Question

who is most likely to lead to the birth of a child who has a lethal gene

Answer 1

One of both of the parents have the same gene.

Answer 2

If the gene is recessive, both parents must contribute the lethal gene.

If it is dominant, assuming that the lethality does not take effect until after the parents are young adults, only one parent needs to pass on the gene. Of course, if the lethality takes effect before they reproduce, neither can pass on the lethal gene.

I hope this helps a little more.