Acrocentric. Term used to describe a chromosome in which the centromere lies close to one end, producing one long and one short arm. Allele (allelomorph). Alternative form of a gene occupying the same locus on a particular chromosome.
Aneuploid. Any chromosomal number that is not the exact multiple of the normal haploid number.


Any chromosome that is not a sex chromosome or mitochondrial chromosome; there are 22 pairs of autosomes in humans.
Bacteriophage. A bacterial virus. These are modified and used as vectors for DNA cloning.
cDNA. DNA synthesized from an mRNA template by the enzyme reverse transcriptase.

Centromere. The point at which two chromatids of a chromosome are joined and also where the spindle fibres become attached during mitosis and meiosis.
Character (trait). An observable phenotypic feature of an individual.
Chromatid. One of the two strands, held together by the centromere, that make up the chromosome as seen during cell division.
Chromosomal aberration. An abnormality in the number or structure of a chromosome.
Chromosome. A thread-like body containing DNA and protein, situated in the nucleus, and carrying genetic information.
Clone. Cells having the same genetic constitution and derived from a single cell by repeated mitoses.
Codon. Three adjacent nucleotides in a nucleic acid that code for one amino acid.
Concordance. The occurrence of the same trait in both members of a pair of twins.
Deletion. Loss of a part of a chromosome.
Diploid. The number of chromosomes found in somatic cells, i.e. two sets.
DNA ligase. The enzyme that joins two DNA ends together.
DNA polymerase. The enzyme that replicates DNA.
Dominant. Term used to describe a trait expressed in individuals who are heterozygous for a particular gene.
Exon. A segment of a gene that is represented in the final spliced mRNA product.
Expressivity. The degree to which the effect of a gene is expressed.
Gene. Part of a DNA molecule that directs the synthesis of a specific polypeptide chain.
Gene pool. The total genetic information contained in all the genes in a breeding population at a given time.
Genetic marker. A genetically controlled phenotypic fearure used in inheritance studies.
Genetics. The science of heredity and variation.
Genome. The total amount of genetic material in the cell.
Genotype. The genetic constitution of an individual.
Haploid. The number of chromosomes found in germ cells, i.e. one set.
Heterozygote. An individual possessing two different alleles at the corresponding loci on a pair of homologous chromosomes.
Homozygote. An individual possessing identical alleles at the corresponding loci on a pair of homologous chromosomes.

Hybridization. The pairing of complementary DNA or RNA strands to give DNA-DNA or DNA-RNA strands;
for example, it is used to search for particular DNA fragments after Southern blotting.
Intron. A segment of a gene not represented in the final mRN A product because it has been removed through splicing together of exons on either side of it.
Karyotype. The number, size and shape of the chromosomes in a cell.
Linkage. The co-segregation of two unrelated DNA sequences which are physically close together on the chromosome.
Linkage disequilibrium. The association of particular alleles at two linked loci more frequently than expected by chance.
Locus. The site of a gene on a chromosome.
Metacentric. Term used to describe a chromosome III which the centromere lies in the middle.
Monosomy. A state in which one chromosome of a pair is missing.
Mosaics. Patients with two different cell lines in their constitution.
Non-disjunction. Failure of a chromosome pair to separate during cell division, resulting in both chromosomes passing to the same daughter cell.
Nucleotide. The basic unit of nucleic acids, which is made up of a pyrimidine or purine base, a pentose sugar and a phosphate group.
Oncogenes. Genes which when altered in their structure or expression contribute to the abnormal growth of cancer cells.
Penetrance. The proportion of individuals with a particular genotype who also have the corresponding phenotype.
Full penetrance occurs when a dominant trait is always seen in an individual with one such allele, or when a recessive trait is seen in all individuals possessing two such alleles.
Phenotype. The appearance of an individual, resulting from the effects of both environment and genes.
Plasmid. A simple circular DNA molecule derived from bacteria which can be modified and used as a vector for DNA cloning.
Ploidy. Term that describes the number of chromosome sets, namely 23 = haploid (1 set), 46 = diploid (2 sets) .
Polymerase chain reaction (PCR). Technique for rapid analysis of DNA. Oligonucleotide primers corresponding to each end of DNA of interest are synthesized and amplified in genomic DNA using DNA polymerase.
Positioned cloning (or reverse genetics). Methodology used to isolate genes whose protein products are not known but whose existence can be inferred from the disease phenotype. Pulsed field gel electrophoresis. Technique for separation of large fragments of DNA.
Recessive. Term used to describe a trait expressed in individuals who are homozygous for a particular gene but not seen in the heterozygote.
Restriction fragment length polymorphisms (RFLPs).
When variations in non-coding DNA sequences affect restriction enzyme cleavage sites, DNA fragments of different sizes (RFLPs) will result from enzyme digestion.
RNA polymerase. The enzyme that synthesizes RNA, based on a DNA template.
Sex linkage. Genes carried on the sex chromosomes.
‘Somy’. Term referring to the number of copies of an individual chromosome per cell, e.g. ‘trisomy’ = three copies.
Splicing. Removing the introns from an unprocessed RNA molecule.
Synteny. Term used to describe genes on the same chromosome.
Transcription. The process by which an RNA molecule is synthesized from a DNA template.
Translation. The process by which genetic information from MRNA is ‘translated’ into protein synthesis.
Translocation. The transfer of a piece of one chromosome to another non-homologous chromosome.
Trisomy. Representation of a chromosome three times rather than twice, giving a total of 47 chromosomes.
tRNA. Transfer RNA, a molecule which carries a single amino acid (depending on its anticodon) and which brings the amino acid to the ribosome.
Vector. A DNA molecule used to carry DNA regions of interest.

Further reading
Brock DJH (1993) Molecular Genetics for the Clinician.
Cambridge: Cambridge University Press.
Brostoff 1, Scadding GK, Male D & Roitt 1M (1991) Clinical
Immunology. London: Gower Medical Publishing.
Brown TA (1990) Gene Cloning: An Introduction, 2nd edn. London: Chapman and Hall.
Chapel H & Heaney M (eds) (1993) Essentials of Clinical Immunology. Oxford: Blackwell Scientific Publishers.
Conner JM & Ferguson-Smith MA (1991) Essential Medical Genetics, 3rd edn. Oxford: Blackwell Scientific Publishers.
Gelehrter TD & Collins FS (1990) The Principles of Medical Genetics. Baltimore: Williams & Wilkins.
Weatherall DJ (1991) The New Genetics in Clinical Practice,
3rd edn. Oxford: Oxford University Press.

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