It’s in your genes.  That’s how scientists explain the physical characteristics, personality traits, and behaviours that make each human unique.  The clues carried in our genes - in our DNA - are used to determine criminal guilt or innocence, resolve paternity or maternity questions, predict the chance of inheriting a disease or medical condition, and even trace the long-distant ancestors of the human family tree.  

Our bodies are made up of tiny units called cells – as many as 100 trillion of them.  Within the nucleus of every one of these cells is a set of instructions that tell the cell what role it will play in the body.  These instructions, essentially a blueprint or recipe for building different parts of the cell, come in the form of a molecule called DNA.  Short for deoxyribonucleic acid, DNA consists of two thread-like strands that are linked together in the shape of a double helix.

Base
The chemical building blocks of DNA, abbreviated A, T, C and G, these bases pair up to form the “stairs” of the DNA double helix and always combine in the same patterns: A with T, and C with G.

Cell
The smallest unit of living matter that can operate independently.

Chromosome
Within the nucleus of each cell, the DNA molecules are coiled around proteins into tiny structures called chromosomes.  In humans, each cell normally contains twenty-three pairs of chromosomes, for a total of forty-six.  One chromosome in each pair is inherited from the mother, and the other from the father.  Twenty-two of these pairs, sometimes called autosomes, look the same in both males and females.  The twenty-third pair, called the sex chromosome because it determines gender, is the one that differentiates males and females.  Females have two copies of the X chromosome, one from each parent, while males have one X chromosome from their mother, and one Y chromosome from their father.  It is the father that determines the sex of his child.

DNA (Deoxyribonucleic Acid)
DNA is made up of four chemical bases: Adenine (A), Cytosine (C), Thymine (T), and Guanine (G).  These bases are combined into pairs – adenine with thymine and cytosine with guanine – to make up the “rungs” of the DNA ladder.  Each “rung,” more accurately called a base pair, is one of three billion such pairs that work together to provide the instructions for building and maintaining a human being – the human genome.  The exact order in which these base pairs are combined is called the DNA sequence.  Much in the way letters of the alphabet are combined to form words and sentences, the sequence of these bases are the “letters” which spell out the genetic code.

Double helix
The shape of DNA, much like a spiral staircase or twisted ladder.  The railings are composed of sugars and phosphates.  Its sides contain the patterned base pairs A, T, C and G.  When a cell divides for reproduction the helix unwinds and splits down the middle like a zipper in order to copy itself.

Gene
Genes are sections or segments of DNA that form the individual units of heredity.  They are carried on the chromosomes and contain instructions for making molecules, called proteins.  Each protein enables a cell to perform its own special function.  The hemoglobin in red blood cells for example, is responsible for transporting oxygen throughout your body.  Another protein, insulin, helps you metabolise your food.  The keratin protein is what helps your hair and nails to grow.  If you look at DNA as a recipe for creating a living thing, then genes and proteins are the ingredients which work together to build, repair, and run your body.  

The traits which make us each unique are also inherited from our ancestors.  Physical characteristics such as curly hair, blue eyes, and a tendency for acne are all determined by our genes.  Scientists also believe that many emotional and behavioural traits, at least in part, are influenced by an individual’s genetic makeup.  Eating habits, intelligence, a penchant for aggressiveness, and even sleeping patterns all have their roots in our DNA.  

Because genes are carried on the chromosomes, humans have two copies of each gene, one inherited from the mother and one from the father.  The two copies aren’t necessarily the same, however.  Just like snowflakes, genes come in variant forms.  These variations are known as alleles.  Different alleles are what produce variations in inherited traits.  This is why your individual traits such as hair colour or blood type may not match those traits in either of your parents.

Genetic markers
A marker is a segment of DNA with known genetic characteristics.  These markers, which can be found at specific locations, or loci, on the chromosome, are essentially places where the same pattern repeats a number of times – sort of a "stutter" in the DNA.  The number of repeats in a marker is known as an allele, basically a variant form of a specific gene.  Since the number of repeats within these sequences is inherited, they make useful mileposts for genetic testing.  

A special type of marker known as a Short Tandem Repeat (STR) is the one most often used for hereditary and forensic testing.  STRs are short sequences of DNA (usually 2-5 base pairs) that are repeated as many as 100 times along the DNA strand.  For example, the four-base pattern CAGT might be repeated four times: CAGTCAGTCAGTCAGT.  STRs are chosen for their tendencies to display variations, caused by mutations, among different people, allowing scientists to differentiate between individuals.  

To determine a connection between two individuals, specific markers on the DNA strand are analysed for the number of repetitions at each marker.  Because mutations happen randomly however, a mutation that appears at a specific marker may have begun with the current generation, or it may have been handed down through five generations.  This is why a number of different markers are tested and compared.  The number of markers examined varies from test to test and company to company, but most ancestry DNA tests are typically in the 12-40 marker range.  The DNA test results provide you with the number of repetitions at each of the specific markers tested.  The more locations that match, the more likely it is that the two individuals are related.

The genetic marker for haemochromatosis is a mutation called C282Y. Someone needs to inherit two copies of this defective gene, one from each parent, in order to be susceptible to the disease; when they do, they’re called “C282Y homozygotes.”

Previous studies have shown that about 1 in 250 people overall have this genetic marker for haemochromatosis, as do about 1 in 200 people with northern European ancestry.  But those studies were not clear as to what percentage of those with the marker would go on to develop the disease.  Some estimates had put the percentage at less than 1 percent.

Genome
The total DNA sequence that serves as an instruction manual for all proteins created in the body.  Two copies of the genome are found inside each of our cells.

Haplogroup
Branches on the tree of early human migrations and genetic evolution.  Haplogroups are defined by genetic mutations or “markers” found in Y-chromosome and mtDNA testing.  These markers link the members of a haplogroup back to the marker’s first appearance in the group’s most recent common ancestor.  Haplogroups often have a geographic relation.

Haplotype
A person’s individual footprint of all tested genetic markers.  Even the difference of a single genetic marker delineates a distinct haplotype.

Heredity
The total sum of genetic information that humans pass on down through the generations.

Melanin
The skin’s brown pigment Melanin is a natural sunscreen that protects tropical peoples from many harmful effects of ultraviolet (UV) rays.  When UV rays penetrate the skin they also produce beneficial vitamin D, so some exposure to UV is necessary.

Mitochondria
A remnant of an ancient parasitic bacteria that now helps to produce energy inside the cell.  A mitochondrion has its own genome, present in only one copy, which does not recombine in reproduction.  This genetic consistency makes mitochondrial DNA a very important tool in tracking genetic histories.

Mitochondrial DNA (mtDNA)
Genetic material found in the mitochondria.  It is passed from females to their offspring with recombining and thus is an important tool for geneticists.

MRCA
Most recent common ancestor.

Mutations
As DNA passes from one generation to the next, it acquires small changes, known as mutations.  The most common is a change to a single base, for example a change from a T to a C.  Other possible changes include the loss or addition of one or more bases.  The effect of a mutation depends upon the type of changes and their location in the sequence.  Just like one single letter can change a word or even a sentence, a mutation can change the instructions in a gene.  Most mutations are considered to be neutral, having little to no impact.  Serious mutations can actually cause a protein to stop functioning properly.  

Mutations in the DNA can be inherited or acquired.  When a mutation is inherited from a parent it is present in almost all of the body’s cells.  Acquired mutations are changes in the DNA that develop throughout a person’s life.  They arise in the DNA of individual cells, either spontaneously or in response to environmental factors such as radiation or viruses.  Spontaneous mutations are the most common, caused by copying mistakes in the DNA code as cells form and divide.  Most of the time the cell recognises the mistake and repairs it, but sometimes it passes the mutation on as it divides and creates new cells.  

DNA doesn’t have long-term memory, so any mutations that develop in a gene are reproduced and passed down to future generations.  By comparing the mutations of two individuals, it is possible to calculate how closely they’re related.  By calculating the mutation rate, researchers can deduce how far back in time different groups split apart.

Nucleotide
A DNA building block that contains a base, or half of a “staircase step”, and sugars and phosphates which form the “railing”.  Nucleotides join together to form DNA’s distinctive double helix shape.

Nucleus
The part of the cell in which chromosomes reside.

Phylogeny
The evolutionary development of a species.  Phylogeny is sometimes represented as a tree that shows the natural relations and development of all species.

Population genetics
The study of genetic variation in a species.

Proteins
Linear sequences of amino acids that are the building blocks of cells.  Each protein has a specific function that is determined by the “blueprint” stored in DNA.

Recombination
The process by which each parent contributes half of an offspring’s DNA, creating an entirely new genetic identity.  This process mixes genetic signals, so that non-recombining DNA, passed intact through the generations, is most important to population genetics.

Replication
The process by which two DNA strands separate, with each helping to duplicate a new strand.  During reproduction, the DNA double helix unwinds and duplicates itself to pass on genetic information to the next generation.  Because bases always form established pairs (A, T and C, G) the sequence of bases on each strand will attract a corresponding match of new bases.  Only occasional errors
occur – about one for every billion base-pair replications.

Sequencing
Determines the order of nucleotides for any particular DNA segment or gene.  The order of a DNA string’s base pairs determines which proteins are produced and thus the function of a particular cell.

Sexual selection
Special form of natural selection based on an organism’s ability to mate.  Some animals possess characteristics that are more attractive to potential mates, e.g. male birds’ plumage.  Individuals with such characteristics mate at higher rates than those without, ensuring more next generation offspring will inherit the desirable trait.  As generations procreate, the desirable trait becomes increasingly common, further boosting the sexual disadvantage for individuals that lack the desired trait.  The effect can be particularly dramatic when one individual controls mating with a larger number of potential partners.

Single nucleotide polymorphism
Small, frequent changes that help to create an individual’s own unique DNA pattern.  When a single nucleotide (A, T, G or C) is altered during DNA replication due to a tiny “spelling mistake”, the genome sequence is altered.

Trait
Physical characteristics, like eye colour or hair texture, which are determined by inherited genes.

X and Y chromosomes
The two chromosomes that determine sex.  Females have two X chromosomes while males have one X and one Y.  When chromosomes pair, the mismatched Y determines male gender.  Because of the mismatch, part of the Y chromosome does not recombine with the X during reproduction.  The non-recombining part of the Y chromosome contains a sequence of DNA passed intact from males to their sons through the generations, giving population geneticists a useful tool for studying human history.

DNA is passed down from one generation to the next: some parts remain almost unchanged, while other parts change greatly.  This creates an unbreakable link between generations and it can be of great help in reconstructing our family histories.   While it won't provide us with our entire family tree or tell us who are our ancestors, DNA testing can:

Determine if two people are related
Determine if two people descend from the same ancestor
Find out if we are related to others with the same surname
Prove or disprove our family tree research
Provide clues about our ethnic origin

DNA tests have been around for a long time but it is only recently that the cost involved has become within the reach of the average person interested in tracing their roots.  Home DNA test kits can be ordered through the mail or over the Internet at a cost averaging between $100-$400 per test.  They usually consist of a cheek swab or mouthwash to easily collect a sample of cells from the inside of your mouth.  You send back the sample through the mail and within a month or two you receive the results - a series of numbers that represent key chemical “markers” within your DNA.  These numbers can then be compared to results from other individuals to help you determine your ancestry.  

There are two basic types of DNA tests available for genealogical testing:
mtDNA Tests - Mitochondrial DNA (mtDNA) is contained in the cytoplasm of the cell, rather than the nucleus.  This type of DNA is passed by a mother to both male and female offspring without any mixing, so your mtDNA is the same as your mother's mtDNA, which is the same as her mother's mtDNA.  mtDNA changes very slowly so it cannot determine close relationships as well as it can determine general relatedness.  If two people have an exact match in their mtDNA, then there is a very good chance they share a common maternal ancestor, but it is hard to determine if this is a recent ancestor or one that lived hundreds of years ago.  It is important to keep in mind with this test that a male’s mtDNA comes only from his mother and is not passed on to his offspring.  

Y Line Tests - More recently, the Y chromosome in the nuclear DNA is being used to establish family ties.  The Y chromosomal DNA test (usually referred to as Y DNA or Y-Line DNA) is only available for males, since the Y chromosome is only passed down the male line from father to son.  Tiny chemical markers on the Y chromosome create a distinctive pattern, known as a haplotype, that distinguishes one male lineage from another.  Shared markers can indicate relatedness between two men, though not the exact degree of the relationship.  Y chromosome testing is most often used by individuals with the same last name to learn if they share a common ancestor.  

Markers on both mtDNA and Y chromosome tests can also be used to determine an individual’s haplogroup, a grouping of individuals with the same genetic characteristics.  This test may provide you with interesting information about the deep ancestral lineage of your paternal and/or maternal lines.  

Since Y-chromosome DNA is found only within the all-male patrilineal line and mtDNA only provides matches to the all-female matrilineal line, DNA testing is only applicable to lines going back through two of our eight great-grandparents - our father's paternal grandfather and our mother's maternal grandmother.  If you want to use DNA to determine ancestry through any of your other six great-grandparents you will need to convince an aunt, uncle, or cousin who descends directly from that ancestor through an all-male or all-female line to provide a DNA sample.  Additionally, since women don’t carry the Y-chromosome, their paternal male line can only be traced through the DNA of a father or brother.

DNA tests can be used by genealogists to:

Link specific individuals - e.g. test to see whether you and a person you think may be a
cousin descend from a common ancestor

Prove or disprove the ancestry of people sharing the same last name - e.g.  test to see if
males carrying the same surname are related to each other

Map the genetic origins of large population groups - e.g.  test to see whether you
have European or African American ancestry

To best use DNA testing to learn about your ancestry you should start by narrowing down a question you are trying to answer and then select the people to test based on the question.  For example, you may wish to know if two families with the same name are related.  To answer this question with DNA testing, you would then need to select several male descendants from each of the lines and compare the results of their DNA tests.  A match would prove that the two lines descend from a common ancestor, though would not be able to determine which ancestor.  The common ancestor could be their father, or it could be a male from over a thousand years ago.  This common ancestor can be further narrowed down by testing additional people and/or additional markers.  

Most Recent Common Ancestor (MRCA)
When you submit a DNA sample for testing, an exact match in the results between you and another individual indicates that you share a common ancestor somewhere back in your family tree.  This ancestor is referred to as your Most Recent Common Ancestor or MRCA.  The results on their own will not be able to indicate who this specific ancestor is, but may be able to help you narrow it down to within a few generations.  

An individual’s DNA test provides little information on its own.  It is not possible to take these numbers, plug them into a formula and find out who your ancestors are.  The marker numbers provided in your DNA test results only begin to take on genealogical significance when you compare your results with other people and population studies.  If you don’t have a group of potential relatives interested in pursuing DNA testing with you, your only real option is to input your DNA test results into the many DNA databases starting to spring up on the Internet, in the hopes of finding a match with someone who has already been tested.  Many DNA testing companies will also let you know if your DNA markers are a match with other results in their database, provided that both you and the other individual have given written permission to release these results.  

Understanding the Results of Your Y-Chromosome DNA Test (Y-Line)
Your DNA sample will be tested at a number of different data points called loci or markers and analysed for the number of repeats at each of those locations.  These repeats are known as Short Tandem Repeats (STRs).  These special markers are given names like DYS391 or DYS455.  Each of the numbers that you get back in your Y-chromosome test result refers to how many times a pattern is repeated at one of those markers.  The number of repeats is referred to by geneticists as the alleles of a marker.

The effect of adding more markers
Adding additional markers increases the precision of DNA test results, providing a greater degree of probability that an MRCA (most recent common ancestor) can be identified within a lower number of generations.  For example, if two individuals match exactly at all loci in a 12-marker test, there is a 50% probability of an MRCA within the last fourteen generations.  If they exactly match at all loci in a 21-marker test, there is a 50% probability of an MRCA within the last eight generations.  There is a fairly dramatic improvement in going from 12 to 21 or 25 markers, but after that point the precision starts to level off making the expense of testing additional markers less useful.  Some companies offer more precise tests such as 37 markers or even 67 markers.

Results of Your Mitochondrial DNA Test (mtDNA)
Your mtDNA will be tested on a sequence of two separate regions on your mtDNA inherited from your mother.  The first region is called Hyper-Variable Region 1 (HVR-1 or HVS-I) and sequences 470 nucleotides (positions 16100 through 16569).  The second region is called Hyper-Variable Region 2 (HVR-2 or HVS-II) and sequences 290 nucleotides (positions 1 though 290).  This DNA sequence is then compared to a reference sequence, the Cambridge Reference Sequence, and any differences are reported.

The two most interesting uses of mtDNA sequences are comparing your results with others and determining your haplogroup.  An exact match between two individuals indicates that they share a common ancestor, but because mtDNA mutates extremely slowly this common ancestor could have lived thousands of years ago.  Matches that are similar are further classified into broad groups, known as haplogroups.  An mtDNA test will provide you with information about your specific haplogroup which may provide information on distant family origins and ethnic backgrounds.

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