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) [1]. 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 [2] [3]. 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 [2] [3].

Typically, children with PT21 exhibit fewer features of DS or none at all. Some cases only experience mental impairment [4].

DS is the most frequently reported birth defect, with the incidence increasing with maternal age [5]. 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.

• Trisomy 21

  Although trisomy 21 is not sufficient to cause CHD, three copies of at least part of chromosome 21 (Hsa21) increases the risk for CHD. [ncbi.nlm.nih.gov]

  Partial Trisomy 21 in a Patient without the Down Syndrome Phenotype. A. Umrigar 1, T. J. Chen 2, F. [ashg.org]

  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. [symptoma.com]

• Multiple Congenital Anomalies

  Abstract A patient with a double partial trisomy 20 and 21 with mild mental retardation and multiple congenital anomalies is presented. [ncbi.nlm.nih.gov]

• Inguinal Hernia

  Bilateral sandal gaps, right inguinal hernia and bilateral hydrocele of testis have also been found. The complete description of dysmorphic features observed in our patient is shown in Tables 1 and 2. [omicsonline.org]

• Receding Chin

  A female patient with mild mental retardation with spatial perceptual difficulties, microcephaly, depressed nasal root, receding chin, webbed neck, low hairline, shield chest, cubitus valgus, scoliosis and dermatoglyphic findings not characteristic of [ncbi.nlm.nih.gov]

• Heart Murmur

  Many phenotypic features of DS were present including hypotonia, flat occiput, flat facies, up-slanted palpebral fissures, epicanthic folds, flat nasal bridge, macroglossia, open mouth, small ears and a heart murmur. [ncbi.nlm.nih.gov]

• Sparse Hair

  Upon genetic consultation the growth parameters were noted: body weight – 2730 g, length- 51 cm and typical Down syndrome gestalt was observed including: large fontanel, sparse hair, low posterior hairline, prominent forehead, microophtalmia, flat occiput [omicsonline.org]

• Widely Spaced Nipples

  Some affected infants may also have a short, webbed neck; a highly arched roof of the mouth (palate); and/or widely spaced nipples. Teeth may erupt later than expected and may emerge crooked. [rarediseases.org]

• Large Fontanel

  They presented only a few common features, such as: large fontanels, ear abnormalities and mental retardation, which we have also seen in our patient. These symptoms seem to be nonspecific and not relating to any particular chromosomal localisation. [omicsonline.org]

• Prominent Cheeks

  The proband postnatally displayed by dysmorphic features of a round flat face with prominent cheeks and high forehead, upward slanting palpebral fissures, epicanthic folds, hypertelorism, a short nose, a broad and depressed nasal bridge, anteverted nares [ncbi.nlm.nih.gov]

• Thin Lips

  lips, retrognathia and high arched palate, along with a short neck. [omicsonline.org]

• Headache

  Depending upon the age at symptom onset and other factors, associated symptoms may include sudden episodes of uncontrolled electrical activity in the brain (seizures), irritability, vomiting, headache, loss of coordination, deteriorating mental functioning [rarediseases.org]

Prenatal diagnosis of Down's syndrome, and hence some cases of partial trisomy 21, can be achieved through amniocentesis or chorionic villus sampling [6]. Abnormal features can be distinguished in a fetus, through ultrasound, usually between 14 and 24 weeks gestation [7]. These include flattened nasal bridge, thickened nuchal fold and dilated ventricles. The prenatal screening is extremely effective in detecting DS [8].

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 [9]. A correlation of the clinical characteristics of DS is needed in order to make a clinical diagnosis.

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