If both parents express a particular trait, but their child does not, what does this indicate about the trait?

This indicates that this is an autosomal recessive trait. An autosomal recessive trait is the one that both parents have but do not show but their child can show it. This is the same for either traits or diseases. Everyone has pairs of genes, one gene that cames from the mother one from the father. There is 25% chance that a child will be born with normal genes, 50% that the child will have be born with one normal and one abnormal gene, and 25% to have both abnormal genes.