A marker chromosome (mar) is a small fragment of a chromosome which generally cannot be identified without specialized genomic analysis due to the size of the fragment.
[3] Marker chromosomes occur sporadically about 70% of the time, with the remainder being inherited from a parent.
About 50% of cases involve mosaicism, which affects the severity of the condition.
[4] 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 intellectual disability and syndromic facies.
[1] The small supernumerary marker chromosome (sSMC) page contains examples of other birth defects, syndromes, and tumors that are associated with various types of sSMCs.