Down syndrome and trisomy 21, is it different?


Down syndrome and trisomy 21, is it different?

Down syndrome, Klinefelter syndrome, trisomy 21 or trisomy 18, it is sometimes difficult to differentiate these different pathologies. Trisomy 21 is characterized by the existence of an additional chromosome in the pair of chromosomes 21. This type of trisomy is also called Down’s syndrome, named after the first physician to have carried out a detailed clinical study of this chromosomal abnormality.

Down syndrome and trisomy 21, is it different?
Down syndrome and trisomy 21, is it different?

What is trisomy?

  • Trisomy is a genetic disorder characterized by the presence of a supernumerary chromosome. We all have 46 chromosomes, divided into 23 pairs. Each chromosome forming a pair of chromosomes comes from our father and mother. A trisomic person has 47 chromosomes. This anomaly is due to a gametes problem in the father or mother. Indeed, to make spermatozoa and ova, the cells must undergo distinct divisions.
  • When these divisions occur abnormally, a gamete (spermatozoa or ovule) has an abnormal number of chromosomes. This anomaly can thus be found in the egg cell (cell resulting from the fusion between a spermatozoon and an ovule).
  • In the case of trisomy, the supernumerary chromosome can affect any pair of chromosomes. If it is the 21 pair that is affected, we will talk about trisomy 21. If on the contrary, it is the pair 18 that is touched, one will speak of trisomy 18.
  • The human species consists of 23 pairs of chromosomes divided into 22 pairs of autosomes and a pair of sex chromosomes. Sometimes, trisomy affects the pair of sex chromosomes. In Klinefelter syndrome, for example, trisomy involves the pair of sex chromosomes that have two X chromosomes and one Y chromosome.

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What is Down Syndrome?

  • Down Syndrome, named after the physician who described trisomy at the end of the 19th century, is the other name for trisomy 21. Down syndrome has 47 chromosomes distributed in pairs except at the level of Pair 21 which has three chromosomes.
  • The existence of this supernumerary chromosome causes characteristic symptoms: weak muscle tone, flattened nose, stumpy stature, narrowed eyes, round face, mental retardation, and motor. The signs of trisomy 21 are very variable from one individual to another: some people may have significant mental retardation and motor while others may be perfectly autonomous in adulthood.
  • The existence of this additional chromosome also increases the risk of cardiac abnormalities, ENT or digestive infections or certain cancers such as leukemias. People with Down’s syndrome should, therefore, receive much more extensive medical care.
  • The risk of trisomy 21 can be determined from the first ultrasound (between 11 and 13 weeks of amenorrhoea) by measuring nuchal translucency and craniocaudal length. A blood test, carried out during this same period, to establish the dosage of two serum markers also makes it possible to evaluate the potential risk. If the risk is estimated to be more than 1/250, two diagnostic tests can be proposed: trophoblast biopsy or amniocentesis.

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