3. Genetics & Heredity

Why do you resemble your parents? The answer is DNA — a molecule shaped like a twisted ladder (a 'double helix') that stores instructions in a four-letter code (the bases A, T, C, G). A segment of DNA that codes for a trait is a gene.

Key vocabulary:

• Allele — a version of a gene (for example, a 'brown eye' allele vs a 'blue eye' allele).
• Dominant (written capital, e.g. B) — shows up even if only one copy is present.
• Recessive (lowercase, b) — only shows when BOTH copies are recessive.
• Genotype — the actual alleles (BB, Bb, or bb). Phenotype — the visible trait.

A Punnett square predicts offspring. Crossing two Bb parents:

The result is a 3 : 1 ratio — about 3 offspring show the dominant trait for every 1 that shows the recessive trait. Notice this is exactly the probability skill from math: each box is one equally likely outcome.

⚠️ Common misconception: 'dominant' does not mean 'more common' or 'stronger'. It only means it needs just one copy to appear.

💡 Tip: an organism showing a recessive trait must be bb — two recessive copies, no other way.