Sequencing the human genome (and the genomes of many animals, particularly mice) has produced enormous advances in understanding how a number of brain dysfunction syndromes depend on particular genetic substitutions, deletions, or additions. Following is a list of several well-known genetic disorders that have serious cognitive or neurological effects:
- Down syndrome: Down syndrome is also called trisomy 21 because it is caused by the presence of an extra 21 chromosome or part of it (humans have 48 chromosomes numbered 1 through 48). Down syndrome occurs in more than 1 in 1,000 births, and it is more common in children born of older parents due to the statistically greater chance of their having had this particular random chromosome damage. The syndrome is associated with not only moderate to severe mental learning disability, but complications in other organ systems besides the brain which reduce the life expectancy. Those who live into their 50s have a significantly increased risk of early Alzheimer's disease.
- Fragile X syndrome: Fragile X syndrome, so called because it results from a mutation on the X chromosome, is the most common inherited cause of intellectual disability. It is also the most commonly known single gene cause of autism. Fragile X syndrome is characterized by learning disabilities and a number of noted physical, emotional, and behavioral features. Physical features include an elongated face, large protruding ears, flat feet, and low muscle tone. Fragile X individuals exhibit social anxiety, including particularly gaze aversion. Fragile X syndrome is neurologically associated with reduced function in the prefrontal regions of the brain.
- Rett syndrome: Rett syndrome is a developmental brain disorder characterized by abnormal neuronal morphology and reduced levels of the neurotransmitters norepinephrine and dopamine. Physical features include small hands and feet and a tendency toward microcephaly. Behavior traits include repetitive hand wringing, poor verbal skills, and a tendency to have scoliosis. The syndrome affects females almost exclusively (affected males typically die in utero).
- Williams syndrome: Williams syndrome is a rare neuro-developmental disorder characterized by learning disabilities, except for strong language skills. Individuals with Williams syndrome tend to be highly verbal and overly sociable. They have an "elfin" facial appearance with a low nasal bridge. This syndrome is caused by a deletion of a number of genes on chromosome 7.
- Autism: Autism is a spectrum disorder with multiple genetic causes and a range of characteristics in different individuals, ranging from severe learning disabilities to slight social ineptitude. Asperger's syndrome is typically included in the autism spectrum as autism without significant language delay or dysfunction. Autistic individuals may have above-average intelligence and display unusual aptitude in certain technical or artistic areas. Many savants (people with extraordinary calculating, memorization, or artistic skills) are autistic. In its severe form, autism is severely debilitating, characterized by a total inability to engage in social interactions, repetitive behaviors such as continuous rocking, and extremely poor language abilities. Although it is clear that autism has multiple genetic causes due to its high heritability, the mechanisms of its effects remain poorly understood.