Under which condition(s) would genetic testing for predisposition to an inherited disorder in a minor child be considered reasonable?

Genetic predisposition to an inherited disorder an increased likelihood of developing the particular disease based on a person's genetics.
Genetic testing for predisposition to an inherited disorder in a minor child would be considered reasonable when the risk is high and prophylaxis to reduce disorder severity is available. 

The basic building blocks of dna and rna are nucleotides