Meiotic error leads to the deranged genetic material and so is the formation of an isochromosome. An 'isochromosome' is an unusual chromosome with two identical arms, either two short (p) arms or two long (q) arms produced by transverse rather than normal longitudinal splitting of a replicating chromosome. Each isochromosome does not have one chromosome arm but have the other arm doubled. 'Ring chromosomes' (denoted as 'r') are the unusual chromosomes in which the end of each chromosome arm (telomere) has been lost and the broken arms have been reunited in the ring formation. Chromosomes shaped like rings form in one out of twenty five thousand conceptions. They can involve any chromosome and may occur in addition to a full diploid chromosome set, or account for one of the forty six chromosomes.
Abnormalities associated with Isochromosomes
• Pallister-Killian mosaic syndrome - It is a growth disorder that affects many parts of the body. This state is described by very weak muscle tone (hypotonia) in infancy and early childhood, mental disability, unique facial features, thin hair, areas of atypical skin pigmentation, and other birth defects. This syndrome is normally due to the existence of an unusual extra chromosome known as isochromosome (12p). Isochromosome 12p is a variant of chromosome 12 which is made up of 2 p arms. In general cells have 2 copies of each chromosome, one inherited from each parent. Persons with Pallister-Killian Mosaic Syndrome contain 4 copies of the short arm (p) of chromosome 12 in place of the typical 2. The additional genetic content from the isochromosome alters the regular course of development, causing the distinctive symptoms of this disorder.
• Isochromosome 18p syndrome - It is a innate chromosomal abnormality connected with mental disability. Other general abnormalities are constrained growth, epilepsy (nervous system disorder) and some malformations. The Isochromosome 18p syndrome is due to an extra isochromosome, comprising of 2 copies of the short arm of chromosome 18. In regular cell division the chromosome divides longitudinally, creating two identical daughter chromosomes. If the chromosome instead divides transversely, one daughter chromosome has both short arms that is isochromosome 18p, and the other daughter chromosome has of the two long arms that is isochromosome 18q.The isochromosome is then transmitted as an extra chromosome along with 2 normal copies of chromosome 18, implying that individuals with isochromosome 18p syndrome have cells with 4 copies of the short arm of chromosome 18 instead of the general 2.
Abnormalities associated with ring chromosomes
• Ring chromosome 20 syndrome - It is a state that alter the usual growth and function of the brain. The main characteristic of this state is repeated seizures during infancy and the seizures may happen during the day or at night while asleep. Ring chromosome 20 syndrome is due to an unusual chromosome 20 referred to as r(20). A ring chromosome is a round configuration that occurs when a chromosome breaks in two places and its broken ends join together. Individuals with ring chromosome 20 syndrome will have a copy of this unusual chromosome in some or all of their cells.
• Ring chromosome 14 syndrome - It is due to a chromosomal anomaly called as a ring chromosome 14 denoted as r (14). Individuals having ring chromosome 14 syndrome contain a copy of this unusual chromosome in a few or each cell. Scientists think that various crucial genes close to the end of the long arm (q) of chromosome 14 are gone when the ring chromosome forms. The missing of these genes is fairly accountable for several of the chief features of ring chromosome 14 syndrome, including mental disability and belated development. Scientists are still working to establish which missing genes contribute to the signs and symptoms of this disorder.
• Ring chromosome 13 syndrome - It is a uncommon genetic condition caused by having an unusual chromosome 13 that forms a ring. In humans with ring chromosome 13, one chromosome 13 is normally complete while the other forms a ring. Symptoms include growth delay, feeding difficulties, abnormal formation and positioning of one or both feets and sometimes toe, high palate etc.
Changes that have an effect on the structure of chromosomes (isochromosomes and ring chromosomes) cause problems with growth and development and normal functioning of body's mechanisms.
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