Answer
The mutation is most likely to be dominant.
Work Step by Step
Around half of the offsprings that were born from an affected parent has the mutation. If it were to be recessive, it would be unlikely that the left parents that do not have the mutation can produce that many offspring that do not have the mutation (See explanation below).
A = dominant
a = recessive
Left side unaffected parents (If it was recessive):
A_ x A_
Possible combinations: Aa, AA or aa
However the pedigree shows that NONE of the offspring is affected and therefore this is unlikely to be a recessive linked mutation.
Left side unaffected parents (If it was dominant):
aa x aa
Possible offspring: aa ONLY
This combination is likely to appear as shown in the pedigree since all of their offspring are unaffected between these two unaffected parents.
