15 May 2013

DNA Basics

After much back and forth on how to continue my DNA series, I have come to one conclusion: if this series doesn't kill me, I am immortal. There is so much info, so many companies, so many misconceptions...... if I so chose, I could write about it for the rest of my natural life. My brain would explode from overwork, but as long as you understand what we're doing, right? As previously mentioned, I am in the waiting process on a few DNA tests and I will share those results when available. I also plan to discuss the different companies available in it's own post (along with how to avoid disreputable websites). I am writing three posts, one for each of the common DNA tests used by genealogists as well. On top of that, I also want to discuss privacy laws concerning DNA results and may have to make a separate post so as to not overwhelm you. And yet.... and yet today I find myself writing about what DNA is. I have been reading forums, blogs, Facebook pages, FAQs for a dozen companies, etc. and I have learned one thing: there are people who don't know enough of the basics of genetics to have any use for these tests. Today, I plan to give you something of a glossary or reference post that you can use to help you navigate words and concepts you may have not seen since high school (if at all).

Starting with the Base Pairs

Nucleotide pair
A Nucleotide pair (or Base Pair) is the starting point of DNA. There are four nucleotide molecules: Adenine, Thymine, Cytosine, and Guanine. Adenine pairs with Thymine, Cytosine with Guanine. This is important, because the order that the nucleotides are put in is the blueprint of everything. If a nucleotide is paired with the wrong partner during copying, the error becomes known as a Single Nucleotide Polymorphism (SNP). It's these SNP's that are reviewed during a DNA test to determine your ancestors' possible origins and connect you with potential cousins. A gene is what we call a series of nucleotides. An allele is what we call versions of a gene. For example, there is a gene for the color of your eyes (a few actually, but let's keep this simple). While brown eyes are dominant, there is a mutation that allows for light colors like grey, blue and green. The color of your eyes indicates which allele you have (what version of the gene). Another way to put it: if computer operating systems were a gene, Windows 8, Vista, and Windows ME would be alleles. They all do the same thing. They all started off in the same place. But every new generation changed the basic code a little. (And just like a gene, if it doesn't make life easier, the organism dies.)

Deoxyribonucleic Acid (DNA)
DNA is a series of base pairs held together by a sugar bond. When DNA is copied (for use in making proteins, or in making new DNA for new cells), the base pairs separate. New nucleotides attach to the "unzipped" DNA to copy the information. The most common shape that people recognise is the "Double Helix". When human DNA is compressed, it twists into the helix shape. However, DNA can come in many shapes. In the mitochondria, DNA is shaped into a circle.

Technically, the picture to the right is a Chromatid. However, many sources will use "chromosome" interchangeably with a single chromatid and a pair of chromatids, so it's important you know what we're really talking about. DNA is compressed into the chromosome shape when it's preparing to make a new cell (mitosis) or a gamete (meiosis). A gamete is the science-y term for a sperm or an egg. When a sperm or an egg is made, they get 23 chromosomes (chromatids). A human requires 46 total chromosomes in 23 pairs. You receive one set from your dad, and it's twin set from your mom. The first 22 chromosome pairs are what we call "autosomal". When we discuss ethnicity tests, we're going to be talking about the information found on these 22 pairs. The last pair is your sex chromosomes. Females only have X chromosomes. Men have one X and one Y. So, if you are a male, you received your father's Y chromosome. If you are a female, his X.

So if we want to know more about your father's specific line, a Y chromosome test would be helpful. But only if you are a male (since women don't have a Y chromosome to test). This test usually looks for Short Tandem Repeats (STR). DNA can carry a lot of "junk" and repeats. When a base pair is repeated in a connected sequence, it's known as an STR. Y chromosome testing that looks at the length and frequency of particular STRs helps to determine a man's haplotype. A haplotype is considered the more recent ancestry markers. There's also a haplogroup. Haplogroups are usually determined by looking at SNPs. A haplotype is usually grouped with similar haplotypes into a haplogroup. A haplogroup, therefore, is considered more ancient DNA. Now, because X chromosomes are passed down from both parents, it's not considered helpful for finding out about a mother's ancestry specifically. Mitochondria, however, is passed down only by women. So an mtDNA test will test the DNA inside of mitochondria (that circular DNA) to determine a person's maternal ancestry.

How DNA Can Change
During the copy process, portions of DNA can accidentally be skipped. This is deletion. As long as what is deleted isn't necessary for proper functions of the organism, it's not lethal.

Insertion is when new sequences of DNA is added to the strand. This is how STRs are born. For whatever reason, nucleotides are repeatedly copied in the same order, leading to an insertion of genes that are superfluous. Again, unless it's going to affect the proper functions of the organism, it's not lethal. Insertions are usually considered "junk" DNA because it's also not going to improve the functions of the organism.

This is the really important one and I left it for last so you could let it sink in tonight after reading this post. When a gamete is being made, the chromosome pairs are copied. So there are 92 chromosomes. These 92 chromosomes are separated into four gametes. The crazy thing is, that the "autosomal" chromosomes have this stupid habit of swapping genes. So it becomes a genetic dice roll as to what chromosome will be in what gamete and which gamete will make one half of you. You can end up with 0% of any particular ancestor. And the farther back that ancestor is, the more likely you'll not receive any of their ethnicity markers. Also, because of this swapping, autosomal tests don't differentiate between your mother's side of your DNA and your father's. It's just not possible.

So you take an autosomal test hoping to prove you're Native American and it doesn't show up, or your test reveals more West African than your tree would suggest. Does that mean your family is wrong about your tribal heritage? Not really. It just doesn't help prove it. And really, if you have Native American DNA, it doesn't necessarily prove your great grandmother was actually a Cherokee. It can be an error (I'll talk about accuracy in testing ethnicities later) or ancient DNA that you were lucky enough to receive. While DNA can be fun, it's useless to a genealogist without documentation.

But that is a tale for another day,

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