Learning the DNA Lingo
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.
Mitochondrial DNA (mtDNA): genetic material both males and females inherit from their mothers. Because it’s passed down mostly unchanged from mothers to daughters, mtDNA can tell you about your maternal line—but because this type of DNA mutates infrequently, the results reveal only “deep ancestry,” not definitive links to recent generations.

Modal: the set of most-common DYS values in a group of closely related haplotypes. A particular branch of a surname, for example, might have a slightly different modal from another branch of the name.

MRCA (Most Recent Common Ancestor): the most recent paternal ancestor of two males. Every male on earth shares an MRCA with every other male, although some will have an MRCA thousands of years ago and others will have an MRCA within the last few generations. Y-DNA results can reveal how many generations have passed between two participants and their MRCA.

Mutation: a usually harmless change in the DNA sequence. A mutation can change the value of a DYS marker, for example. Although mutations are random, they typically occur at a known rate and thus provide a rough molecular “clock” useful for surname studies.

NPE (non-paternal event): a break in the Y-chromosome line resulting from adoption, infidelity or another cause. NPEs (also known as non-paternity events or false paternity) can be detected by DNA testing.
Recombination: the exchange of DNA segments at conception. Due to recombination, you inherit less autosomal DNA from each generation going back in time.   

SNP (Single Nucleotide Polymorphism): the mutation of a single nucleotide in the Y-DNA sequence. One of the nucleotides, represented by the letters A, T, C or G, replaces another at that location in the sequence. Haplogroups are defined by SNPs.

STR (Short Tandem Repeat): a repeat of a short nucleotide sequence on the Y-chromosome. The DYS390 marker, for example, is an STR with between 19 and 28 repeats of the short nucleotide sequence. Closely related males will have a similar number of repeats.

TMRCA (Time to Most Recent Common Ancestor): an estimate of the amount of time between two males and their most recent paternal ancestor, calculated using differences between the two haplotypes.
Y-DNA: genetic material passed down from father to son. Because surnames also pass from father to son, Y-DNA tests can confirm (or disprove) genealogical links through a paternal line. 
Planning to add genetic genealogy to your family history research strategy? Or have you already taken a DNA test with Ancestry DNA, Family Tree DNA, 23andme or another company? Visit Family Tree University to find convenient, online genetic genealogy classes to help you choose the right test for your needs, analyze your test results, and use your results to answer questions about your family tree.