Down Syndrome: What It Is, Types, Risk Factors and Incidence

Down Syndrome, also known as Trisomy 21, is a condition characterized by the presence of an extra chromosome 21. This additional genetic material alters the course of development and leads to the characteristics associated with Down Syndrome.

Types of Down Syndromes

In a typical human cell, there are 46 chromosomes organized in 23 pairs. These chromosomes carry the genes that regulate various characteristics of the body. Of these pairs, 22 are homologous chromosomes, meaning they contain the same genetic information. The final pair, the sex chromosomes, differ in males and females.

However, in individuals with Down Syndrome, there's a deviation from this standard chromosomal organization. Instead of the usual 46 chromosomes, they possess 47. This extra chromosome is due to an error in cell division during development.

There are three types of Down Syndrome, classified based on the characteristics of the additional chromosome 21:

1. Free Trisomy 21
   This is the most common form, accounting for about 92-94% of all cases. In this type, an entire extra chromosome 21 is present in all cells.
2. Translocation Trisomy 21
   This type accounts for around 4-5% of all cases. Here, the extra chromosome 21 is attached or "translocated" to another chromosome.
3. Mosaic Trisomy 21
   In this rare form, present in 2-3% of cases, some cells have the normal 46 chromosomes, while others have an extra chromosome 21, totalling 47.

Risk Factors

The risk of having a child with Down Syndrome increases with maternal age, particularly after the age of 35. In 90-95% of cases, the extra chromosome originates from the mother's egg cell. This is due to an error in cell division called non-disjunction, which occurs sporadically and unpredictably.

The risk of having another child with Down Syndrome after having one is approximately 1%, but this risk increases with maternal age. The exact causes behind non-disjunction are still unknown, but advanced maternal age remains a significant risk factor.

Translocation Trisomy 21 can be inherited, accounting for about 4-5% of all cases. Approximately half of these cases are hereditary.

Incidence

Currently, the prevalence of Down Syndrome is estimated at around one case in every 750 live births. The incidence would be much higher if not for the fact that many pregnancies with Trisomy 21 end in miscarriage, typically in the first trimester. Live births with Down Syndrome constitute only about 25% of all Trisomy 21 conceptions.