Glossary
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Allele
Alleles are the different types of a single gene. One gene can have two or more alleles (multi-allelic).
topChromosome
A specific arrangement of genes and other DNA. The arrangement of DNA can differ in different organisms - bacteria have a circular shaped DNA structure called a plasmid, viruses have simple linear DNA, and humans have tightly wound up DNA which is bound to proteins and other molecules. Humans have 46 chromosomes, allocated into 23 pairs, and each one contains many genes.
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DNA
DNA stands for deoxyribonucleic acid. This is a double-stranded helix shaped molecule that encodes all the aspects of the human life (such as our physical characteristics). DNA is made up of many nucleotides joined together and consists of an "alphabet" of four bases. A (adenine) binds to T (thymine), and C (cytosine) binds to G (guanine). This is why DNA looks like a twisted ladder in pictures - the two strands are the vertical parts and the bases that bind to each other are the "rungs" of the ladder that hold it together. DNA is very long and so to fit into the cells of the body, it is packaged into chromosomes.
topDown's Syndrome
An abnormal human phenotype that includes mental retardation, slanting eyes and a small mouth are present due to a trisomy of chromosome 21.
topGene
The region of DNA that gets read (by a process called "transcription") and converted ("translated") into protein which can then be used by the body. It is the functional unit which codes for an individual's phenotype and is passed from one generation to the next. Humans are currently thought to have an estimated number of about 30,000 genes!
topGene Therapy
The process involving the correction of the effects of an abnormal gene that is reduced in function (i.e. it is inactive or makes less of its protein product than what is needed for a normal life) in cells by the addition of functional DNA that can reduce/eliminate the phenotype related to the defective gene.
TopGenome
The entire number of genes in an organism. Humans have approximately 30,000 genes in all 46 chromosomes.
TopGenotype
The specific set of alleles in an organism's genome.
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Nucleotide
A subunit of DNA or RNA. (ribonucleic acid). It is made up of one sugar molecule, at least one phosphate group molecule and one base molecule.
TopMapping
Gene mapping involves techniques showing the relative positions of genes or markers on a DNA segment (e.g. in a chromosome) and the distance between them. There are two types of maps:
A "linkage map" (or "genetic map") is determined by how often two genes on the same stretch of DNA analysed are inherited together. Distance is measured in centimorgans (cM).
A "physical map" of the genome is different to a genetic map in that it determines the locations of genes using identifiable markers, regardless of inheritance. In this case, distance is measured in base pairs (bps).
topMarfan's syndrome
A human genetic disorder that consists of an abnormal phenotype including heart defects and very long limbs & digits.
TopMarker
An identifiable DNA sequence that can be used to trace inheritance of either itself to the next generation (if it is an identifiable gene) or the inheritance of neighbouring genes (if it is not a gene but non-expressed DNA). Examples include restriction sites (areas where certain enzymes can cut) and repeats of the same base (e.g. TGCAAAAAGTA). Markers are also used in mapping techniques.
TopMolecular Imprinting
The case in which there is differential expression of a gene depending on the parent involved. Paternal imprinting means that an allele (a gene variant) inherited from the father is not expressed in offspring. Maternal imprinting is the same but for the mother's inherited allele.
TopMutation
A change in the DNA coding. There are four types of mutations: deletions (one or more base is removed from the DNA), duplications (one or a series of bases is copied in the DNA), insertions (one or more base is added to the DNA at a random place), and inversions (a chunk of DNA is flipped the opposite way round to what it should be). Changes in our DNA occur often and most are harmless, but if a particular region of DNA in a gene has a mutation, this can result in an abnormal phenotype.
TopPhenotype
An individual's observed characteristics that usually (but not always) a result of their genotype.
TopPrenatal diagnosis
The detection of foetal genetic abnormalities during pregnancy through examining foetal cells taken from the amniotic fluid, placenta or umbilical cord. Ultrasound scanning is also used to detect visible abnormalities and other properties (e.g. sex).
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Sequencing
One or more techniques that determines the exact order of the bases in DNA. The genetic "alphabet" is made up for four "letters" (bases), which - when in a particular order - makes up a gene. Different genes are encoded for by different orders of bases and make different proteins.
TopScreening
A technique that determines what alleles of a gene are present in a DNA sample from an individual. The individual does not always exhibit physical characteristics until later in their life, so screening is often used as a preventative measure or to find out what is wrong with a person when their symptoms are difficult to interpret. For example, currently screening is used on newborns to see if they have the condition PKU, which means they have to have a special diet to survive - this is not detectable in their appearance but is still crucial for their survival.
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