Ethics Of Genetic Engineering

It is a sad fact of life that parents can pass on genetic defaults such as cystic fibrosis, cancer, diabetes and Alzheimer's to their children.
Since science managed to map the human genome in 2000, the idea of genetic engineering has been raised numerous times but the ethics of tinkering with the make-up of a foetus is a minefield.
The ability to halt a child suffering from life threatening or life altering diseases should be a no-brainer and if we have the ability then yes, we should be doing it.
If a certain harmful genetic trait like a form of cancer or deafness can be switched off before birth then gene manipulation should be offered to the parents who undoubtedly would agree to it.
The pause for thought comes when you consider the reverse side of the argument that comes with gene manipulation, designer babies.
If it is just a case of simply 'turning off' certain genes to stop terible diseases and conditions developing, it is the same case for a scientist to turn on the gene that gives perfect hair, skin or choice of eye colour and if that was offered, not many parents would refuse.
Who would turn down the chance that their child could gain a gene upgrade and a small tweak could mean an advantage at school or playing sports or having acne free, perfectly smooth skin.
The ethics of genetic engineering are extraordinarily complex but it seems we can't open the door for one without also throwing it open for the other.
We could start down that road and it will then be impossible to stop and the future of humanity could be a split between those engineered to be healthier and smarter and those that aren't but then the flipside is, we also have the power to stop painful and fatal conditions and shouldn't that be the overriding factor, not how the knowledge could be abused?  
A moral dilemma that anyone with a debilitating disease would probably find easy to answer.