Final answer:
An essential gene codes for products crucial for an organism's survival, covering functions like cellular respiration or DNA replication. Constitutive genes are a type of essential gene that are constantly needed, and nonfunctional alleles of these genes can cause lethality.
Step-by-step explanation:
A gene that codes for a product that is necessary for the survival of an organism is called an essential gene. These genes are integral to an organism's ability to live and reproduce, as they code for vital cellular functions and processes. Essential genes may include constitutive genes, which produce products continuously needed by the cell, and can sometimes be involved in adaptations that increase an organism's fitness in a specific environment.
Examples of essential genes are those involved in the basic cellular functions that support life, such as genes coding for enzymes involved in cellular respiration or DNA replication. A nonfunctional allele of an essential gene can result in lethality if present in a homozygous state, since it would fail to produce the necessary gene product. This reflects the critical role of essential genes in an organism's survival and emphasizes the importance of their presence, regulation, and expression within living cells.