a repost from 06 April 2016, but an excellent refresher
Taking a DNA test for genetic genealogy research? We’ll help you understand some of the scientific terms you'll come across.
Autosomal DNA (also called atDNA or admixture DNA): genetic material inherited equally from mother and father. It's genealogically useful for ancestry back through about five to seven generations. Beyond that, you may not have inherited enough DNA from any one ancestor for that person to be represented in your autosomal DNA.
Centimorgan (cM): a measurement of the distance between genetic markers on the DNA based on the expected frequency of recombination with each generation. On average, one cM equals one million base pairs. In general, the more centimorgans you share with a genetic match, the closer your relationship (although individuals related through multiple ancestors also may share a high number of centimorgans).
Chromosome: a threadlike strand of DNA that carries genes and transmits hereditary information.
Genome: All the genetic material in the chromosome set of an organism. 46 chromosomes make up the human genome.
Genotype: The genetic makeup of a particular individual.
DYS (DNA Y-chromosome Segment): DYS followed by a number identifies a short segment of Y-chromosome DNA, also called a Short Tandem Repeat (STR) or a marker. A Y-DNA test reveals how many repeats of a particular nucleotide sequence are found at that DYS marker. For example, DYS390 is one of the most commonly tested Y-DNA markers, and values for the marker typically range from 19 to 28 repeats.
Genetic cousins: Individuals whose DNA test results match one another. You may have cousins who aren't genetic cousins—that is, you and your cousin don't match on DNA tests because you didn't inherit enough of the same DNA from the same ancestor.
Haplogroup: a collection of related haplotypes with a common ancestor. The haplogroup (also called a clade) is usually defined by a single nucleotide polymorphism (SNP) mutation that arose in an ancestor hundreds or thousands of years ago, and is found in all of the descendant haplotypes.
Haplotype: an individual’s set of single nucleotide polymorphisms (SNPs) or DYS markers. Males who are recently related through their paternal line will have similar haplotypes and belong to the same haplogroup. The more diverse two haplotypes are, the more time has passed since their most recent common ancestor.
