Down syndrome is a set of phenotypic manifestations related to the presence, partial or complete, of a supernumerary chromosome 21. For this reason, Down syndrome is also known as "trisomy 21".
Index Section
Classification and causes of origin Risk factors and incidence Causes and risk of having an affected child Clinical phenotype (signs and symptoms) Symptoms Down syndrome Screening test in pregnancy PAPP-A Duo test Combined test in pregnancy Three tests in pregnancy Nuchal translucency Care and Down Syndrome Diet Treatment Down Syndrome, between myths and truthsClassification and Causes of Origin
The genetic patrimony of man is based on 46 chromosomes, organized in pairs. Inside these clusters of DNA are contained the genes that regulate the various characters of the organism; we speak of homologous chromosomes when both contain the same genetic information, related to the same phenotypic characters (eye color, skin color, etc.). As mentioned, in man there are 23 chromosomal pairs, of which 22 are made up of homologous chromosomes, while the last pair - that of sex chromosomes - it is made up of different chromosomes.
In summary, all * cells of the human organism contain 46 chromosomes, divided into 23 pairs, of which 22 homologous (autosomes) and a heterologous one involving the sex chromosomes (heterosomes).
* The rule just expressed, only the germ cells escape, which are the spermatozoa in the man and the egg cells in the woman. These cells, made up of 23 chromosomes, unite during fertilization; thus, at birth, each of our chromosomal pairs it is composed of a chromosome inherited from the mother and a chromosome inherited from the father.
In Down syndrome the chromosomal organization just listed is missing; in fact, in every cell of the organism, we find an extra chromosome (we go from 46 to 47 chromosomes).
In relation to the characteristics of the supernumerary chromosome that characterizes the genome of people with Down syndrome, we can distinguish three different forms of trisomy.
As explained above, Down syndrome is also known as "trisomy 21", given that chromosome 21 is present 3 times in all forms. The best known name, "Down Syndrome", derives from the name of the English doctor, J. Down, who described it for the first time in 1866, using the term Mongoloidism because of the facial features of the patients, which recalled precisely those of the Asian populations of Mongolian ethnicity.
Risk factors and incidence
As known to most people, the risk of conceiving a child with Down Syndrome increases with the advancing age of the mother at the moment of conception, becoming consistent after 35 years of age. In 90-95% of cases it is in fact the mother who has, in the gamete object of conception (egg cell), two chromosomes 21 instead of a single copy of the same. All this due to a lack of disjunction of the same during meiosis; this event occurs sporadically, therefore in an unpredictable way. Consequently, the mother of a child with Down syndrome can give birth to a perfectly healthy child in the subsequent pregnancy, but unfortunately the opposite event is also possible. In particular, in the first case, the risk of giving birth to another affected child. from Down syndrome it is around one percent, while in the second it goes hand in hand with the age of the mother.
The causes responsible for the lack of chromosomal disjunction have not yet been specified, so much so that as we have been able to recall, the "only important risk factor is a" maternal age over 35 years. At this age, in fact, the risk is 0.25%, while at 45 the probability of giving birth to a child with Down syndrome rises to 2.8%.
Down syndrome inheritance is accepted only for translocation trisomy 21 (4-5% of cases, of which about half are inherited).
Currently, the neonatal prevalence of the disease is estimated to be around one case for every 750 live births. The incidence of Down syndrome would still be much higher, were it not for the fact that most of the trisomic pregnancies for chromosome 21 go against spontaneous interruption, generally in the first trimester. Data in hand, live births with Down syndrome they constitute only about 25% of the total trisomic conceptions for chromosome 21.
Other articles on "Down Syndrome"
- Down Syndrome: Screening and Testing in Pregnancy
- Down Syndrome - Causes and Risk of Having an Affected Child
- Diet and Down Syndrome
- Medicines to Treat Down Syndrome
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