Final answer:
Dominant lethal alleles result in death in both homozygous and heterozygous individuals, with examples like Huntington's disease, while recessive lethal alleles cause death only in homozygous recessive individuals, such as in Tay-Sachs disease.
Step-by-step explanation:
A dominant lethal allele is one that results in the death of an organism regardless of whether the organism is heterozygous or homozygous for that allele. One example of a dominant lethal condition in humans is Huntington's disease, where symptomatic individuals carry one copy of the lethal allele, but symptoms only appear in adulthood, often after they have had children, thereby passing on the allele to the next generation despite its lethality.
On the other hand, recessive lethal alleles cause death only when an individual is homozygous recessive for that allele. An individual heterozygous for a recessive lethal allele typically does not show any signs of the lethal phenotype and can unknowingly pass the allele to their offspring. For example, when two carriers of the Tay-Sachs disease allele have a child, if the child inherits two copies of the lethal allele, the disorder manifests and is usually fatal before age 5.