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

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

While the occurrence of Down syndrome is generally random and not directly correlated with most health, personal, or socioeconomic factors, certain parental characteristics can increase the likelihood of a child being born with the condition. The two primary risk factors are advanced maternal age and having one or both parents who carry a chromosomal abnormality or have Down syndrome.

Risk Factors for Down Syndrome

Down syndrome is caused by a chromosomal abnormality involving an extra copy of chromosome 21, resulting in an excessive number of chromosomes (47 instead of the typical 46). Although variations of this chromosomal abnormality exist and it is the most common chromosomal condition, occurrences of Down syndrome are generally random and not linked to socioeconomic patterns. Its etiology can be further understood by examining the specific genetic types, inheritance patterns, and associated risk factors.

University of Virginia, Charlottesville

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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

Down syndrome presents with a wide variety of physical, cognitive, and developmental characteristics, as well as several common comorbidities. Because the syndrome is caused by atypical chromosomal composition—specifically an extra copy of chromosome 21—these characteristic manifestations affect multiple developmental pathways throughout an individual's life.

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

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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