Base
In DNA, this can be either adenine (A), cytosine (C), guanine (G) or thymine (T). They are
derived from either purine or pyrimidine.
Base-pair
Two complementary bases joined by hydrogen bonds usually as part of the DNA molecule;
base-pairing occurs between (purines and pyrimidines) i.e. between A and T and between G
and C.
Base sequence
The order of nucleotide bases in a DNA molecule e.g AGTACGTA etc.
Chromosome
The structure by which hereditary information is physically transmitted from one generation
to the next. Humans have a total of 46, found in the nucleus of each cell.
Each chromosome is formed when the DNA strand combines with lots of small proteins
called histones. The DNA and histones automatically wrap themselves up into a shape that
looks like an elongated letter X.
Complementary sequences
DNA is made up of two long strands connected together along the middle (a bit like a zip),
where bases on one strand are bonded to bases on the other side. If the base sequence is G-
T-A-C on one side, the other side is always C-A-T-G, thus complementing each other.
Cytosine
Designated by the letter C, it is a pyrimidine base. One of the four base molecules present in
DNA.

DNA
DeoxyRibonucleic Acid. The genetic material of organisms, usually double-stranded; a
class of nucleic acids identified by the presence of phosphate, deoxyribose (a sugar), and
the four bases. Often forms the familiar double-helix. Within DNA are the code-words
needed to form proteins, although much of the DNA is termed 'junk DNA' and has no
known function.
DYS
D = DNA: Y = Chromosome; S = Single copy sequence. The DYS numbering scheme (e.g.
DYS388, DYS390) for the Y-STR haplotype markers are controlled and administered by an
international standards body called HUGO - Human Gene Nomenclature Committee -
based at University College, London.
Double helix
The familiar shape that two linear strands of DNA with complementary sequences assume
when bonded together.
Enzyme
A protein that can speed up a specific chemical reaction without being changed or
consumed in the process.
Gene
The basic unit of heredity; a sequence of DNA nucleotides on a chromosome.
Genetic genealogy
The new field of family history research utilising the Y-chromosome and mtDNA to prove
or disprove biological connections. The Y-chromosome links the strictly paternal line
(father to son), whilst mtDNA links the mother to offspring line (mother to sons and
daughters).
For the vast majority of family history studies, the Y-chromosome is studied.

Genetics
The study of the patterns of inheritance of specific traits.
Genome
All the genetic material in the chromosomes of a particular organism; its size is generally given as
the total number of base-pairs. In humans, 46 chromosomes make up the genome, with a total of 3
billion bases pairs of approx. 2 metres in length packaged into a tiny nucleus.
Genome projects
Research and technology development efforts aimed at mapping and sequencing some or all of the
genome of an organism. The improvements and lower cost of technology that genome projects
have brought make possible the new field of genetic genealogy for genealogists.
Guanine
Designated by the letter G, it is a purine base. One of the four base molecules present in DNA.
Haplotype Your Y-DNA haplotype is a set of 21 numbers (if using 21 STR markers), each number
representing your allele at a given STR marker. Two or more haplotypes may be compared as a
check for a genetic relationship. There are many haplotypes to be found from many different
populations around the globe that may be used to compare haplotypes.
Haplotype diversity A measure of the uniqueness of a particular haplotype in a given
population.
Haplogroup
Haplogroups are large groups that can be used to define genetic populations and are often
geographically orientated. For example, when the Y-chromosome is tested, many males who live
along the western coasts of Europe belong to Haplogroup R1b (used to be called HG1), which belies
the historical movement of males along that coast.
The Y-chromosome haplogroup of an individual is not defined by their haplotype (because
haplotypes are determined by STR markers). Instead, they are determined by SNP's (single
nucleotide polymorphisms) where a single base changes e.g. from A to G. These happen so rarely as
to be considered unique.
However, there is a strong correlation between haplogroups and haplotypes so given any particular
haplotype, it may be possible to correctly determine the haplogroup without any further tests
Heredity
The transmission of characteristics (hair colour, facial features, genetic disorders etc.) from one
generation to the next.
Junk
DNA Most of the DNA in our bodies is termed 'junk DNA'. It is non-coding (isn't used for making
proteins) and thus does not control medical function but is probably involved in genomic
organization. Genetic genealogy only looks at the markers within junk DNA and thus the Y-DNA
test cannot derive any medical information. Locus (pl. loci) Latin for place. See marker.
Marker
Also called a Locus (pl. loci). A gene of a known location on a chromosome. See STR marker. The
DNA Heritage Y-DNA test uses 21 STR markers.

Microsatellite See STR marker Mitochondrial DNA
mtDNA - The circular DNA contained inside the mitochondria. The mitochondria are small
organelles residing in animal cells which provide the power to the cell. They occupy about one-
fifth of each cell. The mtDNA is passed from mother to her offspring (both sons and daughters),
but only the daughter will pass it on.
The DNA sequence can be read and compared against a standard sequence (the Cambridge
Reference Sequence) and deep (i.e. several thousands of years), but very broad genealogies can be
deduced. A mtDNA haplogroup can usually be assigned for any given sequence.
MRCA
The Most Common Recent Ancestor between two people. For example, for two 1st cousins, their
shared grandparent is the MRCA. If the cousins were both boys, they would share their
grandfather's Y-chromosome.
mtDNA See Mitochondrial DNA
Multiplex reaction
Used in the PCR method, it utilises a 'cocktail' of chemicals so that many different markers (or
loci) may be copied and tagged at the same time.
Mutation
Any inheritable change in DNA sequence. Mutations usually occur as a result of miscopying by
cell enzymes. Because the Y-chromosome has relatively few regions that control genetic function
and is mostly 'junk-DNA', mutations in this 'junk DNA' cause no adverse effects and can therefore
occur quite frequently.
A mutation at an STR marker may cause a particular allele to change from 11 repeats to 12 repeats
- the son will therefore carry the 12 allele.
Mutation rate
The estimated rate of mutational change is estimated to be approximately one mutation per 500
transmission events (0.2%) for any given STR marker. Using 21 different markers, it is reasonable
to expect one mutation every 24 transmission events. However, these changes are random and thus
can take place at any time.
Nucleic acid
DNA and RNA are both nucleic acids. They are long polymer structures made up of nucleotides.
Nucleotide
A single unit of nucleic acid. In DNA they are composed of phosphate, deoxyribose (a sugar), and
either a purine (for adenine and guanine) or pyrimidine (for cytosine and thymine) base.
Nucleus
The central structure of a cell that contains the genetic material.
PCR
Polymerase chain reaction - A process carried out in a test tube that produces millions of copies of
small sections of DNA. Using a heat resistant enzyme (DNA polymerase) and a mixture of other
chemicals, cycles of hot and cold temperatures essentially photocopy a particular marker (or locus)
of the DNA many times.
Fluorescent tags are added to each copy so that they may be detected using laser analysers. A
technique called multiplexing enhances the process.
Population A group of individuals residing in a given area at a given time.

Restriction enzyme
A protein that recognizes specific, short nucleotide sequences and cuts DNA at those sites. Bacteria
contain over 400 such enzymes that recognize and cut over 100 DNA sequences. Snake venoms are
sometimes also used.
Sequencing
Determination of the order of nucleotides (base sequence) in a DNA or RNA molecule or the order of
amino acids in a protein.
Sex chromosomes
X and Y-chromosomes - Chromosomes involved in sex determination. Females have two X
chromosomes, males have one X and one Y-chromosome. The Y-chromosome is thus only found in
males.
STR marker
Short Tandem Repeats marker - A stretch of DNA where a small base sequence (usu. 2-6 base-pairs)
repeats itself several times, giving a particular allele. For example, at the STR marker DYS391, the
base sequence may read TCTA TCTA TCTA TCTA TCTA TCTA TCTA TCTA where TCTA is
repeated eight times.
Choosing markers that have been proven to have high variation between and within populations is
desirable.
Thymine
Designated by the letter T, it is a pyrimidine base. One of the four base molecules present in DNA.
Transmission event
The birth of a baby boy i.e. the effective passing on of the Y-chromosome.

Y-chromosome
The sex chromosome that instructs a foetus to grow into a baby boy. It is passed down from
generation to generation only through the male line i.e. from father to son, father to son, etc. It is
approx. 60 million base-pairs long
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