The cause of the extra full or partial chromosome is still unknown. Most of the time,<unk>  is caused by a random mistake in cell division during early development of the fetus, but not inherited, and there is no scientific research shows that environmental factors or the parents’ activities contributes to<unk> . The only factor that has been linked to the increased chance of having a baby with<unk>  is maternal age.Karyotype for trisomy<unk> : notice the three copies of chromosome 21<unk>  is caused by having three copies of the genes on chromosome 21, rather than the usual two. The parents of the affected individual are typically genetically normal. Those who have one child with<unk>  have about a 1% possibility of having a second child with the syndrome, if both parents are found to have normal karyotypes. The extra chromosome content can arise through several different ways. The most common cause (about 92–95% of cases) is a complete extra copy of chromosome 21, resulting in trisomy 21. In 1.0 to 2.5% of cases, some of the cells in the body are normal and others have trisomy 21, known as mosaic<unk> . The other common mechanisms that can give rise to<unk>  include: a Robertsonian translocation, isochromosome, or ring chromosome. These contain additional material from chromosome 21 and occur in about 2.5% of cases. An isochromosome results when the two long arms of a chromosome separate together rather than the long and short arm separating together during egg or sperm development. Trisomy 21 Trisomy 21 (also known by the karyotype 47,XX,+21 for females and 47,XY,+21 for males) is caused by a failure of the 21st chromosome to separate during egg or sperm development (nondisjunction). As a result, a sperm or egg cell is produced with an extra copy of chromosome 21; this cell thus has 24 chromosomes. When combined with a normal cell from the other parent, the baby has 47 chromosomes, with three copies of chromosome 21. About 88% of cases of trisomy 21 result from nonseparation of the chromosomes in the mother, 8% from nonseparation in the father, and 3% after the egg and sperm have merged. Mosaicism Mosaicism is diagnosed when there is a mixture of two types of cells: some cells have three copies of chromosome 21 but some cells have the typical two copies of chromosome 21. Mosaicism is the least common form of<unk>  and accounts for only about 1% of all cases of<unk> . Children with mosaic<unk>  may have the same features as other children with<unk> . However, they may have fewer characteristics of the condition due to the presence of some (or many) cells with a typical number of chromosomes. Translocation The extra chromosome 21 material may also occur due to a Robertsonian translocation in 2–4% of cases. In this situation, the long arm of chromosome 21 is attached to another chromosome, often chromosome 14. In a male affected with<unk> , it results in a karyotype of 46XY,t(14q21q). This may be a new mutation or previously present in one of the parents. The parent with such a translocation is usually normal physically and mentally; however, during production of egg or sperm cells, a higher chance of creating reproductive cells with extra chromosome 21 material exists. This results in a 15% chance of having a child with<unk>  when the mother is affected and a less than 5% probability if the father is affected. The probability of this type of<unk>  is not related to the mother's age. Some children without<unk>  may inherit the translocation and have a higher probability of having children of their own with<unk> . In this case it is sometimes known as familial<unk> .
Down syndrome
Alzheimer's disease
Baboon syndrome
Apert syndrome
Ring chromosome 14 syndrome
Ring chromosome 20 syndrome
TORCH syndrome
Williams syndrome