Down syndrome is a genetic condition caused by the presence of an extra full or partial copy of 21st chromosome. The incidence is estimated in 1 per 700 live births and represents the most common reason for mental retardation in the world. Although the origin of this condition is still unknown, it is not related to nationality, race, religion or socioeconomic status. The most important development delays in individuals with Down syndrome occur in the cognitive and motor areas. However, as is true for all people, each one has their own abilities and personality that makes them different from one another. For that reason, specialized care and education services should be provided shortly after birth in order to stimulate the acquisition of abilities and skills that support their development and contribute to improve their quality of life.

1. Trisomy 21

Also named nondisjunction, Trisomy 21 is the result of faulty chromosome separation during cell division which makes that either the sperm or the egg has two 21st chromosomes instead of one, so the resulting fertilized egg has three 21st chromosomes.

As the new embryo develops, the extra chromosome is transmitted and replicated in every cell of the body, in such a way that all of cells contain the additional chromosome 21. This error in cell division is responsible for 90 to 95 percent of all cases of Down syndrome.

2. Translocation

In translocation, there is also a triplication of the 21st chromosome material. The difference is that this additional chromosome is rearranged and attached to another, usually the 14th or to another 21st chromosome.  Approximately the fourth part of translocations spontaneously occurs during fertilization. The extra genetic material is inherited from one of the parents; therefore, this is the only case in which Down syndrome is caused by a parental genetic condition. Translocation accounts for 5 to 7 percent of all cases of Down syndrome.

3. Mosaicism

In Mosaicism, the random error takes place in one of the initial cell divisions after fertilization, causing individuals to have 46 chromosomes in some of their cells and 47 in others. Similar to Trisomy 21, there is a factor that causes the unequal chromosome division. When this happens during the second or third cell division, only some of the resulting embryo cells contain the additional 21st chromosome. This means that not all the embryo cells contain the additional chromosome; and as result, it is possible that the newborn may not have all the physical characteristics of this syndrome. Mosaicism occurs in only 1 to 3 percent of all cases of Down syndrome.