You can express the outcome of a genetic cross using probability, direct proportion or ratios.
Two parents are both carriers of ophenylketonuria (PKU). This means that they are both heterozygous.
P = normal
p = phenylketonuria
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.