Maths - Using probability, direct proportion and simple ratios

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

Worked example

aa
AAaAa
aaaaa

Direct proportion

Half the offspring have the combination Aa and half have aa.

OR

Out of four offspring two have the combination Aa and two have aa.

Probability

The probability of the offspring being Aa is 50%

The probability of the offspring being aa is 50%

Ratio

The ratio of Aa to aa is 1:1

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

Each of the four possible offspring combinations is as likely to happen during every fertilisation event, no matter how many events happen.