As is the case with the mutated hemoglobin gene that causes sickle-cell anemia, certain dominant alleles are crucial to normal functioning (or development). Some are so vital that when the mutant recessive alleles are homozygous, the combination is lethal and death results before birth or early in life. However, such recessive alleles can be passed on by heterozygotes (Ll). In many cases, these are not phenotypically different from homozygous normals (LL). If two heterozygote parents mate (Ll x Ll), what is the probability that any of the children will be heterozygous?