This is the simplest expression of “heredity”, that is, the difference of seeds; different seeds grow different species, and the seeds of the same species grow different things, such as the seeds in the same pod are not exactly the same, and they may germinate and flower in different colors, which is the role of “genetic variation”. This is the role of “genetic variation”; on the other hand, the size and shape of the melons produced on the same vine will not be exactly the same, which has the role of “environmental factors”, such as: different soil and sunlight environment to produce different fruit size, sugar, etc. This is the role of “environmental factors”, such as: different soil and sunlight, different fruit size, sugar content, etc. As the basic unit of inheritance, genes are passed from generation to generation and control everything from physical characteristics to the occurrence of hereditary diseases. In a broad sense, all diseases are related to heredity; however, the common concept of genetic diseases is that they are caused by alterations in the genetic material, that is, diseases that have chromosomal alterations or mutations and deletions of genes. What is hereditary deafness? In a narrow sense, hereditary deafness is deafness caused by mutations in a gene (including base substitutions, base deletions or additions, etc.) and abnormalities in the chromosomal region containing a gene. And these genes associated with deafness are called deafness genes. Broadly speaking, we all have different genes. Some genetic differences, while not directly causing deafness, can alter an individual’s ability to tolerate certain factors and make them more susceptible to deafness under certain conditions, and deafness that occurs in this way is also genetic deafness, the most typical example being drug deafness. So how important is hereditary deafness? How many people with deafness are genetically deaf? To answer this question, let’s first look at a set of data: The population of people with hearing and speech disabilities in China has reached more than 27.8 million, of which 20.04 million are simple hearing disabilities, the highest among all kinds of disabilities. In every 1000 newborns, there are 1~3 deaf children, 65% of which are genetically deaf, and this percentage is increasing as more deafness genes are being discovered. It is easy to see from the above data that hereditary deafness is the leading cause of congenital deafness. In fact the vast majority of congenital deafness and late-onset deafness are genetic conditions. ”Both of us have normal hearing and there are no deaf people in either family, so why is our child’s deafness a genetic condition?” This is the question that most parents of children with congenital deafness have, and it is the most common case of hereditary deafness. The answer to this question is: recessive inheritance. 80% of non-syndromic deafness is inherited in an autosomal recessive manner, which simply means that both parents carry a disease-causing mutation (due to the invisible mode of inheritance, each parent carrying a disease-causing mutation will not develop the disease, so the hearing is normal). In this case, the child has a 25% chance of acquiring both parents’ disease-causing mutations, resulting in pure or compound heterozygosity for the disease-causing mutation, which leads to deafness. In addition, if such a couple has another child, the next generation will still have a 25% risk of developing deafness. How many genes are associated with deafness? It is generally believed that there are more than 300 genes in the human genome that are associated with the auditory system, which means that they are all potentially deafness-related genes. To date, 80 genes for non-syndromic deafness have been identified and confirmed (non-syndromic deafness is defined as a patient having only deafness and no other systemic disease), scattered throughout the human chromosome (see pdf file for image. Title: Distribution of selected deafness genes in 23 pairs of human chromosomes, image cited from
Amiel A et al. Neuron 68, October 21,
2010.), and nearly 400 other syndromes can be combined with hearing impairment. Thus, hereditary deafness is highly genetically heterogeneous, which is an important direction and difficulty in the genetic study of deafness.