Final answer:
Genes that are consistently expressed at constant levels regardless of environmental conditions are known as constitutive genes or housekeeping genes.
Step-by-step explanation:
The type of gene that is expressed at relatively constant levels in tissues regardless of variations in an animal's condition is known as a constitutive gene or housekeeping gene. These genes are essential for the basic functions of the cell and are thus always active, providing the necessary components for the cell's survival and maintenance. Unlike other genes that are turned on or off based on environmental conditions and the cell's specific needs, housekeeping genes are continuously expressed to ensure that the cell's genotype contributes to its phenotype consistently.