Final answer:
Genetic disorders can be passed down from parents to their children through mutations in the genetic material.
Step-by-step explanation:
Genetic disorders can be passed down from parents to their children because they are caused by mutations in the genetic material.
In the case of Parker and Jessica, Parker had a genetic disorder that affected how his muscles grew, while Jessica did not have this disorder.
Although Jessica did not have the disorder, she could still carry the gene for it. When Parker and Jessica had children, there was a 50% chance that each child would inherit the gene for the disorder. In this case, their daughter Catherine inherited the gene, while their son David did not.