DNA

This page covers the basics. Please see Links for other websites with more information on DNA, STRs, and SNPs.

DNA is a complex molecule found in the nucleus of most cells in the human body in 23 pairs of chromosomes. Nuclear DNA is inherited from both parents -- with the exception of Y chromosome DNA from the 23rd pair. Y chromosome DNA is only inherited by males from their father.

Y-STRs

Y-chromosome DNA consists of a long string of over 50,000,000 chemical bases, each of which is designated C, A, T, or G. Patterns of bases (e.g. GATA) sometimes repeat themselves on the Y-chromosome. These Y chromsome repeats are called Y-STRs (Short Tandem Repeats), and occur at known "marker" locations on the DNA.

The number of Y-STR repeats at marker locations can be measured and are the values assigned on a typical Y-DNA test. These marker values are passed down relatively unchanged from father to son.

Occasionally, the number of repeats changes between generations. These changes are called "STR mutations", and accumulate at an average rate of about one every 300 generations. By looking at the number of mutational differences between two males, we get a general idea of how long ago they shared a common ancestor.

Y-SNPs

There is another type of mutation that occurs on the Y chromosome called a SNP (Single Nucleotide Polymorphism). A SNP (pronounced "snip") is a change in a single base (e.g. C to T). SNPs are much rarer than STR mutations, but are also passed down from father to son. Males who share a common SNP also share a common ancestor, though that ancestor typically lived hundreds of generations ago in"deep ancestral" time.

Haplogroups

SNPs are used to define "haplogroups" and subgroups. Haplogroups and subgroups define our place on the world Y chomosome tree, and can help trace the migrations of our early ancestors through time and space.

Haplogroups are designated by letter, where the earliest halpogroup starts with "A". As additional SNPs accumulate, the designations progress through the alphabet. Subgroups of specific haplogroups are deignated by further numbers and letters and represent SNPs that are only found in those specific subgroups.

The tree shown below is of the major haplogroups. Note that haplogroup R is shown divided into it's two major sub groups, R1a and R1b, though such subgroups exist for most of the main halpogroups.

The International Society of Genetic Genealogist has a complete tree of Haplogroup R subgroups. Note that the R1b subgroups and S-series markers shown on their site and in the inset below are only tested by EthnoAncestry.

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