Students should be able to use direct proportion and simple ratios to…

The idea

In a monohybrid cross, the Punnett square gives the relative numbers of each genotype or phenotype. A cross between two heterozygotes, Aa × Aa, gives genotype ratio 1 AA : 2 Aa : 1 aa and phenotype ratio 3 dominant : 1 recessive. Direct proportion means comparing one part with the whole, such as 3 out of 4, or 0.75, or 75%. Simple ratios compare parts with each other, such as 3:1. You should be able to convert between these forms and to scale them to a predicted number of offspring, for example 3 out of every 4, or 75 out of 100. Always state what each number represents and keep the ratio in its simplest whole-number form.

What earns the marks

• A genetic cross outcome can be expressed as a ratio comparing genotypes or phenotypes.
• Direct proportion compares a part with the whole, such as 3 out of 4 or 75%.
• A 3:1 phenotype ratio comes from a cross between two heterozygous parents.
• Ratios should be simplified to whole numbers where possible.
• Predicted numbers can be scaled using proportion, for example 3 in every 4 offspring.
• The ratio must be linked clearly to the genotypes or phenotypes it describes.

Common mistakes and how to put them right

• Writing the ratio the wrong way round.

  Put it right by checking which phenotype or genotype each number represents.

• Confusing ratio with proportion.

  Put it right by using ratio for part-to-part comparison and proportion for part-to-whole comparison.

• Leaving a ratio unsimplified, such as 2:2.

  Put it right by simplifying it to 1:1.

Exam tips

• Label each part of the ratio with the genotype or phenotype it represents.
• Convert between ratio and proportion when a question asks for a fraction, decimal or percentage.
• Scale the ratio to the number of offspring stated in the question before giving a predicted count.