DNA Overview

What is DNA?

Deoxyribonucleic acid (DNA) is a nucleic acid which contains the genetic instructions for the development and function of cellular life. 

The main role of DNA in the cell is as a blueprint and is involved in the long-term storage of information and how to construct other components of the cell, such as proteins and RNA molecules.

The DNA segments that carry genetic information are called genes, but other DNA sequences have structural purposes, or are involved in regulating the expression of genetic information.

In eukaryotes such as animals and plants, DNA is stored inside the cell nucleus, while in prokaryotes such as bacteria and archaea, the DNA is in the cell's cytoplasm.

Unlike enzymes, DNA does not act directly on other molecules; rather, various enzymes act on DNA and copy its information into either more DNA, in DNA replication, or transcribe it into protein.

Other proteins such as histones are involved in the packaging of DNA or repairing the damage to DNA that causes mutations.

DNA is a long polymer of simple units called nucleotides, which are held together by a backbone made of sugars and phosphate groups.

This backbone carries four types of molecules called bases and it is the sequence of these four bases that encodes information.

The major function of DNA is to encode the sequence of amino acid residues in proteins, using the genetic code.

To read the genetic code, cells make a copy of a stretch of DNA in the nucleic acid RNA.

These RNA copies can then used to direct protein synthesis, but they can also be used directly as parts of ribosomes or spliceosomes.

Genes and Chromosomes

A gene is a segment of DNA that determines individual characteristics (such as height and hair color). Humans have about 20,000 genes. Genes are passed between generations in the form of chromosomes.

People typically have 23 pairs of chromosomes, with one chromosome in each pair coming from the father and one coming from the mother. Each chromosome contains hundreds to thousands of genes.

Because people usually have 46 chromosomes and only 23 are passed on to a child, half of a parent’s chromosomes are not passed on to his or her child.

During the biological processes that result in the formation of the sperm and egg, DNA is randomly shuffled and then sorted into the resulting sperm and egg. This process has occurred over many generations, resulting in that sperm and egg containing DNA from many different ancestors.

If you think of your parents’ DNA as marbles in jars, you get 50% of the marbles in your dad’s jar and 50% of the marbles in your mom’s jar to make your marble jar.

Because we withdraw whole marbles from these jars, it’s possible that your father has red, blue, green, yellow, purple, gray, turquoise, and orange marbles, but that your scoops don’t have any red marbles from your dad’s side. This doesn’t mean that you didn’t descend from your "red marble" dad, it means only that you didn’t inherit any of your dad’s red marbles.

Inheritance Percentages

Every human inherits approximately 50% of their genome from each of their parents. However, the amounts of DNA which are inherited from generations prior to your parents are not evenly split 50/50 with each successive generation of offspring. Therefore, the following chart displays the approximate percentage of the genome which you have inherited from each ancestral group extending 6 generations into the past. Note that the amount gets increasingly more miniscule the further back you go.  

 

 

 

 

 

 

 

Because inheritance is random, humans don’t necessarily inherit the same amount of DNA from grandparents of the same generational level. An example of this would be a great-great-grandparent passing down 6 percent of his genome to you while another great-great-grandparent only ended up passing down 3 percent to you. Furthermore, this concept also is extended by default to ethnicity being passed down to successive generations meaning that you may not receive any measurable ethnic percentage from a grandparent within your family tree from 6 generations ago. 

By the time the DNA is passed down to you, the DNA segments you’ve inherited are randomized. Generally, the process of estimating one’s ethnic composition is completed by virtually dividing the genome into small regions and assigning each region to a set reference population in a database.

Sibling DNA

From generation to generation, only around half of a parent's genome is inherited by each child. Due to this, siblings end up getting different genes from each parent. For example, if you had several siblings, you would end up receiving half of your parents' total DNA. Your sibling would receive the other half (with around 50% of that amount being identical to yours). Another sibling would show the same pattern of inheritance as the second sibling relative to you, and this pattern would continue for every additional sibling present. Because of this, ethnic percentages are not always distributed evenly among siblings. 

If you are female, you inherit 50% of the nuclear DNA from your mother and 50% from your father, since they each give you an X chromosome. If you are a male, you get slightly more than 50% of your nuclear DNA from mom, because you get her X chromosome and dad's Y chromosome. The X chromosome has 153 million base pairs and about 2000 genes, while the Y chromosome has about 58 million base pairs and about 200 genes. If you calculate on the bases of the whole genome, which is about 3.3 billion base pairs, it ends up being that you have around 2.7% more nuclear DNA from your mom. An interesting point here is that fathers pass down a bit more nuclear DNA to daughters than to sons, because they give the much larger X chromosome to daughters. 

 

Genetic Influence of
Neighboring Regions

 

Over the last several thousand years, there have been times when some groups of people were isolated from neighboring populations. Genetic isolation increases the amount of similarity between group members of that population due to intermarriage over time. Genetic isolation has mainly been driven by geographic restrictions and socioeconomic reasons. However, when individuals from genetically different population clusters intermarry, this results in an admixture which is more difficult to distinguish. High rates of gene flow between populations reduce the genetic distance between these populations and if gene flow is high enough, these populations become a single population from a genetic perspective. High rates of admixture are extremely common throughout the world and has always have been throughout history. Migrations and close geographic proximity are the primary drivers of admixture throughout human populations. For example, we find that a high degree of Greek DNA matches DNA in the neighboring Balkan countries, Turkey, and southern Italy.

 

With typical DNA tests, broad ethnicity estimates are the norm. We try to carefully deduce specifically which ethnic groups your ancestors originated from. Based on this, you will be able to visualize your ancestors homelands more clearly and immerse yourself in the culture of the groups and regions they originated in.