This glossary is an adaptation of entries combined from:
- Understanding Our Genetic Inheritance. The U.S. Human Genome Project: The First Five Years. 1991-1995 from US DOE/ER-0452P
- Mapping the Pig Genome: A Practical Primer by Lawrence B. Schook. University of Minnesota AES Report 234-1995.
-A-
- Allele
- One of the inherited forms of a particular gene or stretch of DNA. An animal carries two
alleles for each gene, one from each parent.
- Autosomal
- Referring to chromosomes other than the sex chromosome.
-B-
- Bacterial Artificial Chromosomes (BAC)
- Plasmid vectors designed to clone large DNA fragments (100 Kb or more,
on average) in Escherichia coli.
- Base pair
- Two nucleotides (adenosine and thymidine or guanosine and cytidine)
held together by weak bonds. Two strands of DNA are held together in the
shape of a double helix by the bonds between base pairs.
-C-
- Centimorgan (cM)
- A unit of measure of recombination frequency. One centimorgan
is equal to a 1 percent chance that a genetic locus will be separated from
another marker due to recombination in a single generation.
A unit of measurement that describes how tightly linked two points on a chromosome
are -- i.e., how likely they will be separated by crossover during meiosis.
One hundred centimorgans, or one Morgan, takes its name
from biologist Thomas Hunt Morgan, who first observed the crossover phenomenon in
the early part of this century.
- Chromosome
- A rod-like structure found in the cell nucleus and containing the
genes. Chromosomes are composed of DNA and proteins. They can be seen in
the light microscope during certain stages of cell division.
They are discrete collections of DNA that contain the information that is passed from one
generation to the next. Each animal species has a characteristic number of chromosomes.
- Chromosome number
- Human chromosomes are ordered and named according to
size, the largest being chromosome 1 and the smallest chromosome 22. Chickens have
39 chromosome pairs; however, only the largest ones can be clearly differentiated
by their size or banding pattern.
- Co-dominant alleles
- Alleles of a given gene or marker in which properties due to both alleles
can be observed in a heterozygote. Microsatellites and RFLP markers are usually
co-dominant.
- Coinheritance
- The passing along of two traits (or, on a genetic level, of two segments of DNA) together.
Loci that are tightly linked have a high probability of being coinherited.
- Contigs
- Groups of overlapping clones representing a continuous region of
DNA.
- Cosmid
- Plasmid vector that carries 30-50 Kb inserts and can be inserted into
Escherichia coli using in vitro packaging with bacteriophage lambda-infected cells.
- Crossover
- A process that occurs during the creation of eggs and sperm in which chromosomes
exchange segments of DNA, enhancing the possible variability in offspring.
- Cytological mapping
- Mapping of genes using DNA probes that bind to the
chromosome at the site of the gene and are visible in a light microscope.
-D-
- Diploid
- Containing a full set of chromosomes, half of which were donated by the mother and
the other half from the father.
- DNA (deoxyribonucleic acid)
- The molecule that encodes genetic information.
DNA is a double-stranded molecule held together by weak bonds between pairs
of nucleotides on opposite strands. There are four nucleotides in DNA:
adenosine (A), guanosine (G), cytidine (C), and thymidine (T). In nature, base
pairs form only between A and T and between G and C, thus the sequence of
each single strand can be deduced from that of its partner.
- DNA sequence
- The order of base pairs whether in a stretch of DNA, a gene, a
chromosome, or an entire genome.
- DNA sequencing
- Determining the sequence of the nucleotides in DNA.
- Dominant allele
- An allele which leads to the identical phenotype whether homozygous or
heterozygous in combination with a recessive allele.
- Double helix
- The shape in which two linear strands of DNA are bonded
together.
-E-
- Economic trait loci (ETL)
- Loci that code for traits of economic interest to producers (litter size, disease resistance,
percent lean, etc.).
- Electrophoresis
- A laboratory process that separates large molecules (such as DNA
fragments or proteins) from a mixture of similar molecules. An electric current
is passed through a medium containing the mixture, and each kind of molecule
travels through the medium at a different rate, depending on its electrical charge
and size. Separation is based on these differences.
- Epistasis
- The process in which one gene causes another (other than its allele) to not be expressed.
- Escherichia coli (E. coli)
- A common intestinal bacterium geneticists have used
for many studies.
-F-
- Fingerprinting
- Generation of a complex polymorphic pattern of DNA framents that can
distinquish the DNA of a specific individual or strain. (See VNTR)
-G-
- Gene
- The fundamental physical and functional unit of heredity. A gene is an
ordered sequence of nucleotides located in a particular position on a particular
chromosome.
- Genes
- Stretches of DNA within a chromosome that contain specific information needed by
the cell to make the proteins that control the growth, development and function of the
organism.
- Gene mapping
- Determining the relative locations of different genes on
chromosomes.
- Genetic code
- The sequence of nucleotides, coded in triplets along the mRNA,
that determines the sequence of amino acids in protein synthesis. The DNA
sequence of a gene can be used to predict the mRNA sequence, and the genetic
code can in turn be used to predict the amino acid sequence.
- Genetic 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. Distance is measured in centimorgans.
- Genome
- All the genetic material in the chromosomes of a particular organism;
its size is generally given as the total number of base pairs.
The set of DNA molecules that specify the inherited characteristics of an animal. With
a few minor exceptions, every cell in an animal's body carries the entire genome within
its nucleus.
- Genome projects
- Research and technology development efforts aimed at
mapping and sequencing some or all of the genome of human beings and other
organisms.
- Genotype
- The genetic makeup of an organism.
- Genotyping
- The process of identifying which alleles an organism carries for a specific trait or traits.
-H-
- Haploid
- Containing half the normal number of chromosomes. Eggs and sperm are haploid.
- Hemizygous
- A diploid organism containing only one copy of a given gene (usually due to
location on a sex chromosome in the heterogametic sex or due to a deletion).
- Heterogametic
- An individual which contains two different sex chromosomes.
- Heterosis
- The tendency of crossbred organisms to be better than either of their parents.
- Heterozygosity
- The degree to which a gene occurs in different alleles in different animals.
- Heterozygous
- In a diploid organism or cell, having two different alleles of a given gene.
- Homogametic
- An individual which contains two identical sex chromosomes.
- Homozygous
- In a diploid organism or cell, having two identical alleles of a given gene.
- Human Genome Initiative
- An initiative whose goal is to map and sequence the
human genome. The concept was first formally proposed in 1986.
- Human Genome Project
- The implementation of the concepts
proposed as the Human Genome Initiative.
- Human Genome Program
- The individual programs, such as those at DOE and
NIH, that make up the Human Genome Project.
-I-
- Informatics
- The study of the application of computer and statistical techniques
to the management of information. In genome projects, informatics includes the
development of methods to search databases quickly, to analyze DNA sequence
and to determine DNA structure from DNA sequence data.
-K-
- kb (kilobase)
- 1000 contiguous bases or base pairs
-L-
- Lambda phage vectors
- Vectors designed to carry foreign DNA in replicating lambda bacterial
viruses of Escherichia coli. Different vector types accept different insert sizes and
have different advantages.
- Library
- A collection of recombinant DNA clones. A genomic library contains cloned
chromosomal DNA isolated from a given organism. A cDNA library contains
complementary DNA copies of mRNA molecules isolated from a given cell or tissue.
A complete genomic library has a high probability of containing at least one
clone of any gene that one might want.
- Linkage map
- A map that describes the location of genes relative to other genes rather than as points
on specific parts of specific chromosomes.
- Locus (loci)
- The location of a particular DNA segment on a chromosome.
-M-
- Major Histocompatability Complex (MHC)
- A length of DNA that contains numerous genes involved in regulating the process by
which the body distinguishes itself from invaders (bacteria, thorns, etc.).
- Marker
- A portion of DNA that appears in more than one form in a population, the variations
of which can be detected in the laboratory and so used to identify patterns of heredity.
- Marker
- An identifiable physical location on a chromosome (e.g., restriction
enzyme cutting site, gene, RFLP marker) whose inheritance can be monitored.
Markers can be expressed regions of DNA (genes) or some segment of DNA
with no known coding function but whose pattern of inheritance can be
determined.
- Marker-assisted selection (MAS)
- Selection of animals for propagation or culling based on the presence or absence of
genetic markers that are linked to desired or undesired characteristics.
- Meiosis
- The process that creates haploid egg and sperm cells.
- Messenger RNA (mRNA)
- A class of ribonucleic acid (RNA) whose role is to
carry the genetic code from the chromosome to the ribosome, the site of protein
synthesis.
- Microsatellite
- A stretch of DNA that contains a repeating nucleotide pattern, the length of which is
polymorphic. Microsatellites are used as markers.
-N-
- Nucleotide
- A subunit of DNA or RNA consisting of a nitrogenous base
(adenine, guanine, thymine, or cytosine in DNA; adenine, guanine, uracil, or
cytosine in RNA) a phosphate molecule, and a sugar molecule (deoxyribose in
DNA and ribose in RNA). Thousands of nucleotides are linked to form the
DNA or RNA molecule.
- Nucleotide
- One of four similar molecules that, when linked together into DNA, can create the code that
determines an organism's inherited characteristics.
-P-
- PAC
- BAC type vectors based on the P1 phage replicon
- P1 phage vectors
- Vectors that carry about 90Kb of foreign DNA and grow as plasmids in Escherichia coli.
Based on P1 phage replication and packaging systems.
- Phenotype
- The expressed characteristics of an organism. Not all genes of the genotype affect the
phenotype.
- Physical map
- A map of the locations of identifiable landmarks on DNA (e.g.,
restriction enzyme cutting sites, genes, RFLP markers). A physical map may also
be a set of overlapping clones, called a contig. Distance is measured in base
pairs. For the human genome, the lowest-resolution physical map is the banding
patterns of the 24 different chromosomes; the highest-resolution map would be
the complete nucleotide sequence of the chromosomes.
- Polymerase chain reaction (PCR)
- An enzymatic reaction that precisely and
rapidly amplifies a small segment of DNA millions of times or more. The
reaction can start with one molecule of DNA.
- Polymerase chain reaction (PCR)
- A biochemical technique that allows laboratory workers to produce millions of copies
of a relatively short (100-300 nucleotide) sequence of DNA. PCR is an invaluable
technique for distinguishing which allele of a microsatellite marker is present in a blood
or tissue sample.
- Polymorphism
- One of the various forms of a gene, or a genetic marker.
- Probe
- A selected labeled nucleic acid used to hybridize to a DNA sample and
identify or isolate those sequences to which it hybridizes.
- Pulsed-field gel electrophoresis (PFGE)
- A type of gel electrophoresis in which
pulses of current are applied to the sample at various angles, enabling scientists
to separate and order by size extremely large segments of DNA.
-Q-
- Quantitative Trait Loci (QTL)
- Genes that contribute to the expression of a trait that is expressed on a continuum
(e.g., weight gain).
-R-
- Radiation hybrid
- A somatic cell hybrid that contains pieces of human
chromosomes generated by irradiation.
- Random Amplified Polymorphic DNA (RAPD)
- DNA fragments that are polymorphic in size generated by PCR using one or
two randomly selected oligonucleotide(s) or primer(s). RAPDs are dominant
markers and RAPD patterns can be used in strain identification.
- Recessive allele
- An allele of a given gene which does not affect the phenotype when present
together with a dominant allele of that gene in a heterozygote. A recessive
phenotype is expressed when the allele is homozygous.
- Recombinant DNA
- The hybrid DNA produced in the laboratory by joining
pieces of DNA from different sources.
- Recombinant DNA technology
- Techniques for cutting apart and splicing
together pieces of DNA from different sources.
- Recombination
- The process by which portions of DNA are exchanged or
deleted. Recombination occurs naturally between or within chromosomes,
particularly during the formation of sperm and egg cells.
- Reference families
- Groups of related but genetically varied animals (usually three generations) which are used
by genome mappers to create a linkage (cM) map.
- Resource family
- A group of related animals that show characteristics of interest such as good marbling and
large litter size, for which researchers have both genetic material and information about
which of the related individuals exhibit the characteristic. Useful for mapping QTL and in
developing markers for MAS.
- Restriction enzyme
- An enzyme that recognizes a specific base sequence (usually
four to six base pairs in length) in a double-stranded DNA molecule and cuts
both strands of the DNA molecule at every place where this sequence appears.
- Restriction enzymes
- Proteins that cleave DNA at the site of a particular nucleotide sequence. Because they only
work when their particular, recognizable sequence is present, restriction enzymes can be
used to identify which form of a polymorphic DNA segment an animal has inherited.
- Restriction enzyme cutting site
- A specific nucleotide sequence of DNA at
which a restriction enzyme cuts the DNA. Some sites occur frequently in DNA
(e.g., every several hundred base pairs), others much less frequently (e.g., every
10,000 base pairs).
- Restriction Fragment Length Polymorphism (RFLP)
- The presence of two or
more variants in the size of DNA fragments from a specific region of DNA that
has been exposed to a particular restriction enzyme. These fragments differ in
length because of an inherited variation in a restriction enzyme recognition site,
or an insertion or deletion of DNA between two restriction sites.
- Restriction Fragment Length Polymorpbism (RFLP)
- A polymorphic DNA segment that influences how a DNA segment cleaves when treated
with a restriction enzyme. RFLPs are used as markers.
-S-
- Sequence-tagged site (STS)
- A short DNA sequence that uniquely identifies a
mapped gene or other marker. The order and spacing of these sequences
comprise an STS map.
- Simple Sequence Repeats (SSR)
- Also referred to as microsatellites.
- SLA Types-major histocompatability
- A genetic test for determining the alleles of genes controlling immune responses.
- Stable isotopes
- Nonradioactive isotopes.
-T-
- Technology transfer
- The process of converting scientific knowledge into useful
products.
-V-
- Variable Number of Tandem Repeats (VNTR)
- DNA fragments that hybridize to a repetitive "fingerprinting" DNA probe
and which vary in size due to different numbers of tandem copies. VNTR
polymorphisms can be used in strain identification and as genetic markers.
- Variance
- Ranges in phenotype that result from genetic and environmental parameters.
-W-
- W-chromosome
- The smaller sex chromosome in organisms like birds where the female is
heterogametic (ZW).
-X-
- X-chromosome
- A sex chromosome. Normal human females have two X
chromosomes in each cell, while normal human males have one X and one Y
chromosome in each cell. With other organisms, in which the female is
heterogametic, the convention is to use Z and W to distinguish sex
chromosomes.
-Y-
- Y-chromosome
- A sex chromosome. Normal human males carry one X
chromosome and one Y chromosome in each cell. (See also: X, Z, W chromosomes.)
- Yeast artificial chromosome vectors (YAC)
- Cloning vectors for use in Saccharomyces cerevisiae that contain those portions
of yeast chromosomal DNA needed for replication and maintenance and with
which foreign DNA can be cloned. YAC vectors can accommodate foreign
DNA fragments up to 1 million base pairs in size.
-Z-
- Z-chromosome
- The larger sex chromosome in organisms like birds in which the female
is heterogametic (ZW) and males are homogametic (ZZ).