Down Syndrome Site : Down Syndrome Genetic History

The Down syndrome genetic condition occurs when a person has three copies of chromosome 21, instead of the normal two. This is called "trisomy 21." The additional genetic material is what causes Down syndrome and every cell of a Down baby will then have 47 chromosomes, rather than 46. However, a small percentage of people with Down syndrome have an extra copy of chromosome 21 in some cells but not all, in a condition called "Mosaic Down syndrome." In a third type called "Translocation," part of chromosome 21 attaches to another chromosome during the formation of eggs and sperm, early in fetal development. There are few differences in people with the various Down syndrome types, although they exhibit a wide range of effects.

There are many ways the genetic data of Down syndrome can be damaged or rearranged. For example, in trisomy 21, one parent has two chromosomes located at chromosome #21, rather than one, causing the child to receive two chromosomes from one parent and one from the other parent for a total of three. The Down syndrome child will then have 47 chromosomes, instead of 46. This occurs in 90% of all Down syndrome cases.

With Mosaic syndrome disorder, chromosome 21 gets damaged during cell replication, where two normal chromosomes become three in some cells. People with this disorder have some cells with 46 chromosomes and some cells with 47 and their symptoms may be less severe. This occurs in 2% of all Down syndrome cases. The last type of Down syndrome is called Translocation and occurs when a normal chromosome breaks into two pieces and attaches to another chromosome. This occurs in 3-4% of all Down patients.

Down syndrome genetic malfunctions are not inherited, generally, although trouble with chromosome 21 can be detected as early as the first trimester now. Translocation syndrome is said to be the only inheritable type, which occurs because one parent is a carrier. A carrier will have 45 chromosomes rather than 46, even though they have the correct amount of genetic information and display no effects of Down syndrome. If the carrier is the mother, then there is a 1/5 chance of having a child with Translocation syndrome. However, if the father is the carrier, then there is only 1/20-1/50 odds that the child will inherit the disorder. Prior to pregnancy, both parents can meet with a genetic counselor to determine their risk.

To discover Down syndrome genetic mutations, there is testing that can be done. Non-invasive screening tests can be performed as early as 11-15 weeks, either the Nucha translucency ultrasound or the triple screen blood test. These procedures accurately predict Down syndrome 80% of the time. Diagnostic tests are 99% accurate but are also more invasive and more dangerous. Following amniocentesis, chorionic villus sampling or percutaneous umbilical blood sampling, miscarriages happen 2-3% of the time. Moreover, women who have received a Down syndrome diagnosis often go on to have perfectly healthy babies. Once the baby is born, the doctor can also test with x-rays, cardiographs and can assess by looking.

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