You can express the outcome of a genetic cross using probability, direct proportion or ratios.
Half the offspring have the combination Aa and half have aa.
Out of four offspring two have the combination Aa and two have aa.
The probability of the offspring being Aa is 50%
The probability of the offspring being aa is 50%
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.