An introduction to genetics in a genealogy/historical context Part 1

A non-scientific overview in 7 Parts

Deoxyribonucleic acid (DNA) is really just four chemicals: Thymine (abbreviated as T) pairs with Adenine (A), and Guanine (G) pairs with Cytosine (C).
DNA was discovered in 1953, but the first Human genome (3200 Million base “Letter” pairs, all the Human DNA in one person) was not sequenced (reading each letter) until 2003, and findings first published in 2007. Only about 2% of DNA encodes proteins, and 99.5% of any two random people’s genomes will be identical.

Aside from sequencing a genome (which was on pace to cost $100M in 2001, dropped to $10M in 2007, and was now just a few thousand dollars in 2015, projected to go below $1000 in the next few years), there are some inexpensive (by comparison) consumer level tests one can take by mail order. 3 “Kinds” of DNA can be tested from home easily, in 2 Main “Types” of tests.

Y-DNA tests can be taken only by genetic males, and one type (usually done first) looks at Short Tandem Repeater (STR)- phrases of repeated alleles, for example ‘GATAGATAGATAGATA’ would be “4” at a particular location, called a “DYS” (DNA Y-chromosome Segment). DYS are usually designated with a unique number. Used in Y-DNA to statistically decide what SNP (explained below) to test for, and based on known drift rates, can mathematically estimate number of generations between two men. Changes in STRs is basically a “copy error” that drops off or adds a set of repeats. An average rate is 1 per 500 generations, and there are up to 111 locations or “Markers” that are looked at for current Y-DNA tests.

Another type of Y-DNA test looks for specific Single Nucleotide Polymorphisms (SNP)–When a base pair of chemicals at any given location randomly changes from one chemical to another (like C becomes T, G becomes A, etc).

This mutation is inherited by all descendants of the individual in which the SNP occurred. SNPs occur very rarely, so they can be used to split a “Haplogroup” (think tree trunk) into “Subclades” (think branches). Because you have to know the specific location and mutation to test for, you usually need to have done an STR test to find someone within a comparable amount of generations that has already tested SNPs to know where to start from. Some testing companies like FamilyTreeDNA will also make a prediction of a Haplogroup, and even some subclades for you if you are very “close” to someone with a confirmed SNP. Other than copy error insertion and deletions, and SNPs, Y-DNA is passed unchanged from father to son, since there were humans. By tracking the branches in the tree through the errors, we have learned a lot about male-line-specifics.
A second kind of DNA test looks at Autosomal DNA (atDNA), which is the 22 non-sex chromosomes that are recombined between mother and father when passed on to children. Test can show cousinship within 6-7 generations, and also distant ancestral admixtures. Genealogy DNA atDNA test are for SNPs (Family Tree’s  atDNA test looks for ~700,000 SNPs). The forensic DNA you hear about is also atDNA, but it looks at STRs, so it is totally different. X-DNA (genetic males have one X chromosome, females two) is often tested at same time as atDNA, but the data not used in most genealogy matching systems.

The third kind looks at DNA that is only inherited from the mother– Mitochondrial DNA (mtDNA). mtDNA is non-nuclear DNA, so it is not part of your chromosomes or “genes” and is passed “unchanged” (except for SNP mutations that occur) from mother to all children, but can only be passed on by a mother, so a mtDNA shows the distant female-line descent. Genealogy mtDNA tests are for SNPs. mtDNA SNPs are not very numerous, so the branches are quite infrequent. Perfect matches may still not have had a common female-line ancestor for 500 or 1000 years.


Y-DNA Branches and mtDNA branches in a timeline

Part 2 will continue with some Clan MacLeod specific information


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s