Autism spectrum disorder (ASD) is a complex developmental condition that involves  persistent challenges  in social interaction, speech and nonverbal communication, and restricted/repetitive behaviors.

Down syndrome (sometimes called Down’s syndrome) is a condition in which a child is born with an extra copy of their 21st chromosome — hence its other name, trisomy 21. This causes physical and mental developmental delays and disabilities

These are very different illnesses that affect the mind but they have similar effect in parents