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.
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.
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.
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