Translocation is the type of Down syndrome that occurs when the copy of chromosome 21 attaches to and inhibits one of the pairs of another chromosome, usually chromosome 14. The individual retains 46 chromosomes, but the presence of a third copy of chromosome 21 causes characteristics of Down syndrome. This accounts for 3-4% of known cases.

Translocation

Mosaicism in the type of Down syndrome in which nondisjunction occurs in only one initial cell division instead of all. This results in a mix of cells: some containing 46 chromosomes and no extra copies of chromosome 21, and some containing 47 chromosomes with an extra copy of 21. This mixture of cell compositions causes individuals with mosaic Down syndrome to present fewer characteristics of Down syndrome than the other types. Mosaicism accounts for only 1% of known cases.

Mosaicism 

The type in which the extra chromosome 21 is replicated in every cell of the body. This is a result of either the sperm or egg failing to separate the pair of chromosome 21 before or during conception. 
It is the most common type of Down syndrome and accounts for 95% of known cases.


Trisomy 21/ nondisjunction

There are three types of Down syndrome, distinguished by the form in which chromosome 21 is copied. The three types are: 
- Trisomy 21 (also known as nondisjunction):
- Translocation
- Mosaicism 


University of Virginia, Charlottesville

Down syndrome is caused by the presence of an extra copy of the 23rd chromosome, resulting in an excessive number of chromosomes: 47, as opposed to 46. However, variations of this chromosomal abnormality exist. And though it is the most common chromosomal abnormality, births with Down syndrome are random and not linked to any health or socioeconomic patterns. We can best explore the variations of the occurrence of Down syndrome through:  
- Types of Down syndrome
- Trisomy 21
- Frequency 
- Inheritance 
- Factors that increase risk 

What causes Down syndrome?

“About Down Syndrome.” National Down Syndrome Society (NDSS), https://www.ndss.org/about. 

National Down Syndrome Society (NDSS): About Down Syndrome

The majority of Down syndrome cases are not inherited, but there are two ways in which a fetus can acquire the genetic condition that causes Down syndrome. These two ways occur through:

1. A random event during fertilization or early fetal development 
2. An unaffected parent being a carrier of a particular chromosomal rearrangement 


Inheritance

Types of Down syndrome

Though the occurrence of Down syndrome is random and not directly correlated with any health, personal or socioeconomic factors, there are two certain characteristics of a parent that increase the likelihood of a child being born with Down syndrome. These two characteristics are: 
- The age of the mother 
- If one or both parents have Down syndrome or a chromosomal abnormality 



Factors that Increase Risk 

According to the CDC, approximately 1 in every 772 babies are born with Down syndrome in the U.S. every year, making it the most common chromosomal condition.

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