Students know why alleles that are lethal in a homozygous individual may be carried in a heterozygote and thus

Answer 1

This is one of the science standards for biology.

It means that lethal forms of the gene can stay in the gene pool even though it kills individuals that are unlucky enough to get two copies of the lethal form of the gene.

Any individual with one of the lethal alleles and one of the nonlethal forms or alleles would survive. This surviving individual could pass the lethal allele to the next generation.

It seems like a lethal gene combination would get weeded out of the population and out of the gene pool as the affected offspring die. But since the offspring have to be homozygous in ordered to be affected, the offspring with only one copy of the lethal form keep the allele in the population.