SPD and Weak Muscle Tone

My 4 year old daughter has been receiving OT, PT and ST since she was three month old. At that point, she was unable to nurse and lagged behind basic infant development. As time went on, the therapists that came to my house through EI claimed that she had low muscle tone and that causes her delays. Her pediatrician said that her muscles do feel "interesting and too soft" and also blamed her weakness to her delays.

The past September, my daughter started therapy sessions at a center with top therapists and a state-of-the-art activity center. After a few sessions, her therapist advised me that my daughter is having a real severe case of SPD. I was shocked. And suddenly a new world opened to me. I read up a lot on your website and guess what?! All my doubts were answered. My daughter is the exact image of a child with gravitational sensory disorder, postural instability and a little more of oral and tactile sensory processing disorder.

I am not sure, though, what is her core problem. How are SPD and low muscle tone related? And also, is it hereditary? Now that I read a lot about SPD, I know that her dad might have had a very mild form of SPD. Could it be inherited in such a severe form?