What DNA is and how it's shaped
DNA is the instruction manual for building and running a living thing, written in a code just four letters long: A, T, G and C. Those letters are chemical bases, and they pair under a strict rule — A always pairs with T, G always pairs with C. That pairing rule is what lets DNA copy itself accurately.
Structurally, DNA is a double helix: two strands twisted like a ladder, where the rails are sugar-phosphate backbones and every rung is a base pair. A gene is a stretch of that ladder spelling instructions for one protein, and proteins do the work that creates traits — from eye color to whether you can taste certain bitter foods.
- Four bases: A, T, G, C — A pairs with T, G pairs with C
- Double helix: two twisted strands, base pairs as the rungs
- Gene: a stretch of DNA with instructions for one protein
- Proteins carry out the instructions — they create the traits you see
From genotype to phenotype
Your genotype is the alleles you actually carry; your phenotype is what shows up on the outside — brown eyes, dimples, attached earlobes. You carry two copies of most genes, one from each parent, and each copy is called an allele. The combination of alleles is what a Punnett square tracks.
Alleles can be dominant or recessive. A dominant allele, written as a capital like T for tall, shows its effect even with only one copy. A recessive allele, lowercase t, only shows when both copies are recessive. That's why carriers of a recessive condition often show no sign of it — one healthy dominant allele is enough to mask it.
- Genotype = the alleles you carry (Tt); phenotype = what you show (tall)
- Homozygous = two matching alleles (TT or tt); heterozygous = one of each (Tt)
- Dominant allele: shows its effect with just one copy
- Recessive allele: only visible with two copies
Tt means heterozygous: one dominant tall allele, one recessive short one. The phenotype is tall, because T dominates. Genotype = Tt, phenotype = tall. Writing both words out every single time builds the exact habit tests look for.
Punnett squares: predicting offspring odds
A Punnett square is just a grid that shuffles two parents' alleles to show every possible child. Each parent randomly gives one allele per gene. Put one parent's two alleles across the top and the other's down the side, then fill in the four boxes — each box is an equally likely offspring genotype.
The classic cross is Tt x Tt. The four boxes come out TT, Tt, Tt and tt — so 3 out of 4 (75%) show the dominant trait and 1 out of 4 (25%) shows the recessive. Remember: Punnett squares show probability, not promises. Two Tt parents can absolutely have two tall kids in a row.
- Each box = one equally likely offspring genotype
- Tt x Tt gives a 3:1 phenotype ratio (75% dominant, 25% recessive)
- Tt x tt gives 50/50 — half show the dominant trait, half the recessive
- Ratios are probabilities, not guarantees about real children
Parent 1 can give B or b; parent 2 can only give b. Boxes: Bb, Bb, bb, bb. Genotype ratio: 50% Bb, 50% bb. Phenotype: 50% brown eyes, 50% blue. Quick check — count how many boxes contain at least one capital B, and that count over four is your dominant fraction.
Beyond dominant and recessive
Not every gene follows the simple dominant-recessive script. In incomplete dominance, the two alleles blend — a red flower crossed with a white one gives pink. In codominance, both alleles show fully, like a roan cow with both red and white hairs, or human AB blood type.
Most real human traits — height, skin color, and many disease risks — are polygenic, meaning many genes contribute small effects, which is why people come in continuous ranges rather than two types. For class, master the simple Punnett square first; every fancier variation uses the exact same grid mechanics.
- Incomplete dominance: alleles blend (red + white = pink)
- Codominance: both alleles show fully (AB blood type)
- Polygenic traits: many genes add up (height, skin color)
- All the variations still use the same Punnett square grid
Key concepts to memorize
🎯 Study tips for this topic
- Memorize base pairing as two pairs: A-T and G-C. Quiz it in both directions.
- Always write genotype AND phenotype for every answer — most lost points come from mixing them up.
- Draw every Punnett square, even when you could do it in your head; the grid prevents careless errors.
- Attach vocabulary to one running example (like Tt tall peas) and reuse it all year.
- Say the ratio out loud when you finish a square: 'three dominant, one recessive' — patterns stick when spoken.
