Final answer:
The expected phenotype ratio from a dihybrid cross is 9:3:3:1, attributed to independent assortment and dominance, but this may vary in real organisms due to biological factors like linkage and epistasis, or environmental influences.
Step-by-step explanation:
The expected ratio of phenotypes from a dihybrid cross is typically 9:3:3:1. This ratio comes from the assumption that genes assort independently and follows a pattern of dominance and recessiveness. For instance, in a dihybrid cross considering seed texture and color, you would expect three quarters of the F2 generation offspring to display the dominant phenotype for each trait (round and yellow seeds), leading to 9/16 of them being both round and yellow.
However, when observing real organisms like the shepherd's-purse plant, the ratio may differ due to various biological factors such as linkage and epistasis, which affect the independent assortment of alleles. Additionally, environmental factors might influence phenotype expression, leading to a deviation from the expected dihybrid cross ratio.