Final answer:
Parents can pass on the dominant lethal allele for Huntington's disease without realizing it as the disease often doesn't present symptoms until middle age, after they have had children.
Step-by-step explanation:
Parents can unknowingly pass on a dominant lethal allele for Huntington disease to their offspring because the symptoms typically do not appear until after the onset of reproductive age, sometimes as late as middle age.
Huntington's disease is a prime example of a condition caused by a dominant lethal allele that doesn't manifest until adulthood. The faulty gene causes irreversible nerve cell degeneration with symptoms appearing usually after the individual has had the chance to reproduce, thus passing on the allele.
This late onset means that individuals may not be aware they carry this dominant allele when they decide to have children. In Huntington's disease, someone with one affected allele (Hh) is likely to develop the condition eventually, later in life. However, by that time, they may have already passed on the allele to their children, with each child having a 50 percent chance of inheriting the disease.