Down syndrome is a genetic condition present from birth, and it is not something an individual can acquire later in life. This condition arises from a chromosomal difference that occurs during early development, meaning the genetic alteration is always congenital. A diagnosis might occur at various points, but the condition is always congenital.
What Down Syndrome Is and When It Develops
Down syndrome is a chromosomal condition resulting from the presence of extra genetic material from chromosome 21. This additional genetic material alters the course of development, leading to the characteristics associated with the syndrome. Typically, human cells contain 23 pairs of chromosomes, totaling 46, but individuals with Down syndrome have an extra copy of chromosome 21, or a portion of it, in some or all of their cells.
This genetic difference occurs at or around the time of conception, either during the formation of reproductive cells or in the early stages of embryonic development. The presence of this extra genetic material affects how the brain and body develop, leading to both physical features and developmental challenges. These characteristics are present from birth and become more apparent as a child grows.
The Genetic Variations of Down Syndrome
Down syndrome manifests in three primary genetic variations, each involving an extra copy of chromosome 21.
Trisomy 21
This is the most common form, accounting for about 95% of cases. In Trisomy 21, every cell in the body contains three copies of chromosome 21 instead of the usual two. This typically results from an error in cell division, known as nondisjunction, during the formation of the egg or sperm cell.
Translocation Down Syndrome
This type makes up approximately 3% to 4% of cases. Here, an extra part or an entire extra chromosome 21 is attached to another chromosome, often chromosome 14, instead of being a separate chromosome. While the total chromosome number might remain 46, the presence of the extra chromosome 21 material causes the characteristics of Down syndrome. This form can sometimes be inherited if a parent carries a balanced translocation.
Mosaic Down Syndrome
The rarest form, affecting about 1% to 2.5% of individuals with the condition. In this variation, only some of the body’s cells have an extra copy of chromosome 21, while other cells have the typical two copies. This mosaicism can lead to a wider range of characteristics, sometimes with less pronounced features compared to Trisomy 21, depending on the proportion and distribution of cells with the extra chromosome.
Can Down Syndrome Be Acquired in Adulthood?
Down syndrome is a genetic condition established at conception or very early embryonic development. It is not a disease or illness that can be acquired later in life. The chromosomal changes occur during the formation of reproductive cells or during the first cell divisions after fertilization, making it impossible for an adult to develop Down syndrome.
While a diagnosis of Down syndrome might occur later in life, particularly in cases of mosaicism where physical characteristics may be less evident, this represents a delayed identification of a lifelong condition. It is not the acquisition of the condition itself. The genetic alteration is always present from early development, even if signs are subtle or initially overlooked.
Health and Aging for Individuals with Down Syndrome
Individuals with Down syndrome experience aging and associated health considerations throughout their lifespan. Life expectancy for people with Down syndrome has significantly increased over recent decades, now reaching nearly 60 years of age. This longer life span means that age-related health issues become more relevant.
People with Down syndrome often exhibit signs of accelerated aging, meaning certain health changes may appear earlier than in the general population. There is an increased risk for specific conditions, such as congenital heart defects, thyroid abnormalities like hypothyroidism, and vision and hearing impairments. These conditions may require ongoing management as individuals age.
A primary concern for aging individuals with Down syndrome is a higher risk of developing Alzheimer’s disease, often at a younger age. By age 40, almost all individuals with Down syndrome show brain changes characteristic of Alzheimer’s disease, though not all will develop clinical symptoms of dementia. Around half of all people with Down syndrome may develop Alzheimer’s disease by age 60, with symptoms typically starting in their early to mid-50s. This increased susceptibility is partly due to the extra copy of chromosome 21, which carries a gene involved in producing a protein linked to Alzheimer’s pathology.