what is allelic sequence variation?
Can you please give a direct quote? I am pretty sure your textbook did not say exactly what you said!


  When DNA polymorphism occurs mutations can take place.


In genetics, polymorphism just refer to the existence of genetic variation among individuals in a population (often referring to a specific locus).

Mutations is the ultimate source of polymorphism. It is not polymorphism that allow mutations to occur.


  If these mutations occur in the germ cell and do not destroy its capability to reproduce, it is said it might move on to the next generation.


Indeed!


  My book says it might lead to allele sequence variation. 


'allele sequence variation' is just some kind of another generic term for polymorphism. For this expression to make sense to you, you should start by looking up the definition of allele