Final answer:
A gene that is necessary for an organism's survival is called an essential gene. Constitutive genes are a subset of essential genes, providing consistently needed products for cellular function.
Step-by-step explanation:
A gene that codes for a product that is necessary for the survival of an organism is called a essential gene. These genes play a crucial role in maintaining the basic cellular functions critical for an organism's survival. Constitutive genes, also known as housekeeping genes, are a type of essential genes that produce products that are constantly present within the cell, maintaining the essential functions and supporting survival.
Essential genes are distinct from inducible genes, which produce products as per the need and time, with an inducer molecule stimulating their expression. Moreover, in the context of cell cycle regulation, a gene that codes for a positive cell-cycle regulator is known as a proto-oncogene. When mutated, a proto-oncogene may become an oncogene, potentially leading to uncontrolled cell division.
It is also pertinent to understand the concept of lethality in genetics. A nonfunctional allele for an essential gene could be lethal if an individual is homozygous recessive for this allele, as the necessary gene product would not be produced, leading to fatality.