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A marker chromosome (mar) is a structurally abnormal chromosome in which no part can be identified. The significance of a marker is very variable as it depends on what material is contained within the marker.
It is essentially a partial trisomy. However, sometimes the marker is composed of inactive genetic material and has little or no effect. There are some markers that are passed down through a family with little effect. There are some markers that arise as new events. Often, markers are not passed down as they can be lost during cell division due to their small size. There are some individuals who have multiple markers. When special studies are performed to identify the material more conclusions can be drawn about the effects of the marker. There is a more common marker called inverted duplication of chromosome 15. This is a specific syndrome that has developmental delays and physical variations and often seizures as its features. A difficult situation arises when a marker chromosome is found on a prenatal study such as amniocentesis. If the marker is not one of the more common markers and neither parent has the marker, it can be very difficult to predict the effects of the marker. Usually, if a parent carries the same marker, the effect is much less.
- 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 - dead link