The Passage of Genetic Information from Parent to Offspring — Study Notes

Two children in the same family can inherit totally different eye colours from the same parents. Nothing mysterious is going on — it comes down to which version of an instruction each child happened to receive, and O-Level examiners expect you to name that idea precisely.

Gene, Allele, and Why One Copy Can Beat Another

Picture your DNA as a long set of instructions, split into sections called chromosomes. A gene is a single instruction within that set — it sets the plan for one particular characteristic, like fur colour or petal shape. That instruction can come written in more than one way, and each written version is an allele. So "black fur" and "white fur" aren't separate genes; they're two alleles of one fur-colour gene.

Because every individual inherits two copies of a gene — one from each parent — the two alleles present can differ. When they do, one often wins out visibly: a dominant allele (shown with a capital letter) produces its effect even when only one copy is present, while a recessive allele (lowercase) only shows up when both copies match it. Pairing two identical alleles is homozygous; pairing two different ones is heterozygous. What you actually observe — the visible outcome — is the phenotype, while the underlying letter pair itself is the genotype.

Worked Example — Hamster Fur Colour

In hamsters, allele F (dark fur) is dominant over allele f (pale fur). A breeder crosses two hamsters and gets an offspring with genotype Ff.

1. Compare the two letters in the genotype: F and f are different symbols, so this hamster is heterozygous for the fur-colour gene.
2. Decide which allele controls the outcome: F is written as a capital, marking it dominant, so its instruction is followed whenever it's present — even alongside a lowercase partner.
3. Read off the phenotype: because the dominant F copy is present, the hamster's fur is dark, even though it also carries the recessive f.

Notice the pale allele hasn't vanished — it's simply masked this generation, and could reappear if this hamster later pairs with another f carrier.

Sex determination, codominance, and the reasoning behind every step above get the full audio walkthrough, the worksheet, and further worked examples in the full lesson below.