Читать книгу Neurobiology For Dummies - Frank Amthor - Страница 85

We know that many nervous system disorders are genetically based, even though we may not always know the genetic origin. So, how do genetic disorders happen?

Genetic mutations — which you probably know something about if you’ve ever read a comic book — are one cause of these disorders. Reproduction starts with germ cells (egg or sperm cells) being generated during meiosis. In this process, alternating partial segments from the parents’ double-stranded DNA produce the offspring’s unique DNA.

In the complex process of assembling a single chromosome of DNA from the two parental chromosomes, errors happen. These errors are one kind of mutation. Here are the three most common single chromosome mutations:

 Deletion: In deletion, a piece of a chromosome or sequence of DNA is missing. A partial sequence from neither parent ends up in the single chromosome of the egg or sperm, making that DNA shorter and missing the gene altogether.

 Duplication: In duplication, a partial sequence of DNA from one parent is inserted twice.

 Inversion: When inversion happens, a partial DNA sequence from one parent is inserted upside down in the offspring. As we discuss earlier in this chapter, DNA is always transcribed from the same direction, so when a partial sequence is inserted upside-down, the protein coded is completely different when it’s transcribed.

Here are a few well-known examples of genetic disorders:

 Down syndrome: The most well-known example of a disorder with a known genetic cause is Down syndrome, which is caused by an extra chromosome 21 (trisomy 21). Down syndrome occurs in about 1 in 1,000 births.

 Fragile X syndrome: This disorder is the most common inherited cause of intellectual disability. It results from an X chromosome mutation. The features of Fragile X syndrome are mental retardation and a number of noted physical, emotional, and behavioral issues.

 Rett syndrome: Another inherited developmental brain disorder, Rett syndrome is characterized by abnormal neuronal morphology and reduced levels of neurotransmitters norepinephrine and dopamine.

 Schizophrenia: Rather than being due to a single mutation, schizophrenia has many genetic causes. It is a serious mental disorder that is characterized by severely impaired thinking, along with emotional and behavioral issues.

 Autism: This neurological disorder also has multiple genetic causes. As such, individuals with different mutations have somewhat different disorders of varying severity. Autism is called a “spectrum” disorder because its multiple genetic causes create a range of phenotypic characteristics, depending on the person. Characteristics range from severe retardation to slight social ineptitude.

 Asperger’s syndrome: Typically, Asperger’s is included in the autism spectrum as autism without significant language delay or dysfunction. Like autism, Asperger’s appears to arise from multiple mutation locations, and correspondingly very different types and severities of symptoms.