FREQUENTLY ASKED QUESTIONS
What is a SNP?
A SNP is a genetic marker at a particular position within a DNA sequence, consisting of a single nucleotide variation in DNA code. It is the most common type of stable genetic variation. SNPs can result from a base transition (A for G, T for C), a transversion (G or A for T or C) or single base deletion. SNPs are the smallest possible change in DNA. Most changes do not change an organism's appearance, though some do. On an individual level a mutation may cause an inherited disease. Geneticists have now continued to map the occurrence of SNPs on the Y chromosome and have the ability to detect unique SNPs in Crawford individuals. The occurrence of SNPs have enabled geneticists to divide the entire male population into haplogroups lettered A through T and determine their origin and migration patterns across the world. They have now been able to evaluate sub-units known as clades and sub-clades. This breakdown of the world population coupled with STRs has enabled geneticists to begin to link near term genealogy with ancient major haplogroups and clades.
What Is an STR?
The most recent evaluations of Y-chromosomes center around short tandem repeats (STRs) which are DNA segments that are repeated similar to a stutter in speech. What this means is that the coding on the Y chromosome at the STR markers will be passed down from generation to generation exactly the same unless a mutation occurs. A mutation is an error in replicating a portion of the chromosome. It is very rare. Research conducted by a variety of institutions and individuals and used by Family Tree has set the general mutation rate at 1/500 generations per marker where a generation is 20 years (Walsh, 2001)(Zhivotovsky, et al., 2004)(Kerchner, 2008). This equates to a mutation rate of 0.002 or 2 X 10³.
What is TMRCA?
It can be seen that it is important to know the rate at which mutations occur if it is to be useful in examining the relationships of surnames and lineages to each other. This relationship has been called Time Till Most Recent Common Ancestor (TMRCA) and is used to estimate how long ago two people with similar Y-DNA may have shared a common ancestor. It is based on how many differences (mutations) are expressed in their gene markers. Since we do not have DNA sampled from ancestors going back to 1100, scientists have estimated the mutation rate by a different method. Weber and Wong (1993) used 20,000 contemporary parent-offspring samples at 28 STR loci on chromosome #19 to check for mutations. They found 47 mutations in the 20,000 events per STR loci per generation (1/833 generations). This assumes that the mutation rate has remained constant through the centuries. This research and other related studies are the basis for determining mutation rates. They have subsequently been refined for most loci.
Why should I test more STR markers?
The estimate of TMRCA is based on a probability distribution. As one uses more and more markers, the distribution becomes tighter. What this means is that each marker is a probability test in that at that marker there is one chance in 500 that there will be a mutation in a specific generation.However, the more markers tested the greater the likelihood that a mutation will occur somewhere. So if there are 37 markers being tested there are 37 chances in 500 that a mutation will occur. This equates to about one chance in 13.5 generations or 13.5 X 20-25 years equals about 300 years. Since surnames were developed in the 1100s only about 2-3 mutations should be expected within surname lineages since that date. Therefore, if you are testing 111 locations on the chromosome the probabilities of detecting a mutation becomes 111/500. This equates to about one chance in 4.5 generations or about every 112 years. Therefore, relationships with other individuals become more certain if there are few mutations.
What is more useful STR markers or SNPs?
What is a haplogroup?
How do I know where I fit on the Genetic Human Tree?