If two individuals who are heterozygous for a single trait have children, the expected phenotypic ratio of possible offspring is 3:1. This means that for every three offspring that show the dominant trait, one offspring will show the recessive trait.

This can be illustrated using a Punnett square. Let's say that the dominant trait is represented by the letter "A" and the recessive trait is represented by the letter "a". If the two parents are both heterozygous (Aa), then the possible offspring genotypes are AA, Aa, aA, and aa.

The AA and Aa offspring will both show the dominant trait, while the aA and aa offspring will both show the recessive trait. Therefore, the expected phenotypic ratio is 3:1.