My brother has something passed down from his genes. We’ve been told that by a specialist but neither of my parents, or

Question: My brother has something passed down from his genes. We've been told that by a specialist but neither of my parents, or ancestors, had it. I don't understand. How can this be?
- Keywords:
  family,
  gene,
  inheritance
Asked by Helena Potter to Gaia on 9 Mar 2016.
- Gaia Andreoletti answered on 9 Mar 2016:
  
  Hi Helena,
  
  I am sorry to hear about your brother.
  It happens that in some cases disease results from a new (de novo) accidental mutation that occurs during the formation of egg or sperm cells or early in embryonic development. In these cases, the child’s parents and other family members are unaffected, but the child may pass on the condition to his or her own children.
  
  I think the specialist will help you to better understand the situation.
  I hope you’ll find this answer useful,
  Gaia

Question: My brother has something passed down from his genes. We've been told that by a specialist but neither of my parents, or ancestors, had it. I don't understand. How can this be?