What is Down syndrome and what are the most common types? Down syndrome is a genetic disorder that is, a disorder arising from an abnormality in an individual’s genetic material4. Human cells typically consist of 23 pairs of chromosomes. 1 chromosome in each pair comes from your father and the other comes from your mother, this results in the person having 3 copies of chromosome 21, instead of the usual 2 copies, in all cells. Some common physical traits of Down syndrome are lack of muscle tone, small stature, an upward slanted eyes, and one single deep crease across the middle of the palm although each person with Down syndrome is a special individual and may obtain these characteristics to different levels, or not at all. Down syndrome is the most common chromosomal disorder found in humans but the three types that can be found in humans are Trisomy 21, Translocation Down syndrome and Mosaic Down syndrome.
Trisomy 21 is one of the most common human chromosomal aberrations, occurring in about one out of every eight hundred live births. Even before the chromosomal basis for the disorder was determined, the frequency of Down syndrome births was correlated with increased maternal age4. It takes place when there are 3, rather than 2, number 21 chromosomes existing in every cell of the human body. Instead of the typical 46 chromosomes, a person that has Down syndrome has 47. Because of this additional genetic material that changes the course of development and causes the characteristics associated with the syndrome. Children with trisomy 21 often suffer from severe, prolonged respiratory infections, which is an immune system defect5. Only two other trisomy’s occur with any significant frequency: trisomy 13 (Patau syndrome) and trisomy 18 (Edwards syndrome). Both of these disorders are accompanied by multiple severe malformations, typically resulting in death within a few months of birth4. Trisomy 21 makes up to 95% of cases.
Translocation is accounted for 4% of all cases of Down syndrome. In translocation, part of chromosome 21 branches off during the cell division and latches to another chromosome, normally chromosome 14. It should be noted that not all defects associated with Down syndrome are found in every affected individual. About 40 percent of Down syndrome patients have congenital heart defects, while about 10 percent have intestinal blockages. While the whole number of chromosomes in the cells remains 46, the existence of an extra part of chromosome 21 causes the characteristics that make up Down syndrome. Whenever translocation is discovered in children, doctors study the parent’s chromosomes to determine if the translocation was genetic or not. If one of the parents has a translocation chromosome, then doctors knows the baby genetically inherited the translocation chromosome from that parent. When a person has a rearrangement of chromosome material he or she has a balanced translocation or a balanced translocation carrier. Parents with balanced translocations can have fertility problems like having trouble to become pregnant, have an increased chance of having a child with health problems, or having miscarriages. Even though the parent can have the ability to donate the appropriate amount of the 23 chromosomes to a pregnancy, he or she can also have a risk factor of donating too much or little genetic material to that pregnancy. This isn’t anything the parent can predict or control.
Mosaicism, or Mosaic Down syndrome, is one of the rare forms of Down syndrome. Mosaicism happens when nondisjunction of the chromosome 21 takes place in one, but not all, of the primary cell divisions after fertilization and makes up about 1% of all cases of Down syndrome. When this happens, there is a mixture of 2 types of cells, some containing the normal 46 chromosomes and the others containing 47. People with this often, but not in all cases, have less symptoms of Down syndrome because some are normal cells. This mosaic of abnormal and normal cells is created by abnormal cell division after fertilization. When children are born with Down syndrome, the doctor takes a sample of blood to do a chromosome study. Although many children with Down syndrome have happy dispositions, a significant number have behavioral problems that can consume the energies of the parents, to the detriment of other children.3 Mosaicism is typically explained by a percentage. Usually , they analyze 20 different cells in a chromosome study. A baby could be said to have mosaic Down syndrome if 5 out of 20 cells have the usual number of 46 chromosomes. The other cells 15 have a number of 47 chromosomes due to one extra chromosome 21. There are developing studies that will discover if there is anything in common with in the health of people with mosaic Down syndrome, based on how the mosaicism occurred. Studies have also been looking at if the percentage of mosaicism can tell if the baby’s IQ or tell if there will be a defect in the heart. Those studies have showed that the percentage of mosaicism doesn’t predict outcome accurately. The risk of having a chromosome defects in a pregnancy is widely based on the women’s age at the time they had the baby. It goes up every year with increasing age.