The terms gene, allele, dominant, recessive, homozygous, heterozygous, genotype and phenotype, and using Punnett squares to predict the ratios and probabilities of offspring in a monohybrid cross.

What this dot point is asking

CCEA wants you to define the key genetic terms, explain dominant and recessive alleles, distinguish genotype and phenotype, and use Punnett squares to predict the ratios and probabilities of offspring in a monohybrid cross.

The key genetic terms

Using a Punnett square

A Punnett square predicts the offspring of a cross. You write the alleles of each parent's gametes along the top and side, then fill in the combinations.

Ratios and probabilities

A cross of two heterozygotes (Tt x Tt) gives a genotype ratio of 1 TT : 2 Tt : 1 tt and a phenotype ratio of 3 dominant : 1 recessive. Remember that a ratio is the expected average over many offspring, while each individual birth is a separate probability.

Examples in context

Example 1. Why two brown-eyed parents can have a blue-eyed child

Brown eye colour (B) is dominant to blue (b). Two brown-eyed parents who are both heterozygous (Bb) can each pass on a b allele. A Punnett square of Bb x Bb gives a 1 in 4 chance of a bb child, who has blue eyes. This shows how a recessive characteristic can reappear when two carriers have children, a common CCEA question that tests the use of a Punnett square.

Example 2. Identifying a carrier

A carrier is heterozygous: they have one recessive allele but do not show the recessive phenotype because the dominant allele masks it. For example, a Bb mouse looks black but carries the brown allele and can pass it on. Test crosses (crossing with a homozygous recessive) can reveal whether a dominant-looking individual is homozygous or heterozygous, by looking at the ratio of offspring. Understanding carriers links monohybrid crosses to genetic disorders.

Example 3. Why a ratio is only an average

A Tt x Tt cross predicts a 3 to 1 ratio of tall to short plants, but this is the expected average over a large number of offspring, not a guarantee. If only four seeds grow, you might get four tall plants, or two tall and two short, purely by chance, in the same way that tossing a coin four times need not give exactly two heads. The more offspring there are, the closer the actual numbers get to the predicted 3 to 1. CCEA often tests this idea by giving real offspring numbers that do not match the ratio exactly and asking you to explain why, so always describe a genetic-cross ratio as the expected or most likely outcome.

Try this

Q1. What does heterozygous mean? [1 mark]

• Cue. Having two different alleles for a characteristic (such as Tt).

Q2. Cross Tt with tt. What ratio of tall to short offspring is expected (T tall, dominant)? [2 marks]

• Cue. 1 tall to 1 short (Tt and tt in equal numbers).