How may chromosome subtypes are observed in Down syndrome?
There are three main types of chromosome abnormalities in Down syndrome: 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. This condition is called trisomy 21. The second type is called translocation since the extra 21 chromosome is attached or translocated on to another chromosome, usually on chromosome 14, 21 or 22. If translocation is found in a child with Down syndrome, it is important to examine the parents’ chromosomes, since in at least one-third of the cases, a parent may be a carrier of the translocation. This form of chromosome error is found in three to four percent of the individuals with Down syndrome. Another chromosome problem, called mosaicism, is noted in about one percent of individuals with Down syndrome. In this case, some cells have 47 chromosomes and others have 46 chromosomes. Mosaicism is though
