Final answer:
Loss-of-function (LoF) alleles can result in reduced or lost gene function and are sometimes lethal when present in homozygous form, depending on the essential nature of the gene involved.
Step-by-step explanation:
Loss-of-function (LoF) alleles are genetic variations that result in reduced or completely lost gene function. When such alleles are present in homozygous form, the individual has two copies of the nonfunctional allele. These individuals can sometimes live without any apparent effect, suggesting that some genes with LoF alleles are not absolutely essential for survival or have redundant functions in the genome. However, if the affected gene is essential, being homozygous for a LoF allele can be lethal, leading to developmental failure, miscarriage, or early death depending on when the gene's function is required. Examples of lethality patterns include recessive lethal inheritance, where the nonfunctional allele is lethal only when homozygous, and dominant lethal inheritance, such as seen in Huntington's disease, where the nonfunctional allele can be lethal even in heterozygotes, although the effect may not appear until after reproductive age.