Final answer:
The downstream effect of exposure to a morphogen is D. Cellular differentiation, as morphogens direct the development of tissues and organs by forming gradients that lead to cell differentiation.
Step-by-step explanation:
The downstream effect of exposure to a morphogen is typically D. Cellular differentiation. Morphogens are substances that induce the differentiation of cells in a position-dependent manner, often resulting in various cell types forming at different positions within the developing tissue. This is because morphogens form gradients in the developmental space that provide positional information to the embryonic cells, leading to the appropriate developmental outcomes.
Exposure to environmental mutagens may damage DNA, which can potentially lead to issues such as cancer; however, this is not the typical downstream effect of morphogen exposure in developmental processes. When discussing morphogens, the focus is on how they guide the development of tissues and organs through the regulation of gene expression and cell differentiation patterns.
While mutations in signaling pathway genes like those coding for cyclins can lead to cancer by causing cells to divide uncontrollably, this outcome is associated with genetic alterations and not the normal function of morphogens in guiding cellular differentiation during development.