Diploid-triploid mosaicism (DTM) is a chromosome disorder. Individuals with diploid-triploid syndrome have some cells with three copies of each chromosome for a total of 69 chromosomes (called triploid cells) and some cells with the usual 2 copies of each chromosome for a total of 46 chromosomes (called diploid cells).

Having two or more different cell types is called mosaicism. Diploid-triploid mosaicism can be associated with truncal obesity, body/facial asymmetry, weak muscle tone (hypotonia), delays in growth, mild differences in facial features, fusion or webbing between some of the fingers and/or toes (syndactyly) and irregularities in the skin pigmentation.

Intellectual disabilities may be present but are highly variable from person to person ranging from mild to more severe.

The chromosome disorder is usually not present in the blood; a skin biopsy, or analyzing cells in the urine is needed to detect the triploid cells.

A regular human carries 23 pairs of chromosomes in his or her cells. Cells containing two pairs of chromosomes are known as diploid cells. Those with diploid triploid mosaicism have some cells which are triploid, meaning that they have three copies of chromosomes, or a total of 69 chromosomes. Triploidy is distinct from trisomy, in which only one chromosome exists in three pairs. A well-known example of trisomy is trisomy 21 or Down syndrome.

Polyploid cells and organisms are those containing more than two paired (homologous) sets of chromosomes. Most species whose cells have nuclei (Eukaryotes) are diploid, meaning they have two sets of chromosomes—one set inherited from each parent. However, polyploidy is found in some organisms and is especially common in plants. In addition, polyploidy occurs in some tissues of animals that are otherwise diploid, such as human muscle tissues. This is known as endopolyploidy. Species whose cells do not have nuclei, that is, Prokaryotes, may be polyploid organisms, as seen in the large bacterium "Epulopiscium fishelsoni" . Hence ploidy is defined with respect to a cell. Most eukaryotes have diploid somatic cells, but produce haploid gametes (eggs and sperm) by meiosis. A monoploid has only one set of chromosomes, and the term is usually only applied to cells or organisms that are normally diploid. Male bees and other Hymenoptera, for example, are monoploid. Unlike animals, plants and multicellular algae have life cycles with two alternating multicellular generations. The gametophyte generation is haploid, and produces gametes by mitosis, the sporophyte generation is diploid and produces spores by meiosis.

Polyploidy refers to a numerical change in a whole set of chromosomes. Organisms in which a particular chromosome, or chromosome segment, is under- or overrepresented are said to be aneuploid (from the Greek words meaning "not", "good", and "fold"). Therefore, the distinction between aneuploidy and polyploidy is that aneuploidy refers to a numerical change in part of the chromosome set, whereas polyploidy refers to a numerical change in the whole set of chromosomes.

Polyploidy may occur due to abnormal cell division, either during mitosis, or commonly during metaphase I in meiosis. In addition, it can be induced in plants and cell cultures by some chemicals: the best known is colchicine, which can result in chromosome doubling, though its use may have other less obvious consequences as well. Oryzalin will also double the existing chromosome content.

Polyploidy occurs in highly differentiated human tissues in the liver, heart muscle and bone marrow. It occurs in the somatic cells of some animals, such as goldfish, salmon, and salamanders, but is especially common among ferns and flowering plants (see "Hibiscus rosa-sinensis"), including both wild and cultivated species. Wheat, for example, after millennia of hybridization and modification by humans, has strains that are diploid (two sets of chromosomes), tetraploid (four sets of chromosomes) with the common name of durum or macaroni wheat, and hexaploid (six sets of chromosomes) with the common name of bread wheat. Many agriculturally important plants of the genus "Brassica" are also tetraploids.

Polyploidization is a mechanism of sympatric speciation because polyploids are usually unable to interbreed with their diploid ancestors. An example is the plant "Erythranthe peregrina". Sequencing confirmed that this species originated from "E. x robertsii", a sterile triploid hybrid between "E. guttata" and "E. lutea," both of which have been introduced and naturalised in the United Kingdom. New populations of "E. peregrina" arose on the Scottish mainland and the Orkney Islands via genome duplication from local populations of "E. x robertsii". Because of a rare genetic mutation, "E. peregrina" is not sterile.

Polyploid types are labeled according to the number of chromosome sets in the nucleus. The letter "x" is used to represent the number of chromosomes in a single set.

- triploid (three sets; 3"x"), for example seedless watermelons, common in the phylum Tardigrada

- tetraploid (four sets; 4"x"), for example Salmonidae fish, the cotton "Gossypium hirsutum "

- pentaploid (five sets; 5"x"), for example Kenai Birch ("Betula papyrifera" var. "kenaica")

- hexaploid (six sets; 6"x"), for example wheat, kiwifruit

- heptaploid or septaploid (seven sets; 7"x")

- octaploid or octoploid, (eight sets; 8"x"), for example "Acipenser" (genus of sturgeon fish), dahlias

- decaploid (ten sets; 10"x"), for example certain strawberries

- dodecaploid (twelve sets; 12"x"), for example the plants "Celosia argentea" and "Spartina anglica" or the amphibian "Xenopus ruwenzoriensis".

Triploidy affects approximately 1-2% of pregnancies, but most miscarry early in development. At birth, males with triploidy are 1.5 times more common than females.

Triploidy can result from either two sperm fertilizing one egg (60%) or from one sperm fertilizing an egg with two copies of every chromosome (40%).

Not only is obesity associated with miscarriage, it can result in sub-fertility and other adverse pregnancy outcomes. Recurrent miscarriage is also related to obesity. Women with bulimia nervosa and anorexia nervosa may have a greater risk for miscarriage. Nutrient deficiencies have not been found to impact miscarriage rates but hyperemesis gravidarum sometimes precedes a miscarriage.

Caffeine consumption also has been correlated to miscarriage rates, at least at higher levels of intake. However, such higher rates have been found to be statistically significant only in certain circumstances.

Vitamin supplementation has generally not shown to be effective in preventing miscarriage. Chinese traditional medicine has not been found to prevent miscarriage.

The age of the pregnant woman is a significant risk factor. Miscarriage rates increase steadily with age, with more substantial increases after age 35. In those under the age of 35 the risk is about 10% while it is about 45% in those over the age of 40. Risk begins to increase around the age of 30. Paternal age is associated with increased risk.

The cause of neuroblastoma is not well understood. The great majority of cases are sporadic and non-familial. About 1–2% of cases run in families and have been linked to specific gene mutations. Familial neuroblastoma in some cases is caused by rare germline mutations in the anaplastic lymphoma kinase (ALK) gene. Germline mutations in the PHOX2A or KIF1B gene have been implicated in familial neuroblastoma as well. Neuroblastoma is also a feature of neurofibromatosis type 1 and the Beckwith-Wiedemann syndrome.

MYCN oncogene amplification within the tumor is a common finding in neuroblastoma. The degree of amplification shows a bimodal distribution: either 3- to 10-fold, or 100- to 300-fold. The presence of this mutation is highly correlated to advanced stages of disease.

Duplicated segments of the LMO1 gene within neuroblastoma tumor cells have been shown to increase the risk of developing an aggressive form of the cancer.

Neuroblastoma has been linked to copy-number variation within the NBPF10 gene, which results in the 1q21.1 deletion syndrome or 1q21.1 duplication syndrome.

Several risk factors have been proposed and are the subject of ongoing research. Due to characteristic early onset many studies have focused on parental factors around conception and during gestation. Factors investigated have included occupation (i.e. exposure to chemicals in specific industries), smoking, alcohol consumption, use of medicinal drugs during pregnancy and birth factors; however, results have been inconclusive.

Other studies have examined possible links with atopy and exposure to infection early in life, use of hormones and fertility drugs, and maternal use of hair dye.

Based on a series of 493 neuroblastoma samples, it has been reported that overall genomic pattern, as tested by array-based karyotyping, is a predictor of outcome in neuroblastoma:

- Tumors presenting exclusively with whole chromosome copy number changes were associated with excellent survival.

- Tumors presenting with any kind of segmental chromosome copy number changes were associated with a high risk of relapse.

- Within tumors showing segmental alterations, additional independent predictors of decreased overall survival were N-myc amplification, 1p and 11q deletions, and 1q gain.

Earlier publications categorized neuroblastomas into three major subtypes based on cytogenetic profiles:

- Subtype 1: favorable neuroblastoma with near triploidy and a predominance of numerical gains and losses, mostly representing non-metastatic NB stages 1, 2 and 4S.

- Subtypes 2A and 2B: found in unfavorable widespread neuroblastoma, stages 3 and 4, with 11q loss and 17q gain without N-myc amplification (subtype 2A) or with N-myc amplification often together with 1p deletions and 17q gain (subtype 2B).

Virtual karyotyping can be performed on fresh or paraffin-embedded tumors to assess copy number at these loci. SNP array virtual karyotyping is preferred for tumor samples, including neuroblastomas, because they can detect copy neutral loss of heterozygosity (acquired uniparental disomy). Copy neutral LOH can be biologically equivalent to a deletion and has been detected at key loci in neuroblastoma. ArrayCGH, FISH, or conventional cytogenetics cannot detect copy neutral LOH.