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Glossary
Adenine
A nitrogenous purine base found in DNA and RNA. It is paired
with thymine in DNA.
Allele(s)
Alternative forms of a gene found at the same location on
a chromosome pair. A single allele for each locus is inherited
separately from each parent. Examples: At a locus for eye
color different alleles may result in blue or brown eyes.
On chromosome 19, at the apolipoprotein E locus (APOE),
an individual may have different alleles (E2, E3, or E4)
resulting in different risks to develop late-onset Alzheimer
disease.
Amino acid
One of the twenty chemical building blocks that can be linked
together to form a polypeptide chain or a protein. Examples:
phenylalanine, threonine, and alanine.
Anticipation
The tendency for some autosomal dominant diseases to manifest
at an earlier age and to increase in severity with each
succeeding generation. Example: myotonic dystrophy
Anticodon
A triplet of bases in a tRNA molecule that can base pair
with a complementary triplet of bases in an mRNA molecule
during the process of protein synthesis. Each tRNA is capable
of carrying the amino acid that corresponds to the codon
to which that tRNA can base pair. Thus, tRNA's are responsible
for decoding the amino acid sequence specified by an mRNA
based upon the specificity of the codon-anticodon interaction.
Association genetics
A means of establishing an association between a gene and
an observable trait by comparing frequencies of alleles
of the gene in groups of individuals who differ in the expression
of that trait. A statistically significant difference in
the allele frequencies between groups may be due to a causative
effect of the different alleles of that gene in the expression
of the trait. A common study design for detecting associations
is the case/control design, where allele frequencies in
a group of cases (i.e. patients with a particular disease)
are compared to a control group composed of unaffected individuals.
Example: The presence of the APOE4 allele in an individual's
genome and its association with an increased risk to develop
late-onset Alzheimer disease.
Autosome
Any one of the non-sex determining chromosomes. The diploid
human genome consists of 46 chromosomes, 22 pairs of autosomes,
and 1 pair of sex chromosomes (the X and Y chromosomes).
Base pair
A pair of nitrogenous bases, one on each strand of a DNA
or RNA double helix, which hold the two strands together
by virtue of weak, hydrogen bonds between the bases. There
are specific rules that determine which bases can pair:
adenine pairs with thymine (in DNA) or uracil (in RNA),
and guanine pairs with cytosine.
Base sequence
The order of nucleotide bases in a DNA molecule.
Bioinformatics
The science that uses advanced computing techniques for
management and analysis of biological data. Bioinformatics
is particularly important as an adjunct to genomic research,
which generates a large amount of complex data, involving
DNA sequences and hundreds of thousands of genes.
Candidate gene
A gene whose function or location suggests that it may be
responsible for a disease or trait in a population of individuals.
Carrier
An individual who has one copy (allele) of a disease-causing
gene. Carriers do not usually express the condition caused
by the recessive allele, but can pass it on to their offspring.
Examples: A carrier for cystic fibrosis or sickle cell anemia.
Causative gene
A gene that in a variant form is known to be the reason
for developing a specific genetic disease. Monogenic genetic
diseases are due to "causative" genes. Examples: cystic
fibrosis, sickle cell anemia, Huntington disease.
cDNA (complementary DNA)
A DNA strand copied in vitro from mRNA using reverse transcriptase.
It is complementary to the RNA from which it was reverse
transcribed.
Cell
The basic unit of any living organism. This small compartment
contains chemicals, cellular organelles and a complete copy
of the organism's genome.
Centimorgan (cM)
A unit of measure of genetic distance. One centimorgan is
equal to a 1% chance that there will be a recombination
between two loci. In human beings, 1 centimorgan approximates,
on average, 1 million base pairs.
Centromere
The point at which the two chromatids of a chromosome are
joined, and the region of the chromosome which becomes attached
to the spindle during cell division.
Chromosome
The self-replicating structures in the nucleus of human
cells that spacially and functionally organize the DNA in
an individual's genome. The normal chromosome number in
humans is 46. Examples: 46, XX, normal female; 46, XY, normal
male.
Clone
A group of genetically identical organisms. Identical twins
are an example of naturally occuring clones. A bacterial
colony, grown on a petri dish starting from a single bacterium,
is a clone of identical cells. If the original bacterial
cell contained a recombinant DNA molecule, e.g. a human
DNA fragment, all the bacteria in the colony will have identical
copies of that human DNA fragment. That is called a cloned
DNA fragment.
Coding region
The part of a gene which directly specifies the amino acid
sequence of its protein product.
Codominant alleles
Alleles whose phenotypes are both expressed in the heterozygote.
Example: The AB blood group is due to a single gene which
produces a protein that is found on the surface of the red
blood cells. The A and B alleles of this gene produce antigenically
distinct forms of this protein which can be recognized by
antibodies specific to each form. A person who inherits
the A allele from one parent and the B allele from the other
will express both forms of the protein, so their red cells
will react with both the A and B antibodies. We say they
have type AB blood.
Complex disease
A common disease for which there is no simple model of inheritance
or a single disease-causing gene. This category of disease
is often described as multifactorial or polygenic. Examples:
type II diabetes, asthma, or cardiovascular disease.
Complementary sequence
Nucleic acid base sequences that can form a double-stranded
structure by matching base pairs.
Codon
A sequence of three adjacent nucleotides that code for an
amino acid, chain initiation, or chain termination. Example:
GAG = glutamic acid
Cytogenetic map
A map which illustrates where genes are located on each
chromosome.
Cytosine
A pyrimidine base found in DNA and RNA. It is paired with
guanine.
Differential Gene Expression (DGE)
A laboratory technique that compares the body of expressed,
(switched on) genes in different cells under different conditions
by measuring the mRNA produced. Example: comparing the mRNA
in a diseased liver cell with that in a healthy liver cell.
DNA (Deoxyribonucleic acid)
The double stranded molecule that carries the genetic instructions
for making living organisms. DNA is composed of four nitrogenous
bases, adenine (A), cytosine (C), guanine (G), and thymine
(T), which are bonded to a repeating backbone of deoxyribose-phosphate
to form a DNA strand. Two complementary anti-parallel strands,
where all the Gs pair with Cs and As with Ts, form a double
helix held together by hydrogen bonds between the bases.
The DNA bases encode messenger RNA (mRNA), which in turn
encodes amino acid sequences.
DNA library
A collection of recombinant DNA molecules from a particular
source that, in total, represent all, or the vast majority,
of the nucleic acid sequences in that source. A library
made from the DNA of an organism is called a genomic library,
as it represents the entire genome of the organism. A cDNA
library is usually made from the mRNA population expressed
in a particular cell or tissue type, and represents all
of the genes expressed in that cell or tissue.
DNA sequence
The relative order of base pairs in the DNA molecule, whether
it is in a fragment of DNA, a gene, a chromosome, or the
entire genome.
Dominant
A disease or trait that is expressed in individuals even
when it is present in only one of the alleles of a gene
pair. Males and females are equally likely to be affected,
and the trait can be passed on to successive generations
of a family. Examples: neurofibromatosis, Huntington disease.
Double helix
The structural arrangement of DNA that looks like a twisted
ladder. The sides of the ladder are formed by a backbone
of sugar and phosphate molecules and the rungs consist of
nucleotide bases joined in the middle by hydrogen bonds.
Dynamic mutation
A mutation which, when transmitted, is passed on in an altered
form. Example: The triplet repeat, CGG in Fragile X syndrome,
which can increase in number from one generation to the
next.
Enzyme
A protein that acts as a catalyst, speeding the rate at
which a biochemical reaction proceeds but not altering the
direction or nature of the reaction.
ESTs (expressed sequence tags)
A small sequence from an expressed gene that can be amplified
by PCR. ESTs act as physical markers for cloning and full-length
sequencing of the cDNAs of expressed genes.
Eukaryote
A cell or organism with a distinct membrane-bound nucleus
as well as specialized membrane-based organelles.
Exon
A sequence of genomic DNA which becomes part of a messenger
RNA. In eukaryotes, most genes are composed of multiple
exons, arranged on a chromosome in the order in which they
appear in mRNA, but physically separated by intervening
sequences, or introns. The entire gene, including both exons
and introns, is transcribed into RNA, and the introns are
then spliced out of the RNA molecule to produce the final
messenger RNA.
Frameshift mutations
Mutations in DNA, arising from insertions or deletions that
change the reading frame of mRNA. The reading frame determines
which sets of 3 nucleotides are read as codons. Examples:
insertions or deletions.
Gamete
A male or female reproductive cell. In the female, an ovum
or egg; in the male, a sperm. Gametes are haploid; they
consist of a single set of chromosomes.
Gel electrophoresis
A technique used to separate molecules according to size
or charge. The molecules are passed through a gel under
the influence of an electric field.
Gene
The fundamental physical and functional unit of heredity.
A gene is a defined section of DNA that encodes information
for the production of a specific functional product: a protein
or RNA molecule. The are approximately 100,000 genes in
the human genome.
Gene expression
The process by which a gene's coded information is converted
into the structures present and operating in the cell. Expressed
genes include those that are transcribed into mRNA and then
translated into protein and those that are transcribed into
RNA but not translated into protein. Genes in humans are
subject to complex patterns of regulation. Cells express
only about 15% of their genes with different genes expressed
by different cell types. The pattern of gene expression
determines the characteristics of a cell and its role in
the organism. Changes in the pattern of gene expression
drive cell differentiation. Abnormal patterns of gene expression
are associated with the development of tumors.
Gene families
Groups of closely related genes that make similar products
(e.g. proteins).
Gene product
The biochemical material, either RNA or protein, that results
from expression of a gene. The amount of gene product is
used to measure how active a gene is; abnormal amounts can
be correlated with disease-causing alleles.
Gene therapy
A means of treating or correcting genetic disorders by introducing
the normal or functioning gene into the cells of individuals
who lack the normal gene.
Genetic code
The correlation between the codons in genomic DNA or mRNA
and the amino acid that each codon specifies. Codons are
3 nucleotides long, and since there are 4 bases (C, G, A,
and T), there are 64 possible codons (4 x 4 x 4). There
are 20 naturally occurring amino acids in proteins, some
of which can be specified by more than one codon. 61 of
the 64 possible codons specify amino acids, and the remaining
3 codons, called "stop codons", indicate the end
of the protein sequence.
Genetic counseling
A multi-faceted interaction between a health care professional
and an individual in which information about individual
and familial genetic risks is provided along with information
about a diagnosis, cause of the disease, risk of recurrence,
gene specific tests, treatments, support services and family
planning options.
Genetic testing
The analysis of human DNA, RNA, and/or chromosomes to detect
heritable or acquired disease-related genotypes, mutations,
phenotypes, or karyotypes. The information can be used to:
* confirm suspected clinical diagnosis
* detect a carrier for a recessive disease
* prenatal diagnosis
* newborn screening
* susceptibility testing for healthy individuals
* prediction of responsiveness to therapy
Genetics
The study of traits passed on from parent to child and variation
of those traits within and between individuals.
Genome
All the genetic material in the chromosomes of a particular
organism; its size is generally given as its total number
of base pairs. The human genome contains approximately 3
billion base pairs.
Genome map
A reconstruction of the entire set of chromosomes for a
given organism. This map shows the relative position of
the genes.
Genomic(s)
The determination and analysis of the genome (DNA) and its
products (RNAs).
Genomic imprinting
Differing expression of genetic material dependent on the
sex of the transmitting parent.
Genotype
The "type" of gene or genetic marker an individual has at
a specific location in his or her genome. A genotype consists
of 2 alleles since chromosomes come in pairs. However, a
genotype on either of the male sex chromosomes (X or Y)
consists of a single allele.
Germline cells
Cells responsible for the production of gametes. In humans,
the egg and sperm cells. Germline cells are haploid; they
contain 23 chromosomes.
Guanine
A purine base in DNA and RNA.
Haploid
A single set of chromosomes (half the full set of genetic
material) present in the egg and sperm cells of humans.
Human beings have 23 chromosomes in their reproductive cells.
Haplotype
Combinations of two or more gene variants; can be within
the same gene, within same group of genes on one chromosome
or on different chromosomes.
Heterozygote
An individual who has two different alleles at a particular
locus on the same pair of chromosomes.
High-throughput screening
A method of screening a large number of compounds against
drug targets. These screenings are usually performed in
96-well plates using automated technologies.
Histones
Type of protein rich in lysine and arginine found in
association with DNA in chromosomes.
Homologous chromosomes
Chromosomes containing the same linear sequence of genes,
one derived from one parent, and the other derived from
the other parent.
Homozygote
An individual who has two similar alleles at a particular
locus on the same pair of chromosomes.
Human genome project
A major international collaborative effort to map and sequence
the entire human genome of 3 billion base pairs. The ultimate
goal is to discover all of the more than 30,000 human genes
and render them accessible to further study. It is now scheduled
to complete the full sequence by the end of 2003, 2 full
years ahead of earlier projections.
Hybridization
The process of joining two complementary strands of DNA
or one each of DNA and RNA to form a double stranded molecule.
Identified DNA sample
GlaxoSmithKline term used to describe a sample from clinical
trials that is labeled with the patient number from the
clinical trial. These samples are used for genetic research
as defined in the clinical trial protocol.
Indel
A polymorphism caused by insertion or deletion.
Infectious disease
Diseases predominantly influenced by environmental exposures
to a specific bacteria or virus. Genes can make us more
prone to infection or determine how sick we get when infected.
Example: some individuals who are HIV positive never develop
AIDS because of their genetic makeup. Efforts are underway
in the scientific community to identify treatment strategies
based on this finding.
Informed consent
The International Conference on Harmonization on Good Clinical
Practice defines informed consent as: a process by which
a subject voluntarily confirms his or her willingness to
participate in a particular trial after having been informed
of all aspects of the trial that are relevant to the subject's
decision to participate. Informed consent is typically documented
by means of a written, signed and dated informed consent
form.
Intron (intervening sequence)
Region of DNA which, although located within a gene, does
not specify a gene product. Introns are spliced out during
transcription. At present the function of an intron is not
known.
In vitro
In the laboratory; literally, "in glass".
In vivo
In the normal body or animal; literally " in the living
organism".
Karyotype
A photomicrograph of an individual's chromosomes arranged
in a standard format showing the number, size, and shape
of each chromosome type; used in low-resolution physical
mapping to correlate gross chromosomal abnormalities with
the characteristics of specific diseases.
Linkage
The proximity of two or more markers (genes or polymorphisms)
on a chromosome; the closer together the markers are, the
lower the probability that they will be separated by genetic
recombination during meiosis and, therefore, the greater
probability that they will be inherited together.
Linkage disequilibrium
The occurrence together on the same chromosome of specific
alleles at closely linked loci more frequently than would
be expected by chance. Because linkage disequilibrium is
a function of the distance between loci, it can be used
to help infer the order of genes on chromosomes. Using markers
on either side of a gene can increase the accuracy of predicting
the gene's presence, since two chance crossovers would be
very unlikely.
Linkage map
A map of the relative positions of genetic loci on a chromosome,
determined on the basis of how often the loci are inherited
together. The distances are measured in centi-Morgans.
Locus (plural, loci)
The location on a chromosome of a gene or other chromosome
marker.
LOH (loss of heterozygosity)
The loss of a portion of a chromosome in somatic cells.
Since only one of the two copies of the affected chromosomal
region originally present in an individual's genome will
remain in cells which have undergone LOH, all polymorphic
markers within the region will appear to be homozygous;
i.e. these cells will have lost heterozygosity for these
markers. Comparison of marker genotypes in a population
of cells that are suspected of having undergone LOH with
genotypes of normal tissue from the same individual allows
for the identification of LOH, and for mapping the extent
of the loss. LOH is frequently seen in cancer calls, where
it is thought to contribute to the process of tumorigenesis
due to the loss of tumor suppressor genes.
Marker
An identifiable physical location on a chromosome (e.g.,
restriction enzyme cutting site, gene) whose inheritance
can be monitored. Markers can be either expressed regions
of DNA (genes), or some segment of DNA with no known coding
function but whose pattern of inheritance can be determined.
Medicine Response Profile
A test or set of tests which indicate the likely response
(positive or negative) to a drug.
Meiosis
The process by which the diploid chromosome number (46 chromsomes
= 23 pairs in humans) is reduced to haploid (23 unpaired
chromosomes) during the formation of gametes. Meiosis involves
a single round of DNA replication followed by two rounds
of cell division. During the first meiotic division, homologous
chromosomes pair, allowing recombination to occur and ensuring
that one member of each pair segregates into each of the
daughter cells.
Mendelian
Diseases or traits that are the result of a single mutant
gene that has a large effect on phenotype, and that are
inherited in simple patterns similar to, or identical with,
those described by Gregor Mendel. Modes of inheritance reveal
whether a Mendelian trait is dominant or recessive and whether
the gene that controls it is carried on an autosome or a
sex chromosome. Examples of Mendelian diseases: cystic fibrosis,
sickle cell disease, Huntington Disease, and hemophilia.
Microarrays
Two-dimensional array in which genes or gene fragments are
aligned in order to allow them to be probed in a high-throughput
manner.
Microsatellite markers
Polymorphic variants of DNA sequences resulting from variation
in number of tandem repeats of short (2-6 base pair) sequence
motifs.
Mitosis
The process of cell division which produces daughter cells
that are genetically identical to each other and to the
parent cell.
Monogenic
A disease or trait caused by variation in a single gene.
Mutant
A gene that has undergone a change or mutation.
Mutation
A change in the genetic material of an individual. A mutation,
which occurs in the egg or sperm, is an inheritable change;
a change which occurs in the somatic cells is not inheritable.
Changes found in 1% or more of the population are called
polymorphisms. Changes found less frequently than 1% are
called mutations or variants.
Non-coding region
Regions of a gene that are not translated into protein.
These include introns, non-coding sequences at the beginning
and end of the mRNA produced from the gene, and control
regions, such as promoter sequences.
Non-identified DNA samples
A GlaxoSmithKline term for samples obtained from volunteers
for genetic research where the donor of the sample cannot
be identified.
Nucleic acid
A large molecule composed of nucleotide subunits. DNA and
RNA are nucleic acids.
Nucleotide
A subunit of DNA or RNA consisting of a nitrogenous base,
a sugar (deoxyribose or ribose in DNA or RNA, respectively),
and a phosphate molecule. DNA and RNA are long, linear arrays
of nucleotides formed by linking the phosphate of one nucleotide
to the sugar of the next.
Nucleus
The cellular organelle in eukaryotes that contains the genetic
material.
Oligonucleotide
A chain of a few nucleotides.
Oncogene
A gene, one or more forms of which is associated with cancer.
Many oncogenes are involved, directly or indirectly, in
controlling the rate of cell growth.
Peptide
A short sequence of amino acids connected by peptide bonds.
Peptide bond
A covalent bond which forms between the amino group of one
amino acid and the carboxyl group of another amino acid.
PCR (polymerase chain reaction)
A laboratory technique that permits a small DNA section
located between two fixed points on the DNA molecule to
be duplicated many times, yielding many copies of that DNA
section.
Pharmacogenetics
The study of variability in drug response due to hereditary
factors in different populations.
Pharmacogenomics
The determination and analysis of the genome (DNA) and its
products (RNAs) as they relate to drug response.
Phenotype/Trait
The observable or measurable characteristics (physical,
biochemical, physiological) of an individual which results
from an interaction of their genes with their environment.
Polygenic disorder
Genetic disorder resulting from the combined action of alleles
of more than one gene. Although such diseases are inherited
they depend on the simultaneous presence of several alleles;
thus the hereditary patterns are usually more complex than
those of single gene disorders. Example: cardiovascular
disease, Alzheimer disease.
Polymorphism
Difference in DNA sequence among individuals that occurs
in 1% or more of a population.
Polypeptide
An arrangement of amino acids joined together by peptide
bonds.
Positional cloning
The localization of a gene to a particular region of a chromosome
which then leads to its isolation.
Primer
A short pre-existing polynucleotide chain to which new deoxyribonucleotides
can be added by DNA polymerase.
Promoter
A site on DNA to which RNA polymerase will bind and initiate
transcription.
Probe
A labeled, single-stranded DNA/RNA fragment, which hybridizes
with, and thereby detects and locates, complementary sequences
among DNA/RNA fragments on, for example, a nitrocellulose
filter.
Protein
A large molecule composed of one or more chains of amino
acids in a specific order; the order is determined by the
base sequence of nucleotides in the gene coding for the
protein. Proteins are required for the structure, function,
and regulation of the body's cells, tissues, and organs.
Examples: enzymes, antibodies.
Pseudogene
Genes that are similar in DNA sequence to coding genes but
that have been altered so that they cannot be transcribed
or translated.
Purine
A nitrogenous base with fused five- and six-member carbon
rings. Examples: adenine and guanine are purines.
Pyrimidine
A nitrogenous base with a six-member carbon ring. Examples:
cytosine, uracil, and thymine are pyrimidines.
Recessive
A disease or trait that is expressed only when the mutation
occurs in both genes of a gene pair. Males and females are
equally likely to be affected in the case of autosomal recessive
diseases, which result from a gene (allele) residing on
an autosomal chromosome. The disorder can appear suddenly
with no prior history of it in a family. Examples: cystic
fibrosis, sickle cell anemia, color blindness in males.
Recombination
Physical exchange of portions of a homologous pair of chromosomes.
Recombination occurs during meiosis when homologous chromosomes
are aligned in close proximity. It usually produces recombinant
chromosomes that contain a complete complement of genes,
but with a novel recombination of the alleles of genes that
is different from either of the parental chromosomes. Recombination
creates genetic diversity by making new combinations of
existing alleles. The frequency of recombination between
two loci is what is measured in a genetic mapping experiment.
Recombinant DNA
A DNA molecule formed by joining two molecules of DNA from
different sources using a variety of laboratory methodologies.
Example: A human gene may be joined to a small bacterial
chromosome, termed a plasmid. This allows the resulting
recombinant DNA molecule to be propagated in bacteria in
the laboratory, producing large quantities of the isolated
human gene for study.
Restriction enzyme
A protein that recognizes specific, short nucleotide sequences
and cuts DNA at those sites. Over 400 such enzymes that
recognize and cut over 100 different DNA sequences have
been identified in different species of bacteria.
RFLP (restriction fragment length polymorphism)
Variation between individuals in DNA fragment sizes cut
by specific restriction enzymes; polymorphic sequences that
result in RFLPs are used as markers on both physical maps
and genetic linkage maps. RFLPs are usually caused by mutation
at a cutting site.
RNA
Ribonucleic Acid. A single stranded molecule that consists
of a sugar (ribose), phosphate group, and a series of bases
(adenine, cytosine, guanine, and uracil). There are several
types of RNA: messenger RNA (mRNA), transfer RNA (tRNA),
ribosomal RNA (rRNA) and other small RNAs, each serving
a different purpose.
Ribosomes
Large, multi-subunit, macromolecular assemblies composed
of specialized RNA and protein. The site translation of
mature messenger RNA into amino acid sequences.
Sequencing
Determination of the order of nucleotides (base sequences)
in a DNA or RNA molecule or the order of amino acids in
a protein.
Sex chromosomes
The X or Y chromosome in human beings that determines the
sex of an individual. Females have two X chromosomes in
diploid cells; males have an X and a Y chromosome.
Single gene disorder
An hereditary disorder caused by a mutant allele of a single
gene. Example: Huntington disease.
SNP (single-nucleotide
polymorphism)
A SNP is a single-base variation that occurs about every
1,000 bases along the three billion base pairs of the human
genome. The most common SNP is a change from cytosine to
thymine (C — T) on one strand of DNA, with a change
from guanine to adenine (G — A) on the complementary
strand.
SNP Consortium (TSC)
A group of thirteen companies (10 pharmaceutical companies),
five academic centers, and one charitable trust to fund
the mapping of 300,000 SNPs by the year 2002. The pharmaceutical
companies are: GlaxoSmithKline, AstraZeneca, Pfizer, Bristol-Meyers
Squibb, Novartis, Bayer, F. Hoffman-La Roche, Hoechst Marion
Roussel, and Searle. The other companies are IBM, AP Biotech
and Motorola. The academic centers are Washington University
Medical College, Stanford Human Genome Center, Whitehead
Institute at MIT, Sanger Center, Cold Spring Harbor Laboratories.
The trust is the Wellcome Trust.
SNP LD profile
A combination of a SNP profile and a linkage disequilibrium
map that could potentially be used for positional cloning
of disease genes, or predicting disease susceptibility,
or predicting an individual's response to a medicine.
SNP map
A collection of single nucleotide polymorphisms that can
be superimposed over the existing genome map, creating greater
detail, and facilitating further genetic studies and analysis.
Somatic cells
Cells of the body other than those of the gamete-forming
germ line. Somatic cells are diploid; they contain 46 chromosomes.
Susceptibility gene
A gene that confers a risk to develop a disease, but is
not necessary or sufficient by itself to cause the disease.
It can also contribute to age of onset, severity, as well
as protection against developing the disease.
Tandem repeat sequences
Multiple copies of the same base sequence on a chromosome.
Telomere
The most distal portion of a chromosome arm. Telomeres contain
a 10-15 kb sequence composed of tandem repeats of a 6 base
pair DNA sequence. These sequences appear to be important
in protecting the ends of chromosomes from degradation.
Thymine
A pyrimidine base in DNA (replaced by Uracil in RNA). It
base pairs with adenine.
Transcription
The synthesis of an mRNA copy from a sequence of DNA. The
first step in gene expression.
Translation
The process whereby genetic information from messenger RNA
is translated into protein.
Transposon
A mobile genetic element able to replicate and insert a
copy of itself at a new location in the genome.
Uracil
A pyrimidine base normally found in RNA but not DNA; uracil
is capable of forming a base pair with adenine.
Variants
Alleles that are rare; they are found in less than 1% of
a population. Some mutations are variants.
VNTRs (variable number tandem repeats)
Differences in the copy number of repeated sequences. VNTR
sequences are significantly longer than microsatellites.
(6-20+ base pairs in length) and are flanked by restriction
sites.
Wild type
The form of a gene or allele that is considered "standard"
or most common.
X chromosome
One of the sex chromosomes. Females have two X chromosomes;
males have one X chromosome.
Y chromosome
One of the sex chromosomes. Males have one Y chromosome;
females have none.
Zygote
A fertilized egg.
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