Final answer:
The expression of a gene is determined by environmental conditions, such as sunlight and food availability. Transcription factors also play a role in regulating gene expression, and multiple genes can interact to determine a trait.
Step-by-step explanation:
The expression of a given gene is determined by various factors, with the most significant factor being the environmental conditions. Although an individual's genotype remains constant, the expression of genes can be influenced by environmental factors such as sunlight, food availability, and temperature. These factors can affect the phenotype, or observable characteristics, of an organism.
Additionally, the activity of transcription factors can regulate differential gene expression in cells. These factors bind to specific DNA sequences and control the transcription of genes, leading to the development of different cell products and functions.
Furthermore, genes are not the sole determinants of population variation. Phenotypes can also be influenced by epistasis, which is the interaction of multiple genes in determining a trait. For example, eye color in humans is influenced by at least eight genes acting in unison.