mom and dad are both heterozygous for the harmful or deleterious recessive trait what is the probability of their offspring having the illness

There will be 25% chance of the offsprings getting the disease.

Explanation:

A recessive trait is a trait that is masked by a dominant trait. A dominant trait can be described as the one which dominates or masks the effects of a recessive trait.

Heterozygous means that the alleles of a gene are different whereas homozygous means that the alleles of a gene are the same.

According to the punnet square, a cross between heterozygous parents for a disease will result in 25% normal offsprings, 50% carrier offsprings and 25% offsprings with the disease.