Elspeth Gilchrist
Elspeth is a family historian with an actuary's fondness for tables. She writes the genealogy mathematics: cousins and removes, relatedness, pedigree collapse and how many years a generation really is.
Explainers by Elspeth
Second cousin once removed and twice removed, explained with a chart
A second cousin once removed is your parent's second cousin or your second cousin's child; twice removed adds one more generation. A chart and DNA figures.
AncestryDNA vs 23andMe for finding relatives: which match list does more
AncestryDNA vs 23andMe for finding relatives: database size, how matches and shared cM are shown, trees, segment tools, privacy and when to test both.
How many ancestors do I have? Counts for 10, 12 and 20 generations
You have 2^n ancestors n generations back and 2^(n+1) - 2 in all: 1,024 at 10 generations, 4,096 at 12, over a million at 20. Why fewer real people fill them.
First cousin once removed vs second cousin: the difference, with a chart
A first cousin once removed is your parent's first cousin or your first cousin's child. A second cousin shares your great-grandparents. Chart, DNA, degrees.
Teaching kinship with a spreadsheet, one row per person
Build a small genealogy in Excel or Google Sheets: one row per person, formulas that find grandparents, siblings and cousins, and a lookup that names relatives.
Pedigree collapse: why no one has a billion ancestors
Why the doubling of ancestors cannot go on: pedigree collapse worked through with small numbers, the Spanish Habsburgs, and what Chang and Rohde found.
The coefficient of relationship, worked out by hand
Sewall Wright's path method for the coefficient of relationship, worked step by step for siblings, uncles, cousins and double cousins, with what DNA adds.