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

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 


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.

Frequency of Down Syndrome

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 

University of Virginia, Charlottesville

Down syndrome is a developmental disorder that causes varying degrees of physical and cognitive disability. It is the most common genetic condition and is caused by a chromosomal abnormality in which there is an extra copy of chromosome 21. 

Down Syndrome

As a result of atypical chromosomal composition, the characteristics of individuals with Down syndrome present in many aspects of their development, both physical and cognitive. Individuals with Down syndrome are characterized by…
 - Physical characteristics of Down syndrome
 - Cognitive characteristics of Down syndrome
- Developmental characteristics of Down syndrome
- Comorbidities of Down syndrome


Characteristics of Down syndrome

The science, research, and biological evaluation of Down syndrome allow us to understand it as we can today. Ways in which scientists have made progress in Down syndrome studies can be appreciated through
- the neurobiology of Down syndrome
- Genes affected by the triplication of chromosome 21 and their links to Down syndrome symptoms
- Mouse Model of Down syndrome: Ts65Dn
- Emerging treatments of Down syndrome


The Science of Down syndrome

What causes Down syndrome?

Individuals with disabilities and their families and caretakers often identify with the umbrella term of the “Down syndrome community.” To understand this disability, it is important to understand the cultural and social characteristics of the community. We can understand the community by considering how the Down syndrome community prefers to be communicated with, the employment and education trends, disparities in outcomes, how individuals with Down syndrome see themselves and each other, and the experiences of caretakers. 

Culture of Down syndrome

The life expectancy for people with down syndrome has increased from 10 years of age in 1960 to 47 in 2007. It is still lower than people without it because of the additional health conditions and lower birth weight associated with this syndrome. Race has also been seen to impact how long one can live with this condition. One explanation for this increase in life expectancy is that people have stopped institutionalizing those with down syndrome.

Life expectancy for people with Down Syndrome

Such factors may include:
- chemotherapy;
- cigarette smoking;
- chemicals that disrupt hormone-releasing glands;
- hormones the person receives through treatment for example.

Environmental factors that can influence probability of Down Syndrome

Dementia is common in adults with Down Syndrome, but it is difficult to receive this diagnosis. It may be manifested in different ways depending on the individual. 

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