Scientific soul?
We all vibrate continually, each giving off unique pulses of energy in the form of electricity, magnetism and radio frequency. Proof of this is seen and measured with scientific instruments by the scientist or via feelings and psychic talent in the spiritually minded. we are aware of this too by the way we are "drawn" to some people and have feelings of fear/foreboding when we meet others. Imagine this...

"Blood is thicker than water";due to similar dna and/or natural vibrations , we are more likely to easily feel the emotion of a family member than a stranger. Our dna patterns and vibrations may be more closely similar to such people. As the physical body becomes weaker and less able to function, a vibration/feeling is triggered in the dying person which is connected to a body of similar vibrations or feelings.(Perhaps a child/relative has a feeling that today the person will die)
Since memories/trace paths in memory channels and the like are deteriorating it is likely that a new developing energy will have the same kind of weakened or less functioning waves. Perhaps the dying person and a newly developing energy would be seen as baby and grandpa? Let's continue using Grandpa and baby as examples.

As further triggers in the "grandpa" give him the feeling that he knows he is going to die soon, a trigger may also be going on in "baby" using "mum" as the agent that something new is happening within but also that something strange is happening outside. A new emotional state caused or being the cause of a soul reaching out for a new home or electromagnetic and radio pulses being sent and received by server and user. Either way the long and short of it is the same, Grandpa in his waining state is triggering a recieving body . Baby is receiving vibrations and pulses and will indeed receive "soul" charecteristics or scientifically "wave patterns" at a time of development, growth and change.

He's been on this earth before; Often the words to describe the baby who has mannerism, talent, emotional needs of someone known. Perhaps if the whole process could take place at a time when language function sections of the brain are complete, then we would be able to communicate and remember our past lives without a problem?
Unfortunately the time span of emotional triggers etc does not incorporate this. Imagine your life with no language- of any sort- FROM DAY ONE-how much of your life would you know/remember? Emotions/feelings,instinct, wordless spiritual aspects would remain just as in a newly born baby who people are "sure he has been on this earth before". Perhaps he knows but cannot tell you without relearning with language all that he was taught with language unless he has a psychic connection with an understanding agent?

ps:grandpa and baby are an example but not to be taken that only a birth gramps and baby can connect.

So your saying a newborn baby can share a few traits with his grandparents?

Uncanny.

However I suspect that the true method of passing them in is through the families genes, which is a comparitivley simple and well tested method of passing on traits...rather than these unexplained "vibration/feelings". I like how you even suggested the source of these could be DNA and that to get from grandparent to newborn they' have to go through "Mum"( or Dad of course!).

Phlogiston.

the energy vibrations are simply tested and shown- electricity, radio wave,magnetic fields, all tried and tested. I am not saying a direct blood link has to be the only source of transferring energies. If you are a spiritualist the wording is different to the scientist, I am just trying to show people here on this site to trust the bond between science and what is seemingly paranormal. (i personally believe little to be para normal since we have the natural resources within, to project what appears paranormal to the untrained)

ps:my note at the bottom does say I AM USING GRANDPA AND BABY AS AND EXAMPLE , I am not speaking of family genes in this theory

With the ONLY dna thing, why do i have no trait at all connected to my birth parents? thats a tricky one

Maybe your adopted
You must share a few traits with them, like two eyes, two legs, skin etc. Thats passed on in your genes too.
Not all traits are visible...and some can remain dormant thru a few generations. For example: one of your grandparent has say, red hair, the rest are blonde. Your parents might both be blond but will still carry the genes for red hair which then might get passed on to junior.

Its fairly complicated( things that are true usually are!) and tricky to explain and to be honest its a while since I learnt about the basics, but if you go to the library you should be able to find some very big books full of information about it. The library is the big building with all the knowledge in it, worked out by other people so all you had to do was read the book!