A dominant lethal trait is a trait that is expressed if present in the genome and therefore precludes having descendants. All such cases must necessarily be sporadic and must represent new mutations, not inherited ones (because a potential parent with the trait would die before being able to pass it along).
A mendelian trait is one that segregates in accordance with the laws of genetics set forth by Gregor Mendel. Sickle cell trait is a mendelian trait. It refers to the situation in which a person has one copy (and one copy only) of the gene for sickle cell but does not have sickle cell disease (which requires two copies of the sickle cell gene). If two people with sickle cell trait have children together, each of their children has a one in four chance (25%) of having sickle cell disease.
A nonpenetrant trait is a genetic trait that is in the genome but does not manifest itself in the individual. By contrast, a penetrant trait is one that manifests itself. If, for example, if 100 people have a particular genetic trait but only 80 of them express it, the penetrance of that trait is 80%.