Formation of a<unk>  chromosome. Rather than the typical linear pattern of a chromosome, deletion of the endings of a chromosome can lead to<unk>  formation. A chromosome has two arms, one long and one short. Deletion of the short arm of chromosome<unk>  does not appear to result in epilepsy; however, terminal deletion of the long arm is associated with epilepsy. Therefore, some gene loss from the terminal segment could be responsible for the manifestation of epilepsy in<unk> . The most common breakpoint in patients is in the q13.33 region of chromosome<unk> . As chromosomes occur in pairs, the affected individual usually has one normal chromosome and the other replaced by a<unk>  chromosome. However, this is not usually the case for every cell in the patient. In most individuals with<unk> , a certain percentage have two normal chromosome<unk> s and the remainder have one normal and one<unk> <unk>  chromosome. This mixture is called mosaicism. A higher degree (percentage) of mosaicism appears to be associated with earlier age of seizure onset and presence of malformations. Yet, the degree of mosaicism does not determine response to drug treatment. Since the range in age of onset of seizures and IQ for a given mosaic ratio is relatively wide, prediction of phenotype based on the mosaicism ratio is somewhat limited. Occasionally, there is a variation, where the<unk>  is characterized by an extra<unk>  chromosome in addition to the two normal ones and hence gives rise to a partial trisomy or supernumerary<unk> <unk> . This supernumerary<unk> <unk>  can cause multiple anomalies but no epilepsy, and this resulting syndrome should be differentiated from<unk> . This confusion has persisted even in the literature. The genes in the q13.33 region of chromosome<unk>  have yet to be fully delineated. However, this telomeric region includes two genes that are related to other distinct epilepsy syndromes, autosomal dominant nocturnal frontal lobe epilepsy and benign familial neonatal convulsions. The genes identified in these two syndromes are the nicotinic acetylcholine receptor alpha-4 subunit and potassium voltage-gated channel subfamily KQT member 2 respectively. Mutations in these genes lead to the clinical manifestations found in these disorders. It is theorized that deletions in these regions from the<unk> ed chromosome may lead to epilepsy in affected patient. A similar association between a<unk>  chromosome and an autosomal-dominant disease has been reported for<unk>  17 chromosome and Miller-Dieker syndrome. The recurrence risk for<unk>  is very low. The<unk>  formation is typically not identified in parents, and occurs 'de novo' during pre-natal development.
Ring chromosome 20 syndrome
Ring chromosome 14 syndrome
Joubert syndrome
Chromosome 5q deletion syndrome
13q deletion syndrome
Kaufman oculocerebrofacial syndrome
Bardet–Biedl syndrome
2q37 deletion syndrome