 Marker chromosomes typically occur in addition to the standard 46 chromosomes, making it a partial trisomy or 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.Chromosome 15 has been observed to contribute to a high number of marker chromosomes, but the reason has not been determined.