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Table 1 Glossary of useful “technical” terminology

From: Migraine headache: a review of the molecular genetics of a common disorder

Genotype It is the state of the pairs of alleles present at one or more loci associated with a given trait
Phenotype It refers to the observable state of the trait (e.g. blue eyes, red hair)
Dominant It refers to mutations at a given locus occurring in a heterozygote status
Recessive It refers to mutations occurring in homozygosity
Loss of function It refers to the functional consequences of mutations on protein function. It indicates that the amount of normal protein is decreased (as seen in inborn errors of metabolism)
Gain of function It refers to the functional consequences of mutations on protein function. It indicates the case of abnormal gene dosage, as in trisomy of chromosome 21, or when mutations result in a negative effect on normal protein function)
Inappropriate expression It indicates abnormal protein expression as can often be seen for oncogenes
Incomplete or no penetrance It is the case of individuals who may only partly display the characteristic disease phenotype
Variable expression It is the variable consequence of gene mutation on clinical phenotype. It is believed to be due to allelic/locus heterogeneity or to the effects of modifier genes (or even environmental or metabolic factors)
Incomplete dominance It refers to the blending of traits that occurs when two different alleles of a gene pair occur together and neither is dominant
Co-dominance It refers to the condition in which both the alleles in a gene pair are fully expressed, without one being dominant over the other (this results in a third, novel phenotype)
Polygenic inheritance This non-Mendelian inheritance is determined by the alleles of more than one gene. The more genes involved, the greater the number of intermediate phenotypes that will be produced. This modality occurs with a sort of additive effect and the picture becomes even more variegated when the multiple genes interact with environmental factors
Cytoplasmic transmission Another aspect of non-Mendelian inheritance that occurs in the case of variants in the mitochondrial genome (mtDNA). MtDNA is a circular, double-stranded, 16.569 base-pair molecule of DNA which encodes 13 essential polypeptides for the oxidative phosphorylation (OXPHOS) system, two ribosomal RNAs, and 22 tRNAs. The mitochondrial genome is strictly maternally inherited and there are several hundred to several thousands of copies within a single cell [10]. The number of copies present varies between different cell types, depending on the energy demand within the tissue, and it is extremely high in neurons [10]