What types of hereditary deafness are there? In order to answer this question, we must first clarify the concept of “deafness”, followed by the concept of “hereditary deafness” and the related classification. I. The concept of deafness It is customary to call it deafness when the hearing function becomes impaired due to lesions in the sound-transmitting or (and) sound-sensitive parts of the hearing system or (and) the auditory nerve or (and) other auditory centers at all levels (cited in Practical Otolaryngology, Head and Neck Surgery, 2nd edition). In practice, many patients are surprised when they are told by their doctors that they are deaf and ask, “I can still hear, why am I deaf?” This is also a conceptual issue. In clinical practice, there is no strict distinction between the concepts of “deafness” and “hearing loss”, i.e. the aforementioned “… …occur in different degrees of hearing loss, customarily called deafness”, so, generally speaking, deafness does not only refer to the extremely severe deafness that can not hear any speech, but also refers to various types and degrees of hearing loss. It is only in cases such as disability testing that a strict distinction is made between “deafness” and “hearing loss”. There are many ways of classifying and grading deafness, but here we will only briefly introduce the most widely used grading standard, the World Health Organization (WHO 1980) standard: grading by the average hearing threshold of 0.5~2k Hz, 1. Mild hearing loss, pure tone hearing threshold of 16~25dB HL. 2. Mild hearing loss The average hearing threshold is 26~40dB HL. 3.Moderate hearing loss, the average hearing threshold is 41~55dB HL. 4.Moderate to severe hearing loss, the average hearing threshold is 56~70dB HL. 5.Severe hearing loss, the average hearing threshold is 71~90dB HL. 5.Severe hearing loss, the average hearing threshold is 71~90dB HL. 6.Extreme hearing loss, can hardly hear any sound, even the loud shout in the ear, the average hearing threshold is 90dB HL or more. Definition of hereditary deafness Hereditary deafness is an abnormality in ear development or metabolic disorder caused by the genes that cause deafness in the parental generation or the newly occurred mutation of deafness genes, resulting in poor hearing function. In hereditary deafness, there are both conductive deafness caused by developmental abnormalities of the outer and middle ear, and sensorineural deafness caused by inner ear hypoplasia, etc. Among them, sensorineural deafness plays an important role in hereditary deafness (from Practical Otolaryngology, Head and Neck Surgery, 2nd edition). Hereditary deafness can be divided into two categories according to whether it is combined with diseases of other body systems: 1. syndromic deafness 2. Non-syndromic deafness. The term “syndromic deafness” refers to the combination of deafness with diseases of other body systems and organs, such as the eye, skeletal system, nervous system, endocrine system, metabolic diseases, kidney, skin, eye, limb, etc. The term “non-syndromic deafness” means that the patient is only deaf, but not combined with other diseases. “Non-syndromic deafness” is the main type of hereditary deafness, accounting for more than 70% of all congenital deafness detected in newborn hearing screening. Hereditary deafness can also be classified according to the different modes of inheritance: 1. autosomal dominant. 2. 2. Autosomal recessive inheritance, which is the most common mode of inheritance of hereditary deafness. 3. X-chromosome chain dominant inheritance. 4. X-chromosome chain recessive inheritance. 5. Y-chromosome chain inheritance. All three of these are due to changes in the genes carried by the sex chromosomes (X and Y chromosomes), and are collectively referred to as “companion genetics”. 6. Mitochondrial genetic disorders.