Down syndrome is the most common and early identified chromosomal disorder which was first described by a British doctor John Langdo Down in 1887. It wasn’t until 1957, after the Watson and Crick model of DNA (1952) that an extra chromosome was identified as the cause.
It is a combination of physical abnormalities and mental retardation characterized by a genetic defect in chromosome pair 21. It is caused by the presence of all or part of a third copy of chromosome 21. The incidence is 1/650 and increases with maternal age. More than 60% of the babies are spontaneously aborted and 20% stillbirths are seen.
There are 3 main causes of Down syndrome.
- 95% people have three separate copies of chromosome 21-trisomy 21, where the karyotype is 47, +21. Trisomy 21 results from non-disjunction during meiosis 1 during separation of chromosome 21.Most of the non-disjunctions are maternally derived. The phenotype observed is severe.
- 4% people have the extra copy of chromosome 21 because of a Robertsonian translocation. The acrocentric p arms of chromosome 21 and 14 are lost during translocation. The karyotype is 46 but there is an abnormal chromosome with an extra 21 chromosome (chromosome 14 gains a part of chromosome 21). A severe phenotype is seen.
- 1% people have mosaicism with normal and trisomy cell lines and usually have much milder features. Non-disjunction occurs during mitosis (postzygotically).
The clinical features seen are characteristic facial appearance with round face, flat nasal bridge, protruding tongue, small ears and epicanthic folds. Other dysmorphic features include single palmar crease, flat occiput, incurved little fingers, gap between first and second toes and small stature. Structural defects such as congenital heart diseases, duodenal atresia, mental retardation and developmental delay are encountered. Increased risk of leukemia, respiratory infections and hypothyroidism are some of the late medical complications.
Gene therapy is not effective for such chromosomal aberration. For a better life style treatment of phenotypic characteristics, such as surgical treatment of heart anomalies can be carried out. Referral to neurologists, child psychiatrists and speech-language pathologists on a regular basis can help manage the genetic disorder. Maintenance of a healthy diet and exercise can be very helpful too. Keep calm;it’s only an extra chromosome!