Partial trisomy 21 (PT21) is a type of trisomy 21 where only a part of chromosome 21 is present in three copies. Individuals with PT21 may exhibit a smaller number of Down's syndrome (DS) characteristics. The existence of PT21 is helpful in identifying the gene loci responsible for the Down's syndrome phenotypes.
Partial trisomy 21 is a rare clinical entity. It is characterized by the presence of only a part of chromosome 21 as an extra copy. Usually, through non-disjunction during meiosis or mitosis, there are three complete copies of chromosome 21. In the case of mosaicism, some of the cells of the body express the trisomy. PT21 has been used in Down's syndrome research, to try and locate the genes responsible for the typical Down's syndrome phenotype, with the discovery of regions such as the Down syndrome critical region (DSCR) . PT21 shows a heterogeneous expression of phenotypes, as well as genetic variation.
Studies have shown that several genes on chromosome 21 are able to code for abnormalities expressed, such as cognitive impairment  . Furthermore, it has been shown that the DSCR region is not solely responsible for the phenotypes observed, but may have an input in their expression  .
Typically, children with PT21 exhibit fewer features of DS or none at all. Some cases only experience mental impairment .
DS is the most frequently reported birth defect, with the incidence increasing with maternal age . The typical features of the condition include intellectual impairment, hypotonia during the neonatal period, flat profile, flattening of the nasal bridge and occiput, epicanthal folds, upward slanted eyes, microcephaly, macroglossia with lack of a central fissure, and small ears. Brushfield spots may be observed in the iris.
In addition, children with DS may have a single palmar crease, short hands and fingers, clinodactyly, sandal gap deformity, and plantar furrow. Growth delay, short stature, decreased intelligence quotient (IQ) may be detected during development, as well as traits of depression, autism, and attention deficit hyperactivity disorder may be seen.
Systemic effects of DS include a higher incidence of duodenal atresia, duodenal stenosis, intestinal obstruction, leukemia, Hirschsprung's disease, Alzheimer's disease, and congenital heart diseases such as ventricular septal defect and atrioventricular canal defect.
Prenatal diagnosis of Down's syndrome, and hence some cases of partial trisomy 21, can be achieved through amniocentesis or chorionic villus sampling . Abnormal features can be distinguished in a fetus, through ultrasound, usually between 14 and 24 weeks gestation . These include flattened nasal bridge, thickened nuchal fold and dilated ventricles. The prenatal screening is extremely effective in detecting DS .
Prenatal biochemical markers include abnormally high levels of alpha-fetoprotein, inhibin, estriol, and beta HCG (human chorionic gonadotropin), which are tested in the first and second trimesters. Fetal DNA present in maternal blood can also be tested for trisomy 21 and PT21 in noninvasive prenatal screening (NIPS).
After birth, features of DS may be present, and additional chromosome or karyotype analysis will reveal PT21 . A correlation of the clinical characteristics of DS is needed in order to make a clinical diagnosis.