Allele: One of a number of alternative forms of the same gene.

Base: A basic component of DNA. There are four bases that form the rungs of the DNA double helix: adenine (A), thymine (T), cytosine (C) and guanine (G). The order of these bases determines a person’s genetic code. Bases are sometimes used interchangeably with nucleotides. 

Cell: The building blocks of the body. Cells provide structure, take in nutrients from food, and perform all the body’s important functions. Almost all of the body’s cells contain a complete copy of a person’s unique genetic code.

Codon: A sequence of three bases (A, C, G, and T) that corresponds to a specific amino acid or a stop codon.

Chromosome: A structure found in most cells that contains all of a person’s DNA, tightly wound around proteins called histones.

Consanguineous: Having a common ancestor. For example, parents who are also first cousins.

DNA: Deoxyribonucleic acid – the substance that contains a person’s unique genetic instructions

Gene: A section of DNA that is the basis of inheritance. Each gene contains the code for a particular molecule (usually a protein) that the body needs to function properly.

Genetic variant: A change in a person’s DNA sequence.

Germline variant: A variant present in egg or sperm cells that is able to be passed from parent to child

Heterozygous: If a person is described as being heterozygous for a particular genetic variant, this means that the variant only affects one copy of a gene. People who are heterozygous for a dominant condition will be affected, while people heterozygous for a recessive condition will be able to pass that condition on to their children, but usually not be affected themselves. 

Homozygous: A person is homozygous for a particular genetic variant when that variant is present in both copies of a gene.

Inversion: Two breaks in the chromosome allow for a chromosome segment to be removed and flipped 180 degrees before rejoining the broken ends. This reverses the DNA sequence (order of bases) within the chromosome.

Mitochondrial DNA (mtDNA): Mitochondria are structures in cells that produce energy. They contain a small amount of DNA (mtDNA) which codes for 37 genes – these are required for normal mitochondria function. Other genes necessary for mitochondrial function are also found elsewhere in a person’s genome. The mitochondria, and thus mitochondrial DNA, are passed only from mother to offspring.

Nucleotide: A basic building block for DNA and RNA. A nucleotide is composed of a base, phosphate group and sugar molecule (deoxyribose in DNA). Nucleotides are sometimes used interchangeably with bases. 

Pathogenic variant: Variants that have proven to be associated with a disease. These type of variants are generally rare in populations.

Somatic variant: A variant acquired at some point during a person’s lifetime that is not able to be passed from parent to child. These variants are not found in the egg or sperm cells.

Stop codon: A codon that instructs for termination of a polypeptide chain (protein) during translation.

Transcription: Transcription is the first step of gene expression, in which a particular segment of DNA is copied into RNA

Translation: The process of reading codons in the messenger RNA sequence to produce a chain of amino acids (protein). This step occurs after transcription.

Translocation: A chromosome segment is broken off and reattached to a different chromosome


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