Heredity and Evolution: From Mendel's Peas to Modern Genetics

Why do children look like their parents? Why did dinosaurs vanish and birds emerge? The answers lie in heredity and evolution — two ideas that completely changed our understanding of life. Together, they form one of the most fascinating chapters in Class 10 biology.

Heredity

Heredity is the transmission of characteristics from parents to offspring through genes. The branch of science that studies heredity is called genetics.

Variation

Variations are differences in traits among individuals of the same species. Variations help species survive in changing environments and form the raw material for evolution.

Mendel — The Father of Genetics

Gregor Mendel, an Austrian monk, conducted experiments on pea plants from 1856 to 1863. He selected seven contrasting characters (tall/short, round/wrinkled, etc.) and discovered the basic rules of inheritance.

Mendel's Laws of Inheritance

1. Law of Dominance

In a pair of contrasting traits, one trait is dominant and the other is recessive. The dominant trait expresses itself in the F₁ generation.

2. Law of Segregation

During gamete formation, the two alleles of a gene separate so that each gamete carries only one allele.

3. Law of Independent Assortment

The alleles of different genes assort independently during gamete formation.

Monohybrid and Dihybrid Crosses

Monohybrid cross: Involves one pair of contrasting traits. Phenotypic ratio in F₂ = 3:1.
Dihybrid cross: Involves two pairs of contrasting traits. Phenotypic ratio in F₂ = 9:3:3:1.

Sex Determination in Humans

Humans have 23 pairs of chromosomes, with one pair being sex chromosomes (X and Y).

Females: XX
Males: XY

Mothers always pass on X; fathers pass either X (daughter) or Y (son). Therefore, the father's chromosome determines the sex of the child.

Genes and DNA

A gene is a segment of DNA that codes for a specific protein and determines a trait. DNA (deoxyribonucleic acid) is the molecule of heredity, organised into structures called chromosomes found in the nucleus of every cell.

Evolution

Evolution is the gradual change in the inherited characteristics of populations over generations. It explains the diversity of life.

Theory of Natural Selection

Proposed by Charles Darwin in 1859 in his book On the Origin of Species. Key ideas:

Organisms produce more offspring than can survive.
Individuals within a species vary.
Those better suited to the environment survive and reproduce more — "survival of the fittest."
Over time, favourable traits accumulate, leading to new species.

Evidence for Evolution

1. Homologous Organs

Organs with similar structures but different functions, suggesting a common ancestor (e.g., the forelimbs of humans, whales, bats, and frogs).

2. Analogous Organs

Organs with different structures but similar functions, suggesting convergent evolution (e.g., wings of insects and birds).

3. Fossils

Remains or impressions of ancient life forms preserved in rocks. Fossils help trace the evolutionary history of life.

4. Vestigial Organs

Organs that have lost their function over time, such as the human appendix or wisdom teeth.

Speciation

The formation of new species over time, often through geographic isolation, genetic drift, mutation, and natural selection.

Human Evolution

Modern humans (Homo sapiens) evolved over millions of years from earlier hominids. Important steps include:

Australopithecus — early bipedal ancestors.
Homo habilis — used simple stone tools.
Homo erectus — first to use fire.
Homo sapiens — modern humans with developed language and culture.

Acquired vs Inherited Traits

Inherited traits: Passed through genes (eye colour, blood group).
Acquired traits: Developed during a lifetime; not passed on (skill in painting, muscular build from gym).

Common Mistakes Students Make

Confusing dominant traits with "stronger" or "better."
Calling analogous organs evidence of common ancestry.
Believing humans evolved from modern monkeys — both share a common ancestor instead.

Frequently Asked Questions

Q1. Why are Mendel's experiments still relevant?
Because they laid the mathematical foundation of modern genetics. The same principles apply to humans, plants, and animals today.

Q2. Why do offspring resemble parents but are not identical?
Because they inherit a unique combination of genes from both parents, plus random mutations and recombination during gamete formation.

Q3. How do vestigial organs support evolution?
They show that organisms once used these organs but no longer need them, indicating evolutionary change.

Q4. Is the appearance of bacteria resistant to antibiotics an example of evolution?
Yes. Resistant bacteria survive and reproduce, passing resistance to future generations — a classic example of natural selection.

Q5. Why are fossils found in rocks of specific ages?
Because sedimentary rocks form in layers, and fossils get trapped during layer formation, indicating the time period in which organisms lived.

Key Takeaways

Heredity transmits traits through genes, and evolution shapes the diversity of life through variation and natural selection. Together they explain why a child has its mother's eyes and why elephants and whales share a common ancestor with rats.