Every living thing — from a bacterium to a blue whale — carries a set of molecular instructions that tell its cells what to build and when. That instruction manual is written in DNA, organised into genes, and passed down through heredity. This is the story of how it all works.
Genetics is the branch of biology that studies genes, heredity, and the variation of inherited traits. It explains why you might have your mother's eyes, your grandfather's height, or a trait that skipped a generation entirely. At its core, genetics answers one big question: how do parents pass biological information to their offspring?
The molecule
A long, double-stranded chemical that stores the instructions for building and running a living thing.
The instruction
A specific section of DNA that codes for one product — usually a protein that affects a trait.
The handover
The process by which genes are passed from parents to offspring, creating family resemblance.
DNA: The Molecule of Life
DEOXYRIBONUCLEIC ACIDDNA stands for deoxyribonucleic acid. It's a long, ladder-like molecule made of repeating units called nucleotides. Each nucleotide has three parts: a sugar (deoxyribose), a phosphate group, and one of four nitrogen-containing bases — adenine (A), thymine (T), cytosine (C), or guanine (G).
The famous double helix shape comes from two strands of DNA winding around each other. The bases pair up in a strict way: A always pairs with T, and C always pairs with G. This complementary pairing is what allows DNA to be copied accurately every time a cell divides.
What DNA actually does
- Stores information: the sequence of bases along a DNA strand spells out instructions, like letters in a book.
- Copies itself: before a cell divides, DNA unzips and each strand serves as a template to build a new partner strand.
- Makes proteins: through transcription and translation, DNA's instructions are used to build proteins that carry out almost every function in your body.
If you stretched out all the DNA in one of your cells, it would be about 2 metres long. Packed into a nucleus just 6 micrometres wide — roughly 300,000 times shorter than the DNA it contains.
Genes: The Instructions for Traits
SEGMENTS OF DNAA gene is a specific segment of DNA that contains the instructions for making a particular product — usually a protein. Genes are the basic units of heredity. Humans have roughly 20,000 to 25,000 genes, and each one plays a role in how your body develops and functions.
Not all of your DNA is made of genes. In fact, only about 1–2% of human DNA actually codes for proteins. The rest helps regulate when and where genes are turned on, or serves structural roles we're still learning about.
Alleles: different versions of the same gene
Most genes come in multiple versions called alleles. For example, the gene for eye colour has several alleles that produce different shades. You inherit one allele from each parent, so you carry two copies of most genes — one from your mother, one from your father.
The combination of alleles you carry is your genotype. The trait that actually shows up — blue eyes, brown eyes, detached earlobes — is your phenotype.
| Term | Meaning | Example |
|---|---|---|
| Gene | A section of DNA that codes for a trait | The gene for eye colour |
| Allele | A specific version of a gene | The "blue eye" allele |
| Genotype | The combination of alleles you carry | One brown allele + one blue allele |
| Phenotype | The trait that actually appears | Brown eyes (because brown is dominant) |
Chromosomes: How DNA Is Packed
ORGANISED DNAIf you unravelled all the DNA in a human cell and laid it end to end, it would be about 2 metres long. To fit inside a nucleus that's only a few micrometres across, DNA is tightly wound around proteins called histones and then coiled into structures called chromosomes.
Humans have 46 chromosomes in most cells — 23 pairs. One chromosome in each pair comes from your mother, and the other from your father. These chromosomes carry all your genes, and the specific arrangement is what makes each person genetically unique.
- Autosomes: the first 22 pairs, which carry most of your genetic information.
- Sex chromosomes: the 23rd pair — XX in females, XY in males. These determine biological sex and also carry other genes.
- Homologous pairs: matching chromosomes from each parent that carry the same genes, though possibly different alleles.
Think of DNA as a huge library. Chromosomes are the bookshelves. Genes are the individual books. Alleles are the different editions of the same book. And the base sequence is the text inside each edition.
Heredity: Passing Traits to the Next Generation
FROM PARENT TO OFFSPRINGHeredity is the process by which traits are passed from parents to their offspring. In sexual reproduction, each parent contributes one set of chromosomes through specialised cells called gametes — sperm in males, eggs in females.
When a sperm and egg fuse during fertilisation, the resulting cell contains a full set of chromosomes — half from each parent. That single cell then divides and develops into a new individual, carrying a unique combination of both parents' genes.
Dominant and recessive alleles
How a trait appears often depends on whether the alleles are dominant or recessive. A dominant allele only needs one copy to show up in the phenotype. A recessive allele needs two copies — one from each parent — to be expressed.
- Homozygous: you carry two identical alleles for a gene (e.g., two brown-eye alleles).
- Heterozygous: you carry two different alleles (e.g., one brown, one blue).
- Carrier: someone who carries a recessive allele but doesn't show the trait — they can still pass it on.
Predicting inheritance with a Punnett square
A Punnett square is a simple grid that shows all the possible gene combinations a child could inherit from two parents. You write one parent's alleles across the top and the other parent's down the side, then fill in the boxes to see the possible genotypes of their offspring.
| Parent 2 → Parent 1 ↓ | B (brown) | b (blue) |
|---|---|---|
| B (brown) | BB (brown) | Bb (brown) |
| b (blue) | Bb (brown) | bb (blue) |
In this example, two brown-eyed parents (both carriers of the blue allele) have a 1 in 4 chance of having a blue-eyed child. This is why a recessive trait can seem to "skip" a generation.
Mutations and Genetic Variation
CHANGES IN THE CODEA mutation is a change in the DNA sequence. Mutations can happen spontaneously when DNA is copied, or they can be caused by environmental factors like radiation or certain chemicals. Most mutations are harmless, some are beneficial, and a few can cause genetic disorders.
Mutations are the raw material for evolution. Without them, every generation would be genetically identical, and populations couldn't adapt to changing environments.
- Substitution: one base is swapped for another — like changing a single letter in a word.
- Insertion: extra bases are added, which can shift how the rest of the sequence is read.
- Deletion: bases are removed, often disrupting the gene's instructions.
Beyond Simple Inheritance
Not every trait follows the simple dominant–recessive pattern. Many traits are polygenic, meaning they're controlled by multiple genes working together. Height, skin colour, and eye colour are all influenced by several genes plus environmental factors.
| Pattern | How it works | Example |
|---|---|---|
| Simple dominance | One dominant allele masks the recessive one | Free vs attached earlobes |
| Incomplete dominance | Heterozygous shows a blend of both traits | Pink flowers from red and white parents |
| Codominance | Both alleles show up fully | AB blood type |
| Polygenic | Many genes add up to one trait | Height, skin colour |
Genetics gives you the blueprint, but the environment helps write the final draft. Nutrition, lifestyle, and chance all shape how your genes are expressed.
Why Genetics Matters
Understanding genetics isn't just academic — it has real-world impact across medicine, agriculture, and conservation.
- Medicine: genetic testing can identify risk for inherited conditions like cystic fibrosis or Huntington's disease.
- Agriculture: selective breeding and genetic modification improve crop yields and disease resistance.
- Forensics: DNA fingerprinting is used to solve crimes and identify remains.
- Conservation: genetic diversity helps endangered species survive environmental changes.