Final answer:
Genes may result in different developmental pathways due to variations in gene expression and environmental influences, leading to different phenotypes even among genetically identical individuals.
Step-by-step explanation:
Related genes may take very different developmental pathways due to changes in gene expression and the influence of environmental factors. While the actual genetic code may be almost identical, as in the case of mice and humans sharing a large percentage of their genes, the vast differences in physical form and function arise from the temporal and spatial expression of these genes during development.
This is further complicated by the epigenetic factors, which can lead to different expressions of the same genotype, resulting in diverse phenotypes even among genetically identical individuals, such as identical twins.