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Different copies of the ame gene. You inherit one allele from your mother and one from your father.
Autosome Any chromosome other than the sex chromosomes.
Structures found in the cell which contain a package of the cell's DNA. 
Chromosomes come in pairs and a normal human cell has 46 chromosomes: 22 pairs of autosomes and two sex chromosomes.
Sub-unit of DNA which codes for a specific protein. The translated protein performs a unique function in the cell.
The entire group of genes possessed by an organism.
The actual genes carried by the individual, not necessarily what is expressed (as opposed to phenotype).
Organism carrying two different alleles of the same gene (adjective is heterozygous).
Organism carrying two of the same allele of a gene (adjective is homozygous).
Incomplete penetrance
Occurs when a gene only sometimes results in disease.
Mutation A change in the nucleotide sequence of a gene.
A sub-unit of DNA or RNA, consisting of one chemical base plus a phosphate molecule and a sugar molecule. There are four different chemical bases: adenine, guanine, cytosine, and thymine. Nucleotides pair up via hydrogen bonds in the DNA strand to form the double helix.
When a gene is inherited in a recessive fashion, the mutated allele will not be expressed if there is a normal copy. You must possess two recessive alleles to show the mutated phenotype. eg. cystic fibrosis (opposite of dominant).
Penetrance The likelihood that a given gene will actually result in disease.
Phenotype The physical characteristics into which genes are translated (eg. a carrier of the CF gene and a non-carrier have the same phenotype).
Polymorphism Different forms of the same gene or allele.
Sex chromosome Chromosomes that contain the genes that distinguish the sexes. In humans, females have two "X" chromosomes, and males have one "X" and one "Y" chromosome.
Tumour-suppressor gene 
A gene that normally suppresses cell growth but, if missing or mutated, a cell is allowed to divide uncontrolled.