Ancestral DNA Percentages

You - 1 - 100%, Parents - 2 - 50%, Grandparents - 4 - 25%, Great-Grandparents - 8 - 12.5%,
2nd-Great-Grandparents - 16 - 6.25%, 3rd-Great-Grandparents - 32 - 3.12%, 4th-Great-Grandparents - 64 - 1.56%,
5th-Great-Grandparents - 128 - 0.78%.

There is a 99.6% chance that you will have DNA from all of your 16 2nd-Great-Grandparents, only a 54% chance of sharing DNA
with all 32 of your 3rd-Great-Grandparents, and a 0.01% chance for your 64 4th-Great-Grandparents.

Pedigree

Parents - 2, Grandparents - 4, Great-Grandparents - 8,
2nd-Great-Grandparents - 16, 3rd-Great-Grandparents - 32, 4th-Great-Grandparents - 64, 5th-Great-Grandparents - 128,
6th-Great-Grandparents - 256, 7th-Great-Grandparents - 512.

What Is a Cousin?

Cousins are people who share a common ancestor that is at least 2 generations away, such as a grandparent or great-grandparent. You and your siblings are not cousins because your parents are only 1 generation away from you.

What Is a Second Cousin?

The number associated with your cousin has to do with how many generations away your common ancestor is.

For example:
First cousins share a grandparent (2 generations)
Second cousins share a great-grandparent (3 generations)
Third cousins share a great-great-grandparent (4 generations)
Fourth cousins share a 3rd-great grandparent (5 generations)

To be “once removed” from a cousin means you are separated by one generation. The number before “removed” will always represent the number of generations you are separated (“removed”) from the cousin. (Source: Family Search).