Down syndrome (DS) is the mostly observed and the best known chromosomal defect.  And it is normally seen once every 700-800 births (less than 1%)  births. It is easily be diagnosed at birth by its specific symptoms. At patients hypotonia, slanting eyes, epicantus, pug nose, low located ears, transverse lines on hands, klinodactylie, wide range between the first and the second tombs at food and mental retardation are observed. Half of the children suffer hearth and/or digestion systems defects.

At 94% of the patients there are three copies of 21. chromosome. Regular trisomia 21 results from a defect at maternal meiosis (85%) and at meiosis 1 predominantly (75%). The repetition of the disease at the families is 1%.

There is Robertsonian translocation  at the percentage of 4 between the long branch of chromosome 21. and one of other acrocentric chromosomes ( group D: 13,14,15; Group G: 21,22 ). While D /21 translocation is rooted ( 40%-45%) from parents, G/21 translocation is rooted from parents just at the percentage of 4%It is strange that evaluated as Robersonian translocation at patients having t(21;21), the real one is i21q.

If a person has 21:21 translocation, All his/her children will be either have down syndrome or all these pregnancies will be ended because of monosomia 21. This situation is valid for all translocations observed among homological acrocentric chromosomes.

There is mosaism at 2% of the cases. Mosaic individuals result from normal zygotes or trisomial zygotes because of the fact that mitoticol chromosoms are not seperated. A this patients phenotype can be more smoother than that at regular trisomia 21. However among mosaic patients, there are a wide spectrum.