Had a great day, meeting wonderful people, delivering a presentation and workshops on emotional intelligence at the European Conference of the Chromosome 18 Charity.
Humans normally have 46 chromosomes in each cell, divided into 23 pairs. Two copies of chromosome 18, one copy inherited from each parent, form one of the pairs. Chromosome 18 spans about 78 million DNA building blocks (base pairs) and represents approximately 2.5 percent of the total DNA in cells.
Identifying genes on each chromosome is an active area of genetic research. Because researchers use different approaches to predict the number of genes on each chromosome, the estimated number of genes varies. Chromosome 18 likely contains 200 to 300 genes that provide instructions for making proteins. These proteins perform a variety of different roles in the body.
Distal 18q deletion syndrome
Distal 18q deletion syndrome occurs when a piece of the long (q) arm of chromosome 18 is missing. The term “distal” means that the missing piece (deletion) occurs near one end of the chromosome arm. The signs and symptoms of distal 18q deletion syndrome include delayed development and learning disabilities, short stature, weak muscle tone (hypotonia), foot abnormalities, and a wide variety of other features.
Proximal 18q deletion syndrome
Like distal 18q deletion syndrome (described above), proximal 18q deletion syndrome is a chromosomal condition that occurs when a piece of the q arm of chromosome 18 is missing. The term “proximal” means that in this disorder the deletion occurs near the centre of the chromosome, in an area between regions called 18q11.2 and 18q21.2. The size of the deletion varies among affected individuals. Proximal 18q deletion syndrome can lead to a wide variety of signs and symptoms among affected individuals, including delayed development and intellectual disability, recurrent seizures (epilepsy), behavioural problems, and characteristic facial features. These signs and symptoms are likely caused by the loss of specific genes in the deleted region.
Tetrasomy 18p results from the presence of an abnormal extra chromosome, called an isochromosome 18p, in each cell. An isochromosome is a chromosome with two identical arms. Normal chromosomes have one long (q) arm and one short (p) arm, but isochromosomes have either two q arms or two p arms. Isochromosome 18p is a version of chromosome 18 made up of two p arms.
Cells normally have two copies of each chromosome, one inherited from each parent. In people with tetrasomy 18p, cells have the usual two copies of chromosome 18 plus an isochromosome 18p. As a result, each cell has four copies of the short arm of chromosome 18. (The word “tetrasomy” is derived from “tetra,” the Greek word for “four.”) The extra genetic material from the isochromosome disrupts the normal course of development, causing intellectual disability, delayed development, and the other characteristic features of this disorder.
occurs when each cell in the body has three copies of chromosome 18 instead of the usual two copies, causing severe intellectual disability and multiple birth defects that are usually fatal by early childhood. (The word “trisomy” comes from “tri,” the Greek word for “three.”) In some cases, the extra copy of chromosome 18 is present in only some of the body’s cells. This condition is known as mosaic trisomy 18.
For more information regarding Chromosome 18 disorders: Chromosome 18