The boxes represent DNA markers. A genetic branching opportunity occurs with each one. But the timeline set down here is based solely on rough averages. For example, mitochondrial (MT-DNA) acquires a mutation about once every thousand years. But that doesn't mean three can't emerge in that time. It varies with each lineage.

Autosomes are robust for five to seven generations. But discovery depends on the extent of the completed genealogy. It's rare that identifiable branching can occur after seven generations or so. The size of the boxes diminish because the possibility that the known genealogy will coincide with matching DNA segments diminishes.

On the other hand, MT-DNA (maternal) and Y-DNA (paternal) clones itself every generation. A tester receives 100% from the respective parent. The Y, however, is not as metronomic as illustrated here. Mutations occur randomly and vary considerably in every lineage. Still, counted over hundreds of lineages, the law of averages makes it a good molecular clock.

Reminders

• Autosomes (chromosomes 1 through 22) come from each parent. Therefore, each person has two copies of each autosome in their cells — one from each person. This halving occurs every generation. Therefore, the maximum amount of autosomal DNA inherited from any one ancestor is determined by the number of generation they're removed. In other words, you might receive no more than 0.25% from an ancestor in the tenth generation. Even far less, even none, in some cases.
• The Y chromosome (Y-DNA), which carries the male sex gene, is passed in full down the father line, making it great for surname studies. And it's why only men can test it.
• The mother's fertilized egg is loaded with mitochondrial DNA. It comes to us straight through the mother line.