Look at a family photo. You have your mother's eyes, perhaps your father's chin. Those features did not appear by accident — they were copied and handed down through a set of chemical instructions called DNA, passed from parent to child.
DNA, genes and chromosomes: nested boxes
These three words are often muddled, so picture them as boxes inside boxes, like an instruction library:
- DNA is the molecule that carries the code — a long twisted ladder (the famous double helix).
- A gene is a short section of that DNA that codes for one characteristic, like eye colour. (One page of instructions.)
- A chromosome is DNA coiled up tightly for storage. (A whole book of pages.) They sit inside the nucleus of every cell — the library building.
So: DNA is the writing, a gene is a paragraph, a chromosome is a bound book.
Alleles: two versions of the same instruction
You have two copies of most genes — one from each parent. Different versions of the same gene are called alleles. For a gene we might label them B and b.
- A dominant allele (capital B) shows its effect if you have at least one copy.
- A recessive allele (small b) only shows if you have two copies (bb) — because with just one, the dominant allele overrides it.
| Alleles you have | Which trait shows? |
|---|---|
| BB | Dominant |
| Bb | Dominant (the B wins) |
| bb | Recessive |
Think of dominant as a loud voice and recessive as a whisper: if even one loud voice is in the room, that is all you hear.
Punnett squares: predicting the children
A Punnett square is a simple grid that predicts the possible offspring of a cross. You write one parent's alleles across the top, the other parent's down the side, then fill each box by combining them.
Memory hook: Dominant Doesn't need a partner; recessive needs a replica (two copies) to show. And a Punnett square is just a game of "combine one from the top with one from the side".