Heredity and Evolution — Class 10 NCERT Chapter 8: quick revision, 18 bites

Genes Are the Units of Heredity

Traits travel from parent to child via genes.

**Heredity** is the transmission of characters from parents to offspring. The physical carrier of each trait is a **gene** — a segment of DNA on a chromosome.

Takeaway. Each trait is controlled by a pair of alleles — one inherited from each parent.

Dominant vs Recessive Alleles

One allele can silence the other.

Takeaway. Tt and TT both look tall, but only tt looks short.

Mendel's Monohybrid Cross: F1 to F2

3:1 is the magic ratio of heredity.

Takeaway. The recessive allele is not lost in F1 — it hides and reappears in F2.

Phenotypic Ratio of Monohybrid F2

3:1 — memorise this ratio.

Takeaway. Phenotypic 3:1 and genotypic 1:2:1 always go together in a monohybrid cross.

Dihybrid Cross: Two Traits at Once

Two traits, one 9:3:3:1 ratio.

Mendel crossed plants differing in **two traits** (seed shape and seed colour) to test whether traits are inherited independently.

Takeaway. The 9:3:3:1 ratio proves traits are inherited independently of each other.

Law of Independent Assortment

Traits don't travel as a package.

Mendel's **second law** states that alleles of different genes assort independently during gamete formation — one trait's allele does not influence another's.

Takeaway. Independent assortment is why siblings can look so different from each other.

Monohybrid vs Dihybrid Cross

One trait or two — know the difference.

Takeaway. Dihybrid is essentially two monohybrid crosses happening together.

Sex Determination in Humans

The father decides the baby's sex.

Takeaway. Sex is determined by which sex chromosome the father's sperm carries.

Dominant ≠ More Common

Dominant doesn't mean frequent.

Takeaway. Dominance is about expression, not frequency.

Why Siblings Look Different

Same parents, different kids.

Takeaway. Independent assortment and random fertilisation create variation even within one family.

Variation Is the Raw Material of Evolution

No variation, no evolution.

**Variation** means differences in traits among individuals of a species. It arises from sexual reproduction, mutations and recombination.

Takeaway. Evolution acts on variation — the more variation, the better a species adapts.

How Natural Selection Works

Survival of the best-adapted.

Takeaway. Natural selection is differential reproduction — not 'trying' to adapt.

Acquired vs Inherited Traits

Learning to swim isn't in your genes.

Takeaway. Only inherited traits (in DNA of germ cells) pass to the next generation.

Mendel's First Law: Segregation

Alleles separate before gametes form.

During gamete formation, the two alleles of a gene **separate** so each gamete carries only one allele. This is the **Law of Segregation**.

Takeaway. Segregation explains why a recessive trait can reappear in the F2 generation.

Score Full Marks on Heredity Questions

Ratios and diagrams win marks.

Takeaway. Examiners reward correct ratios plus a neat Punnett square.

Remember the Dihybrid Ratio

9-3-3-1, easy as pie.

For a dihybrid F2, remember **9 : 3 : 3 : 1** — the two '3's are the new mixed combinations, and the '1' is the double recessive.

Takeaway. 9 dominant-dominant : 3 dominant-recessive : 3 recessive-dominant : 1 recessive-recessive.

Heredity in Everyday Life

Genetics is why you look like your parents.

Takeaway. Mendel's rules explain real inheritance patterns in humans, plants and animals.

Don't Confuse Genotype with Phenotype

Same look, different genes.

Takeaway. Phenotype = what you see; genotype = the allele pair behind it.