Lethal alleles are gene mutations that interrupt the normal functioning of vital cellular processes. These mutations can cause severe abnormalities in an organism, resulting in death or a lethal phenotype. These alleles are usually recessive, and their effects are only visible when an organism inherits two copies of the mutation.



When an organism inherits two copies of a lethal allele, it is considered homozygous for that allele. This means that the organism has two identical copies of the same mutation, which can have different effects on its phenotype.



The expression of a lethal allele's phenotype depends on the severity of the mutation and the stage of the organism's development. In some cases, homozygous individuals may not express the lethal phenotype, and the mutation's effects may be masked or compensated for by other genes. However, in most cases, homozygous individuals will express the lethal phenotype, resulting in death.



Homozygosity for a lethal allele can result in severe abnormalities in the organism's body structure, physiology, or metabolism. These abnormalities can affect the organism's ability to survive and reproduce, leading to its death or inability to pass on its genes to the next generation.



In conclusion, homozygosity for a lethal allele can have severe consequences for an organism's survival and reproduction. While the expression of the phenotype may vary depending on the mutation's severity and timing, most homozygous individuals will exhibit severe abnormalities that lead to death. Therefore, it is essential to understand lethal alleles' effects to prevent the inheritance of these mutations in human populations.

It's option (1) because lethal alleles are fatal in homozygous condition, lethal alleles doesn't even cause death in heterozygous indvidual unless it's dominant.