Ancestry Revealed in DNA

We all have 46 chromosomes, with 23 inherited from our father and 23 inherited from our mother. Forty four of these chromosomes are autosomal (or non-sex) chromosomes. We each have 2 sex chromosomes, one inherited from our father and one from our mother. Males have one X and one Y chromosome whereras females have two X chromosomes. Therefore, we all inherit an X chromosome from our mother. Women inherit a second X chromosome from their father whereas men inherit a Y chromosome from their father. The 46 chromosomes exist within the nucleus of cells. When visualized under a microscope, the 46 chromosomes can be grouped into 23 pairs based on the size and shape of the chromosome. This is referred to as a karyotype. The inset in the figure to the left shows the karyotype of a female human. Their are 22 pairs of autosomal chromosomes and both sex chromosomes are X. The karyotype of a human male whould show, in contrast, one X and one Y sex chromosome.

The diagram below illustrates how family trees are typically represented. In this example, males are represented by squares while females are represented by circles. Parents are indicated by arrows and siblings from each set of parents appear under the same horizontal line.

Since Y chromosomes are exclusively derived from fathers, Y-DNA testing can reveal information about your direct paternal lineage. In the first generation of the above figure, the parents at the top had two sons and a daughter. Each son inherited a Y chromosome (indicated by the solid blue color) from the father whereas the daughter did not (she has two X chromosomes and no Y chromosome). A son in the first generation had three children, two daughters and a son. This son in the second generation inherited from his father the same Y chromosome as his paternal grandfather (his father's father). The daughter in the first generation had children with an unrelated man (on right), and their two sons inherited a Y chromosome which is different from their maternal grandfather (indicated by green squares). DNA analysis of Y chromosomes therefore yields information about your grandfather, great grandfather, great-great grandfather, etc., etc. on your father's side.

In addition to the DNA present in our chromosomes, most cells in our body have a structure outside the nucleus termed mitochondria. Mitochondria have their own genome which is called mitochondrial DNA (or mtDNA). The eggs of females have mitochondria whereas the sperm of males do not contain any mitochondria. Therefore, analysis of mtDNA yields information about maternal lineages. The diagram below depicts maternal inheritance of mt DNA.

The children in the first generation (two boys and one girl) all inherit mtDNA from their mother at the top of the diagram (red color). The daughter in this first generation has three children with an unrelated man (on right). All of her children (two boys and one girl) inherit mtDNA from her which is the same as their maternal grandmother (their mother's mother). One of her brothers has three children with an unrelated woman (on left). All three of these children (two daughters and one son) inherit mitochondrial DNA from their mother (orange color), which is different from that of their three cousins. Analysis of mitocondrial DNA can thus trace direct maternal lineages.

Ancestry genetic tests take advantage of these two types of cellular DNA. Various methods analyze DNA in the Y chromosome of males which is paternally derived (inherited exclusively from fathers), or DNA in mitochondria which is maternally derived. The DNA sequence in certain regions of each of these chromosomes is determined and compared to various databases. These databases contain DNA sequence information generated from many people of different ethnicities and from different geographic regions of the world. By comparison of your DNA sequences with these databases, conclusions may be derived about your ancestry and genealogy.

Another method of ancestry genetic testing is analysis of autosomal DNA sequences.  Autosomal DNA refers to the 22 pairs of chromosomes other than the X and Y chromosomes.  Due to genomic DNA sequencing advances, the databases for ancestry related autosomal DNA sequence variations has significantly improved in recent years. Analysis of autosomal DNA sequenceas can now provide more detailed ancestry related information than X and Y chromosome analysis.

Genomic Express has selected Ancestry.com as a preferred provider of ancestry genetic tests. These genetic methods of ancestry determination are described on our Ancestry Genetic Tests page. We provide information about ancestry genetic testing here on our site, and if you wish to undergo testing you will be directed to DNA.Ancestry.com to purchase your test. Ancestry.com will send you your DNA testing kit, perform the genetic test and provide your ancestry report.

References:

Wells, S. (2006). Deep Ancestry: Inside the Genograsphic Project: the Landmark DNA Quest to Decipher Our Distant Past. (National Geographic Society, publisher).

Sykes, B. (2001). The Seven Daughters of Eve. (W.W. Norton and Company, publisher)