A<unk>  (mar) is a small fragment of a chromosome which generally cannot be identified without specialized genomic analysis due to the size of the fragment. The significance of a<unk>  is variable as it depends on what material is contained within the<unk> . The large majority of these<unk> s are smaller than one of the smaller human chromosomes, chromosome 20, and by definition are termed small supernumerary<unk> s.<unk> s 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. The frequency is approximately 3-4 per 10,000 people, and 1 in 300 people with mental retardation.<unk> s typically occur in addition to the standard 46 chromosomes, making it a partial trisomy or tetrasomy supernumerary chromosome.A<unk>  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<unk> s, but the reason has not been determined. The small supernumerary<unk>  (sSMC) page contains examples of other birth defects, syndromes, and tumors that are associated with various types of sSMCs.
Marker chromosome
Satellite chromosome
Small supernumerary marker chromosome
Neochromosome
Sex chromosome
XY sex-determination system
Y chromosome
Low copy repeats