^c and X | Father gametes: X^c and Y

Daughters: X^c X^c (affected) and X^c X (carrier)

Probability of colour-blind daughter = 1/4 = 25% of all offspring = 1/2 of daughters

Revision Notes

MENDEL'S LAWS:
  Law of Segregation: Alleles separate in gametes (monohybrid)
  Law of Independent Assortment: Alleles of different genes assort independently (dihybrid)

MONOHYBRID RATIOS:
  F₁: all Tt (tall) — dominant expressed
  F₂ genotype: 1 TT : 2 Tt : 1 tt
  F₂ phenotype: 3 Tall : 1 dwarf

DIHYBRID RATIO (F₂): 9:3:3:1

SEX DETERMINATION:
  Male: XY (gives either X or Y sperm)
  Female: XX (gives only X eggs)
  Father determines sex!

DARWIN'S NATURAL SELECTION:
  Variation → Natural selection → Survival of fittest → Evolution

HOMOLOGOUS vs ANALOGOUS:
  Homologous: Same structure, different function (common ancestry)
  Analogous: Different structure, same function (convergent evolution)

ACQUIRED traits are NOT inherited (only DNA changes are)

Common Mistakes:

❌ F₁ ratio is 1:1 — WRONG, all F₁ from TT×tt are Tt (all same)

❌ Mother determines sex — WRONG, father (Y chromosome donor) determines sex

❌ Homologous organs show convergent evolution — WRONG, homologous show divergent; analogous show convergent

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