**Author details**

were revealed, also gene for the DFNX4 locus (i.e., *SMPX*) was identified. Another remarkable achievement obtained by the NGS technology is the identification of genes incorrectly classified

For the clinical diagnostic purpose, there are many commercial tests based on NGS, which differ in technologies and numbers of genes included. Heretofore, at least 20 commercially available tests, based on the NGS technology, focused on genetically related HL may be applied [1]. Due to the constant reduction of costs and availability, diagnostic approach based on the NGS technology in the nearest future will become a standard, which will significantly

**Autosomal dominant inheritance** (AD)—type of Mendelian inheritance of a trait in which a defective copy of a gene (localized on autosome) dominates over the normal one. For the

**Autosomal recessive inheritance** (AR)—type of Mendelian inheritance of a trait in which two copies of defective gene (localized on autosomes) are required in order for the disease to

*DFNB***1 locus**—most common locus causative for nonsyndromic hearing loss, containing

**Direct sequencing**—a technology allowing to determine the sequence of nucleotides in DNA invented in 1977 by Frederic Sanger and Alan R. Coulson, based on the chain-dideoxy termi-

**Genetic pedigree**—illustration of genetic relationship of a family, including information

**Mendelian inheritance**—type of transmission of genes according to Gregor Mendel's set of laws, also called classical inheritance. Mendelian inheritance comprises of autosomal domi-

**Mitochondrial inheritance**—non-Mendelian type of inheritance, occurring when a defective gene is located within the mitochondrial genome, inheritance of a trait encoded by this gene

**Next generation sequencing** (NGS)—also known as high-throughput sequencing, a technology allowing to establish the sequence of DNA larger than 1 million base pairs in a single

**Mitochondrial DNA** (mtDNA)—small circular genome localized in the mitochondria.

**Heteroplasmy**—coexistence of more than one mtDNA type within an individual.

**Homoplasmy**—presence of a uniform type of mtDNA within an individual.

nant, autosomal recessive, and X-linked type of inheritance.

takes place exclusively from mother to offspring.

as pathogenic, the examples of such events are: *MYO*1*A* and *RAB*40*AL* [41–43].

symptoms to occur, presence of only one defective copy is sufficient.

improve the level of patient care.

**Index of technical terms**

40 An Excursus into Hearing Loss

*GJB*2 and *GJB*6 genes.

nator method, also called Sanger sequencing.

about health history of the family members.

develop.

experiment.

Agnieszka Pollak\* and Monika Ołdak

\*Address all correspondence to: a.pollak@ifps.org.pl

Department of Genetics, Institute of Physiology and Pathology of Hearing, Warsaw, Poland
