Final answer:
In a dihybrid cross, the expected ratio of phenotypes is 9:3:3:1. The difference between expected ratios and the observed ratio in certain plants can be due to factors like incomplete dominance or gene interactions.
Step-by-step explanation:
The expected ratio of phenotypes from a dihybrid cross is 9:3:3:1. This means that for every 9 offspring, 3 will have both dominant traits, 3 will have one dominant and one recessive trait, 3 will have one recessive and one dominant trait, and 1 will have both recessive traits.
The difference between the expected dihybrid cross ratio and the ratio observed in the shepherd's-purse plant could be due to factors such as incomplete dominance, multiple alleles, or gene interactions that affect the expression of traits.