What Causes Abnormalties In A Baby?

hi my name is salah i have a son who is 14months old he cannot sit or stand or even grasp he does not have s social smile yet he was diagnosed with infantile spasm he is not seizure free yet and it was associated with hypotonia. however we have done so many tests like lysoenzyme test which came out to be normal as well as other tests. we r in the process of doing whole exome gene test as a final test(since me and my wife r first cousins) we have done two mri i test one was on 15th may 2013 which most doctors said it was normal except for the corpus collusm which was thin. and the latest which was done on 2nd of feb 2014 showed moderate diffuse cereberall atrophy. My question is why the baby came like that since we have four healthy daughters? thank u