Genes are fundamental units of heredity, carrying instructions for an organism's traits. However, some genes can have detrimental effects, leading to conditions classified as lethal or sublethal.

Lethal genes are alleles that, when expressed, cause the death of an organism. The effect can manifest at various stages of development, from embryonic to adulthood. Most lethal genes are recessive, meaning an individual must inherit two copies (homozygous recessive) to express the lethal phenotype. Heterozygous carriers typically survive and may even be phenotypically normal, acting as carriers of the trait. A classic example is Tay-Sachs disease in humans, where homozygous recessive individuals lack an enzyme necessary for lipid metabolism, leading to neurodegeneration and early death. Dominant lethal genes are rarer, as they would typically be eliminated from the population quickly unless their expression occurs late in life, after reproduction, such as in Huntington's disease.

Sublethal genes, also known as detrimental or deleterious genes, do not directly cause death but significantly reduce an organism's viability, fertility, or overall fitness. Individuals carrying sublethal genes may experience reduced lifespan, impaired development, increased susceptibility to diseases, or reproductive difficulties. Unlike lethal genes, their impact is not absolute mortality but a reduction in the organism's ability to survive and reproduce effectively. For instance, certain genetic conditions might lead to chronic health issues or reduced fertility, making it harder for affected individuals to thrive or pass on their genes. Both lethal and sublethal genes play a significant role in population genetics, influencing allele frequencies and contributing to natural selection by removing or reducing the reproductive success of individuals carrying these disadvantageous traits.