DNA Glossary of Terms

atDNA Genetic Distance - the number of mutations difference between yourself and your match
 
Chromosome Mapping/Triangulation – Identifying three or more segments that match.  Allows users to narrow down the ancestral line shared between matches.  
 
Endogamous population-significant amount of pedigree collapse/cousin intermarriages-Ashkenazi Jews, Puerto Ricans, Finns, Acadians, Pacific Islanders, Mennonites, Icelanders, French Canadians, US Colonials, SE US, parts of Mexico.
 
Haplogroups - a group of individuals sharing a common ancestor; can denote the region the ancestor originated from (i.e. Western Europe, Middle East).  There are 153 known haplogroups and subgroups.  For example, R-M269:  R is the global positioning (large section of the world; Europe); M269 is the SNP, the location and time period (Western Europe/early Bronze Age).
 
IBD – Identical by Descent – found to be identical in 2 people who are related to each other and was passed down to both of them from a common ancestor
 
IBS- Identical by State – DNA segments that match due to 2 individuals having similar ancestral origins
 
Pedigree Collapse - cousins marrying cousins – occurs when there is a small pool of possible spouses
 
Pile-up segment is an area where a lot of people share a matching segment of DNA. This might be the result of a common descendancy stretching back prior to the genealogical timeframe or, in some instances, the result of an area of a chromosome that includes an active, protein-producing gene that doesn’t recombine. One example is a stretch on chromosome 6 from 29,750,000 – 33,100,000 in the human leukocyte antigen area that's responsible for regulating the immune system. These areas should be ignored in matching as they are considered false positives. 
 
Pseudo-segment – A segment of DNA that appears to be matching another person’s DNA but is composed of alleles strung together from jumping between maternal and paternal chromosomes, thus not legitimately matching the other person
 
SNPs – Single Nucleotide Polymorphisms - differences (or mutations) from the reference sequence
 
STRs - short tandem repeats - values in the YDNA
 
A mother can pass down a segment of DNA from either her father OR her mother, but not both from both for the same segment.
 
Under 5cM is only a true identical by descent match when ALL grandparents or great-grandparents have been tested.
 
Promising matches:
23&me – over 1%
FTDNA - 2C-3C
Ancestry - 3C-4C
 
Once the relationship is more distant than 3C, you share <1% of atDNA.