Calculate and analyse outcomes (using probabilities, ratios and…

The idea

A monohybrid cross follows one gene with two alleles, one dominant and one recessive. Set out the parents' genotypes, put each parent's gametes along the top and side of a grid, and combine them to give offspring genotypes. From the grid you can calculate outcomes three ways: as a probability (for example 1/4), as a ratio (for example 3:1 dominant to recessive) and as a percentage (for example 75%). Pedigree diagrams show the same inheritance across a family: squares are males, circles are females, shaded shapes show the trait. A recessive trait can appear in a child of two unaffected parents only if both parents are carriers (heterozygous). Use the pattern of affected individuals to deduce genotypes, then calculate the chance of the next child showing the trait.

What earns the marks

• Identify parental genotypes and the gametes each produces for a single gene with dominant and recessive alleles.
• Complete a genetic diagram to obtain offspring genotypes and their proportions.
• Convert the same outcome between probability, ratio and percentage, for example 1/4, 1:3 and 25%.
• In a pedigree, use affected and unaffected individuals to deduce whether a trait is dominant or recessive and to assign genotypes.
• Recognise that two unaffected carrier parents can produce an affected child for a recessive trait, giving a 1 in 4 chance.

Common mistakes and how to put them right

• Mixing up the ratio order.

  Put it right: State which phenotype comes first, for example 3 dominant : 1 recessive.

• Treating a 25% probability as a guarantee that one in four children will be affected.

  Put it right: Correct to a chance per child, independent each time.

• Assuming an unaffected parent cannot carry a recessive allele.

  Put it right by identifying heterozygous carriers in the pedigree.

Exam tips

• Write gametes clearly along the grid edges before combining them to avoid errors.
• Give the answer in the form the question asks (probability, ratio or percentage) and show the conversion.
• For pedigree questions, trace the trait through generations and label known genotypes before calculating.