Glossary
Allele: alternative forms of the same gene
Allelic heterogeneity: the existence of many different disease-causing alleles at a locus
Amino acid: building block of proteins, there are 20 of these
Autosome: any chromosomes other than the sex chromosomes X and Y
Back cross: where a son or daughter is mated back to one of its parents or grandparents
Candidate gene: a gene that is suspected of being the disease gene
Chromosome: Contains the main information carrying single piece of double stranded
DNA
Codon: a nucleotide triplet
Congenital: existing at birth, not inherited
Congenital disorder: may be the result of genetic abnormalities, the inter-uterine
environment, errors of development, infection during pregnancy, chromosomal
abnormalities
Compound heterozygote: an animal with two different mutant alleles at one locus
Constitutional: a genotype, abnormality or mutation that was present in the fertilised egg
and is therefore present in all the cells of that animal
Diploid: having two copies of each type of chromosome, the normal constitution of most
human somatic cells
DNA: Deoxyribonucleic acid
Dominant negative: a mutant gene whose product can inhibit the function of the normal
wild-type gene when present in one copy
Dominant: in genetics any trait that is expressed in the heterozygous form
ES cells: Embryonic stem cells. These are cells that can differentiated into any cell type
found in the body
Exon: a segment of a gene
F1 generation: first generation progeny from a cross
F2 generation: second generation progeny from a cross
Founder effect: high frequency of a particular allele in a population because the
population was derived from a small number of founders
Frameshift mutation: a mutation that alters the normal translational reading frame by
adding or deleting nucleotides such that the normal reading frame of multiples of 3 is
altered
Genotype: the genetic constitution of an individual
Germ cells/gametes: sperm cells and egg cells
Haploid: describing a cell (typically a gamete) which has only a single copy of each
chromosome
Haplotype: a series of alleles found next to each other (linked) on a single chromosome
Hemizygous: having only one copy of a gene or DNA in diploid cells. Males are
hemizygous for most genes on the sex chromosomes
Heterozygote: an individual having two different alleles at a given locus
Homozygote: an individual having two identical alleles at a particular locus
Inbreeding: marrying a blood relative
Intron: non-coding DNA that separates exons
Karyotype: summary of chromosome constitution of cell or individual (male human is
46,XY; female human is 46, XX; male cat is 36, XY; female cat is 46,XX)
Linkage disequilibrium: a statistical association between particular alleles at separate
but linked loci
Matrilineal inheritance: inheritance from just the mother to the children of either sex
(mitochondrial inheritance)
Missense mutation: a change in the DNA that results in the change of one amino acid for
another
Nonsense mutation: a mutation that results in the substitution of an amino acid for a stop
codon
Nucleotide: building blocks of DNA and RNA, there are four for RNA and four for DNA
Penetrance: the frequency with which a genotype manifests itself in a given phenotype
Phenotype: the observable characteristics of a cell or organism
Point mutation: usually a single nucleotide change
Polymorphism: the existence of two or more alleles/variants present at a significant
frequency in the population
Recessive: a character is recessive if it is manifest only in the homozygote form i.e. has to
be inherited from both parents
RNA: ribonucleic acid
Semi-dominant: an allele in the heterozygote produces a phenotype intermediate
between the wild type and the homozygote
Sibs: siblings (brothers and sisters)
Somatic cell: any cell in the body apart from the gametes
Synonymous substitution: a substitution that replaces one codon for another but does
not change the amino acid
Stop codon or termination codon: There are three of these. They act as protein full
stops
Transition: Type of mutation in DNA where G and A are swapped or C and T are
swapped
Transversion: Type of mutation in DNA where G and A replaced by C or T and vice
versa
X inactivation: the inactivation of one of the two X chromosomes in females