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Cerebellum
The cerebellum is a part of the brain that fine-tunes signals mostly received from the cerebrum. It contains numerous inhibitory neurons arranged in a one-way filtering pattern, and these help to pin down and output precise signals. These neurons are also plastic and are thus able to retain information and learn from a trial-and-error process. The cerebellar hemispheres correlate with the same side of the body, as opposed to the cerebral hemispheres.

The cerebellum is crucial for making balance-correcting movements aim correctly, without overshooting, therefore a dysfunctional cerebellum usually leads to difficulty with walking and a characteristic compensatory gait (truncal ataxia) with a wide stance and lurching, unequal steps. It can also cause the eyes to overshoot their target, especially when the vermis is involved. Most of the cerebellum's outputs are to the motor system, but the cerebellum mirrors many regions of the cerebral cortex, so cerebellar dysfunction can have other effects depending on the area affected. Damage to the vermis in particular causes a more global interruption of cerebellar function and typically results in a lack of inhibitory regulation over emotion, manifesting as uncontrolled giggling or crying at inappropriate times.

Allele
is any of two or more known variations at a specific place within a gene or chromosome. The term refers to both the nature of the specific difference and the location of that difference. For example, if there is a guanine at position 76 in a gene, and this gets changed to a thymine, both of these are alleles of that gene. It often refers to a variant in the smallest possible genetic unit (the base pair), called a single-nucleotide polymorphism (SNP), however it can also refer to variations for much larger regions that are several hundred or thousand base-pairs long (such as tandem repeats).

Mechanisms
Genetic disorders can affect any bodily systems and can result from various mechanisms. Some disorders (usually dominant) are caused by deficiencies in genes that regulate cell division at certain points in development (such as neurofibromatosis or achondroplasia), while some others (usually recessive) are caused by deficiencies in enzymes that break down molecules, leading to a progressive build-up of the molecule (such as phenylketonuria or mucopolysaccharidoses). Other mechanisms also exist, such as structurally abnormal proteins that progressively clump together (proteopathies such as Huntington's disease) or overexpression of duplicated genes in chromosomal abnormalities.

Risk factors
Most autosomal dominant genetic disorders and chromosomal abnormalities that happen for the first time in a family occur randomly and are unlikely to re-occur in another child. For some chromosomal abnormalities (especially abnormal numbers of whole chromosomes), the risk increases with maternal age, while for some gene mutations a smaller increase in risk exists with paternal age. Occasionally however, the mutation may be present in some sperm- or egg-producing cells (germline mosaicism), leading to a variable risk of recurrence. For autosomal recessive disorders, there is a higher risk among those of similar ethnicity, especially those of relatively close relation.

Intestinal atresia
Duodenal atresia is usually caused by a failure of the duodenum to recanalise in embryonic development, however other forms of of atresia such as jejunal are caused by an inability of the mesenteric arteries to supply blood to a part of the intestines, leading to ischemia and growth arrest or obliteration of that part of the bowel.