Secondly, they aren't white.. While some double merle dogs are almost pure white, they can go the complete range all the way up to a dog that you would think is a just a normal merle (most fall somewhere in the middle).
In between five and six months, your baby usually starts to sit up with your help, holding his head in an upright position, keeping the back straight. He may even be able to sit for a second or so without taking any assistance from you, before toppling down after a while [1] .
Moreover, if he does not start sitting up on his own even at 9 months of age then you must consult your physician immediately. Do keep in mind that premature babies may learn to sit up, as well as other developmental milestones, later than their peers.