Mathematics - use probability, direct proportion and simple ratios

You can express the outcome of a genetic cross using probability, direct proportion or ratios.

Worked example 3

Two parents are both carriers of ophenylketonuria (PKU). This means that they are both heterozygous.

P = normal

p = phenylketonuria


Direct proportion

A quarter of the offspring would develop phenylketonuria (pp), a quarter would be completely normal (PP) and half would be carriers (Pp).


The probability of the offspring being PP is 25%.

The probability of the offspring being Pp is 50%.

The probability of the offspring being pp is 25%.


The ratio of the genotypes PP:Pp:pp is 1:2:1

It is important to remember during the process of fertilisation, the allele combinations created are a random process, and that is why probability is used - nothing is guaranteed.