What are the Types of Down Syndrome?

Trisomy-21

Translocation

Mosaicism

The vast majority of children with Down syndrome (approximately 95 percent) have an extra 21 chromosome. Instead of the normal number of 46 chromosomes in each cell, the individual with Down syndrome has 47 chromosomes. These children may only have triplication of part of the 21st chromosome instead of the whole chromosome, which is called a trisomy 21.

Three to four percent of all cases of trisomy 21 are due to Robertsonian Translocation. In this case, two breaks occur in separate chromosomes, usually the 14th and 21st chromosomes. There is rearrangement of the genetic material so that some of the 14th chromosome is replaced by extra 21st chromosome. The extra 21st chromosome is attached or translocated on to another chromosome, usually on chromosome 14, 21 or 22. Translocations resulting in trisomy 21 may be inherited, so it is important to check the chromosomes of the parents in these cases to see if either may be a "carrier."

The remainder of cases of Down syndrome (one percent) is due to mosaicism. In this case, some cells have 47 chromosomes and others have 46 (normal) chromosomes. Mosaicism is thought to be the result of an error in cell division soon after conception. In cellular mosaicism, the mixture is seen in different cells of the same type. In tissue mosaicism, one set of cells, such as all blood cells, may have normal chromosomes, and another type, such as all skin cells, may have trisomy 21.