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You can express the outcome of a genetic cross using:

• direct proportion
• probability
• ratios

Worked example

Two parents are both carriers of phenylketonuria (PKU). This means that they are both heterozygous ie each has one normal allele (P) and one phenylketonuria allele (p).

P = normal allele

p = phenylketonuria allele

Direct proportion

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

Probability

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%.

Ratio

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.