Regarding mutations within your own well-researched, well-known, and otherwise proven lineage, it is possible for two distant cousins to exactly match on all markers while two brothers might not match exactly. This is the effect of the random nature of the mutations. We do not know when they will occur but we know that they must occur sometime. And in that generation, when the mutation occurs, the son will have a one step change from the father and a one step change from any brothers.

Because of the random nature of mutations we must use statistics and probability to understand the expected mutations in any given lineage or family of lineages. The understood average rate of mutation is .002 (or 1/500). This assumes that any given marker has a .002 chance of mutating with each birth of a son (DNA transmission). In other words, we could expect any specific marker to mutate once in 500 generations but since we test 25 markers we have 25 times the chances of a mutation. Without fully understanding this, participants may jump to the conclusion that their 24/25 mismatch to their own well-researched, well-known and otherwise proven lineage could only happen once in 500 generations. This is grossly wrong, because the mutation rate is applicable to a count of markers, and DNA transmissions, NOT the participant's DNA matched or mismatched total results. By example, if a 25 Marker participant is the seventh generation (six DNA transmissions) from their earliest known ancestor, there would be 0.3 expected mutations in the 6 births in the line from the MRCA (most recent common ancestor) to the participant. Also by example, if the same 25 Marker participant and four other same-generation descendants from four other sons of the MRCA participate in the 25 Marker test, there would be 1.5 expected mutations in this expanded test of the family.

So, the reality is that several mutations must be expected in families that have more than 20 DNA transmissions and use the 25 marker test. Cases have even been noted to have two mutations in one line and none in four other lines within the same family. The time to become concerned is when you find 3 or more mutations and you have 6 or fewer DNA transmissions in the line.