"...The sire more of the common type."
Do u mean the sire has sloped back as in a showline dog?
Ur observation is in line with my guess.
If we relate with Mendel's pea plant expt, say R =roached back
Suppose the sire is SS and dam RR, then a pup will be of the type SR (am i correct)?
The phenotype of SR is likely to be moderately roached ?
I think the case
"... The mother has a very moderate structure and top line. The sire more of the common type..."
refers to SR bred with RR. The a pup has 3/4 chance of being SR (moderately roached) and 1/4 chance of RR Roached back type.
This perhaps explains the observation
"...Some of the pups have his top line and some were like mom..."
Of course I am assuming in SR , some amount (even if very mild) of roach will be present in the phenotype.
I tested a show line litter yesterday. The mother has a very moderate structure and top line. The sire more of the common type. Some of the pups have his top line and some were like mom. One of the puppies was very nice at testing, but has her dad's top line. I kept telling the breeder, love her temperament, hate her top line.
Anyhow, breeding a more moderate dog can give one some more moderate pups.