**3. The frequency of genetic HL**

Genetic HL occurs 1 in 2000 to 1 in 650 live births [Morton & Nance, 2006]. About 70% of the cases are nonsyndromic [Tekin et al., 2001]. Studies show that 75% of nonsyndromic HL

HL may be associated with other physical problems which are called syndromic HL. Genetic HLs without any other complications is called non-syndromic genetic hearing loss [Willems, 2004]. HL loci are named with the prefix DFN, followed by the mode of inheritance which is indicated by B, A, X and Y for autosomal recessive (DFNB), autosomal dominant (DFNA), X-linked (DFNX) and Y-linked (DFNY), respectively. The order in which loci have been described is denoted by a number after these letters, e.g. DFNB1 is the first

identified locus causing autosomal recessive HL [Guilford et al., 1994].

Presbycusis Age-related HL

Criterion Class Definition and example Age of onset Prelingual HL HL occurs before language acquisition

> Genetic Caused by gene mutation Idiopathic Unexplained cause

> > nerve)

Severity Mild Difficulty in hearing of 26–40 dB sounds

Sex-linked DFNX loci (DFN1-8) Y-linked DFNY loci (DFNY1)

Table 2. Various criteria for the classification of hearing loss.

Moderate 41–55 dB Moderately severe 56–70 dB Severe 71–90 dB Profound >90 dB

Autosomal dominant

Autosomal recessive

**3. The frequency of genetic HL** 

Syndromic associated with other symptoms Nonsyndromic Deafness is the only defect

conductive HL

Conductive HL Caused by a problem transferring sound waves through

Sensorineural HL Caused by damage to the inner ear (vestibulocochlear

Mixed HL Caused by a combination of sensorineural and

DFNA loci (DFNA1-64)

DFNB loci (DFNB1-96)

Mitochondrial 12SrRNA (MT-RNR1), tRNASer(UCN) (MT-TS1)

Genetic HL occurs 1 in 2000 to 1 in 650 live births [Morton & Nance, 2006]. About 70% of the cases are nonsyndromic [Tekin et al., 2001]. Studies show that 75% of nonsyndromic HL

Postlingual HL HL occurs after language acquisition

Etiology Acquired Caused by environmental agents such as viral and

bacterial infections (prenatal, e.g., CMV, toxoplasmosis, rubella; postnatal, e.g, meningitis), hyperbilirubinemia, head trauma, anoxia, noise exposure and ototoxic drugs

the external ear, tympanic membrane or middle ear

**2.5 Signs** 

Clinical phenotypes

Position of damage

Mode of inheritance are inherited as autosomal recessive [Tekin et al., 2001]. 10-20% of cases are inherited as autosomal dominant and 1-5% are X-linked recessive. Approximately, 1% of human genes, i.e 200 to 250 genes are responsible for hereditary HL [Finsterer & Fellinger, 2005]. So far, more than one hundred loci and 55 genes have identified which are involved in nonsyndromic HL (http://hereditaryhearingloss.org).
