Down syndrome is a set of phenotypic manifestations related to the presence, partial or complete, of a supernumerary chromosome 21. Because of this, 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 Nucleus translucency Care and Treatment Down Syndrome Down Syndrome, between myths and truthClassification and causes of origin
The genetic code of man is based on 46 chromosomes, organized in pairs. Within these DNA clusters are 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.). According to what has been said, in man there are 23 chromosomal pairs, of which 22 are constituted by homologous chromosomes, while the last pair - that of the sex chromosomes - is formed by different chromosomes.
In summary, all * the cells of the human body contain 46 chromosomes, divided into 23 pairs, of which 22 are homologous (autosomes) and a heterologous one which involves the sex chromosomes (heterosomes).
* Only the germ cells, which are the spermatozoa in man and the egg cells in women, escape from the rule just expressed. These cells, consisting of 23 chromosomes, unite during fertilization; thus, at birth, each of our chromosomal pairs is composed of a chromosome inherited from the mother and a chromosome inherited from the father.
In Down syndrome the chromosome organization listed above is missing; in fact, in every cell of the body we find an extra chromosome (we pass 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.
| Trisomy 21 free complete | Trisomy 21 from translocation | Trisomy 21 free in mosaicism |
| 92-94% of cases | 4-5% of cases | 2-3% of cases |
| Chromosome 21 is present three times instead of two. | The supernumerary chromosome No. 21 is connected to another chromosome (translocation) instead of to the pair No. 21. | One part of the cells contains the normal set of 46 chromosomes, while the others, due to the added chromosome No. 21, have a total of 47 chromosomes. |
As explained above, Down syndrome is also known as "trisomy 21", since in all forms the chromosome n ° 21 is present 3 times. The name most known, "Down Syndrome", derives instead from the name of the English physician, J.Down, who described it for the first time back in 1866, using the term mongoloidismo due to the somatic features of the patients' faces, which recalled precisely those of the Asian Mongolian populations.
Risk factors and Incidence
As is known to most, the risk of conceiving a child with Down Syndrome increases with advancing age of the mother at conception, becoming consistent after the age of 35. In 90-95% of cases it is in fact the mother that 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 separation 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 following pregnancy, but unfortunately the opposite event is also possible. In particular, in the first case, the risk of giving birth to another child suffering from Down syndrome is around one percent, while in the second case it goes hand in hand with the mother's age.
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 greater than 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%.
Inheritance of Down syndrome is accepted only for trisomy 21 from translocation (4-5% of cases, of which about half are inherited).
Currently, the neonatal prevalence of the disease is estimated around one in every 750 live births. The incidence of Down syndrome would still be much higher, were it not 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 constitute only about 25% of the total trisomic conceptions for chromosome 21.
