A marker chromosome (mar) is a small fragment of a

chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...

which generally cannot be identified without specialized genomic analysis due to the size of the fragment.Thompson & Thompson Genetics in Medicine, Chapter 5, 57-74 https://www.clinicalkey.com/#!/content/book/3-s2.0-B9781437706963000054?scrollTo=%23hl0000654 The significance of a marker is variable as it depends on what material is contained within the marker.Nelson Textbook of Pediatrics, Chapter 81, 604-627 https://www.clinicalkey.com/#!/content/book/3-s2.0-B9781455775668000818?scrollTo=%23hl0003126 The large majority of these marker chromosomes are smaller than one of the smaller human chromosomes,

chromosome 20 Chromosome 20 is one of the 23 pairs of chromosomes in humans. Chromosome 20 spans around 66 million base pairs (the building material of DNA) and represents between 2 and 2.5 percent of the total DNA in cells. Chromosome 20 was fully sequenced i ...

, and by definition are termed

small supernumerary marker chromosome A small supernumerary marker chromosome (sSMC) is an abnormal extra chromosome. It contains copies of parts of one or more normal chromosomes and like normal chromosomes is located in the cell's nucleus, is replicated and distributed into each d ...

s. Marker chromosomes occur sporadically about 70% of the time, with the remainder being inherited from a parent. About 50% of cases involve

mosaicism Mosaicism or genetic mosaicism is a condition in multicellular organisms in which a single organism possesses more than one genetic line as the result of genetic mutation. This means that various genetic lines resulted from a single fertilized ...

, which affects the severity of the condition. The frequency is approximately 3-4 per 10,000 people, and 1 in 300 people with

mental retardation Intellectual disability (ID), also known as general learning disability in the United Kingdom and formerly mental retardation, Rosa's Law, Pub. L. 111-256124 Stat. 2643(2010). is a generalized neurodevelopmental disorder characterized by signifi ...

. Marker chromosomes typically occur in addition to the standard 46 chromosomes, making it a partial

trisomy A trisomy is a type of polysomy in which there are three instances of a particular chromosome, instead of the normal two. A trisomy is a type of aneuploidy (an abnormal number of chromosomes). Description and causes Most organisms that reprodu ...

tetrasomy A tetrasomy is a form of aneuploidy with the presence of four copies, instead of the normal two, of a particular chromosome. Causes Full Full tetrasomy of an individual occurs due to non-disjunction when the cells are dividing (meiosis I or II) ...

supernumerary chromosome. A marker can be composed of inactive genetic material and have little or no effect, or it can carry active genes and cause genetic conditions such as iso(12p), which is associated with Pallister-Killian syndrome, and iso(18p), which is associated with mental retardation and

syndromic facies In medical contexts, a facies is a distinctive facial expression or appearance associated with a specific medical condition. The term comes from Latin for "face". As a fifth declension noun, ''facies'' can be both singular and plural. Types Exam ...

Chromosome 15 Chromosome 15 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 15 spans about 102 million base pairs (the building material of DNA) and represents between 3% and 3.5% of the total DN ...

has been observed to contribute to a high number of marker chromosomes, but the reason has not been determined. The

(sSMC) page contains examples of other

birth defects A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can ...

, syndromes, and

tumors A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...

that are associated with various types of sSMCs.

References

* https://web.archive.org/web/20060926021351/http://www.chromodisorder.org/sytrix/card_list.php3?dbid=63&id=365 An International System for Human Cytogenetic Nomenclature, Shaffer, L.G., Tommerup N. (eds); S. Karger, Basel 2005 {{Chromosomal abnormalities Chromosomal abnormalities