How is it possible that one defect Gene can affect so much of your body?

Question: How is it possible that one defect Gene can affect so much of your body?

Asked by emjelled to Darren on 16 Jun 2010 in Categories: General.
- Darren Nesbeth answered on 16 Jun 2010:
  
  Two scenarios:
  
  1) Every last bit of your body is made of cells. Even the hair, teeth and toe nails.
  
  Every cell in your body has identical genes, inherited from an egg cell from your mum and a sperm cell from your dad. If one gene has a defect at the sperm/egg stage, all your cells inherit that defect. That’s why it will affect so much of your body.
  
  2) When you are healthy, lucky and your immune system is top notch then if one gene is damaged in one cell (by smoking, sunlight, or eating crisps) the cell will send a signal to your immune system saying ‘hello, I’m dodgy, please kill me now’. And your immune system will happily oblige and noone will be any the wiser. It happens all the time – but there’ no reason to know about it if you’re fit and healthy.

Question: How is it possible that one defect Gene can affect so much of your body?