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Down syndrome, Klinefelter syndrome, trisomy 21 or trisomy 18, it is sometimes difficult to differentiate between these different pathologies. Trisomy 21 is characterized by the existence of an extra chromosome at the level of the pair of chromosomes 21. This type of trisomy is also called Down syndrome, named after the first doctor to have carried out a thorough clinical study of this chromosomal anomaly. .
What is trisomy?
- Trisomy is a genetic disease that is 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, in turn, 47 chromosomes. This anomaly is due to a problem of gamete production in the father or mother. In fact, to make sperm and eggs, the cells must undergo specific divisions.
- When these divisions occur abnormally, it happens that a gamete (spermatozoid or ovum) 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 egg).
- In the case of trisomy, the supernumerary chromosome can affect any pair of chromosomes. If it is the pair 21 that is affected, we will talk about trisomy 21. If instead, it is the pair 18 is affected, we talk about Trisomy 18.
- The human species has 23 pairs of chromosomes divided into 22 pairs of autosomes and a pair of sex chromosomes. Sometimes trisomy affects the sex chromosome pair. In Klinefelter syndrome, for example, trisomy refers to the sex chromosome pair that has two X chromosomes and one Y chromosome.
What is Down syndrome?
- Down syndrome, named after the physician who described this trisomy in the late nineteenth century, is the other name of trisomy 21. People with Down's syndrome therefore have 47 chromosomes distributed in pairs, except at the level of pair 21 which has 3 chromosomes.
- The existence of this supernumerary chromosome causes characteristic symptoms: a weak muscular tone, a flattened nose, a stout stature, slanted eyes, a round face, a mental and motor retardation. Signs of trisomy 21 vary greatly from one individual to another: some individuals may have significant mental and motor retardation while others may be fully independent in adulthood.
- The existence of this extra chromosome also increases the risk of suffering from cardiac abnormalities, ENT or digestive infections or certain cancers such as leukemias. People with Down's Syndrome should therefore benefit from a much greater medical follow-up.
- The risk of trisomy 21 can be determined from the first ultrasound (between 11 and 13 weeks of amenorrhea) thanks to the measurement of nuchal translucency and cranio-caudal length. A blood test, performed during this same period, to establish the determination of two serum markers also makes it possible to evaluate the potential risk. If the risk is estimated at more than 1/250, two diagnostic tests may be offered: a trophoblast biopsy or amniocentesis.