Why Do Children Look Like Their Parents (But Not Exactly)?
Have you ever wondered why an elephant always gives birth only to a baby elephant — never to a giraffe or a lion? Or why a mango seed always grows into a mango plant? And yet, if you look closely at any two siblings, you'll find they are not identical — they differ in skin tone, eye colour, height, the way they smile.
Welcome to genetics — the branch of biology that answers two interconnected questions:
- Heredity — how do traits get passed from parent to offspring, so reliably that an elephant always makes elephants?
- Variation — why are offspring not exactly identical to their parents or to each other?
These two phenomena are opposite sides of the same coin. Heredity ensures species continuity; variation provides the raw material for evolution. This entire chapter is about understanding how heredity works at the level of genes, chromosomes, and DNA — what we today call the principles of inheritance.
[Big Picture] Before Mendel (1860s), people had a vague idea that "blood mixes" from parents — the so-called blending theory of inheritance. Mendel proved this wrong: traits are inherited as discrete units (we now call them genes) that stay intact across generations. This was the revolution that founded modern biology.
Heredity Before Mendel — Selective Breeding Through History
Long before genetics became a science, humans already practised it intuitively through selective breeding — deliberately choosing which plants or animals to mate based on desired traits.
Look at how radical the results have been:
- Plants: Wild teosinte (a grass) was transformed by selective breeding over ~9,000 years into modern maize / corn. Wild Brassica has been bred into cabbage, broccoli, cauliflower, kale, kohlrabi, and Brussels sprouts — all the same species.
- Animals: Wild aurochs (now extinct) were domesticated into modern cattle. In India, we have well-known native breeds like Sahiwal and Gir cows (Punjab and Gujarat), Murrah buffalo, Basmati rice, and Alphonso mango — each the result of generations of selective breeding.
This selective breeding worked, but our ancestors had no idea why. The traits seemed to "blend" sometimes (cross a tall and short plant, get a medium one — sometimes), and other times the offspring looked exactly like one parent.
The puzzle: What was being passed on, and how?
It took until 1856 for one man — an Austrian monk named Gregor Johann Mendel — to crack the code by running careful, quantitative experiments with garden pea plants in the gardens of an Augustinian monastery in Brno (then in the Austro-Hungarian Empire, today in the Czech Republic).
The Vocabulary You Need — Key Genetic Terms
Before we dive into Mendel's experiments, lock these 12 terms in your head. The rest of the chapter is built on them.
| Term | Definition | Example |
|---|---|---|
| Heredity | Transmission of traits from parents to offspring | Eye colour passed from mother to daughter |
| Variation | Differences in traits between individuals of a species | Two brothers with different heights |
| Gene | Unit of inheritance — a stretch of DNA coding for one trait | The gene for plant height |
| Allele | Alternative form of a gene | T (tall) and t (dwarf) are alleles of the height gene |
| Locus | Specific location of a gene on a chromosome | Pea height gene at a specific locus |
| Homozygous | Both alleles identical (TT or tt) | TT = homozygous tall (pure-breeding) |
| Heterozygous | Two different alleles (Tt) | Tt = heterozygous tall (hybrid) |
| Genotype | Genetic makeup (the alleles present) | Tt (with one tall + one dwarf allele) |
| Phenotype | Observable trait (the outcome) | "Tall" (what you see, regardless of TT or Tt) |
| Dominant | Allele that masks the other in heterozygous | T (tall) is dominant over t (dwarf) |
| Recessive | Allele masked in heterozygous; expressed only when homozygous | t (dwarf) expresses only as tt |
| F1 / F2 | First / Second filial generation (offspring of cross / offspring of self-pollinated F1) | TT × tt → F1 (Tt all tall) → F2 (3 tall : 1 dwarf) |
[NEET Trap] Many students confuse genotype and phenotype. A simple rule: Genotype = letters (Tt), Phenotype = what you see (Tall).
The cleanest way to picture this:
In a Tt × Tt cross, all three genotypes (TT, Tt, tt) exist, but only two phenotypes are observed (tall and dwarf) — because T is dominant over t.
Memory Capsule — Section 1
3 ideas to take away:
Genetics has two opposite jobs: heredity (continuity, parent → offspring) and variation (differences). Both essential — without heredity, species would dissolve; without variation, evolution would stop.
Selective breeding = "intuitive genetics" practised for millennia. The Indian examples to memorise: Sahiwal & Gir cows, Murrah buffalo, Basmati rice, Alphonso mango.
Vocabulary first, principles second. Master the 12-term table above — every subsequent section uses these terms. If you can't recall heterozygous in under 3 seconds, re-read the table.
The chapter's opening question (to keep in mind):
"What is being passed on, and how?"
This is the question Mendel's pea experiments (Section 2) and laws (Section 3) will answer.
[Cross-chapter link] Chapter 5 (Molecular Basis of Inheritance) will go inside the gene — DNA, replication, transcription, the molecular machinery. Chapter 4 (this one) stays at the outside — how traits track through generations. Both views are needed for a complete picture.
Solved Examples — Section 1
Q1. A plant with red flowers (RR) is crossed with a plant with white flowers (rr). What are the genotype and phenotype of the F1 generation?
Answer: Genotype Rr (heterozygous), phenotype red. Each offspring picks up one R from the red parent and one r from the white parent, and since R is dominant they all look red.
Q2. Define heterozygous. Give one genetic notation example.
Answer: Heterozygous means carrying two different alleles at the same locus on the two homologous chromosomes — for example Tt (one tall, one dwarf allele). Compare TT or tt, which are homozygous.
Q3. Name any three native Indian breeds/varieties developed through selective breeding.
Answer: Any three of: Sahiwal cow (Punjab), Murrah buffalo (Haryana), Basmati rice, Alphonso mango (Maharashtra), Gir cow (Gujarat). Each was shaped over generations of choosing the best individuals to breed.
Q4. A homozygous tall plant (TT) is crossed with a homozygous dwarf plant (tt). The F1 plants are then self-pollinated to produce F2. What are the genotypic and phenotypic ratios of the F2?
Answer: Genotypic ratio 1 TT : 2 Tt : 1 tt, phenotypic ratio 3 Tall : 1 Dwarf. The P cross gives all Tt; selfing Tt × Tt then reshuffles the alleles:
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Q5. Differentiate between genotype and phenotype with one example each.
Answer: Genotype is the allele combination (written in letters, e.g. Tt); phenotype is the observable trait you can see (e.g. tall). The catch is that the same phenotype can come from different genotypes — both TT and Tt look tall — which is exactly why a test cross is needed to tell them apart.