Down syndrome is a genetic condition present from birth, leading to a range of developmental and physical characteristics. It arises from changes in an individual’s chromosomal makeup, specifically in the number or structure of chromosomes within the body’s cells.
The Chromosomal Anomaly in Down Syndrome
The genetic cause of Down syndrome is the presence of an extra full or partial copy of chromosome 21, often referred to as Trisomy 21. The term “trisomy” indicates three copies of a particular chromosome instead of the usual two.
Typically, each human cell contains 23 pairs of chromosomes, totaling 46. These chromosomes carry genetic instructions that guide the body’s development and function. In individuals with Trisomy 21, however, cells contain 47 chromosomes, with the additional one being a third copy of chromosome 21. Chromosome 21 is the smallest human autosome, which is a non-sex chromosome.
Different Genetic Types of Down Syndrome
The extra genetic material from chromosome 21 can arise through different mechanisms, leading to three main types of Down syndrome.
Standard Trisomy 21
This is the most common form, accounting for approximately 95% of cases. It occurs due to an error in cell division called nondisjunction, where a pair of chromosome 21s fails to separate correctly during the formation of egg or sperm cells. As a result, the developing embryo receives an extra chromosome 21 from one parent, and this additional chromosome is replicated in every cell of the body. This form is generally not inherited from parents.
Translocation Down Syndrome
Occurring in about 3% of cases, this type involves an extra full or partial copy of chromosome 21 attaching itself to another chromosome, often chromosome 14, 21, or 22. In this scenario, the total number of chromosomes remains 46, but the genetic material is rearranged. This type can sometimes be inherited if a parent carries a balanced translocation, meaning they have rearranged genetic material but no extra or missing chromosomal content themselves.
Mosaic Down Syndrome
Mosaic Down syndrome is the least common type, affecting about 1% to 2% of individuals with the condition. This form is characterized by the presence of two or more genetically different cell lines within the same person. Some cells have the typical 46 chromosomes, while others have an extra copy of chromosome 21, totaling 47 chromosomes. This mosaic pattern typically arises from an error in cell division that occurs after fertilization, during the early stages of embryonic development.
How Extra Genetic Material Affects Development
The presence of extra genetic material from chromosome 21 influences development through a concept known as “gene dosage.” Having three copies of the genes located on chromosome 21, instead of the usual two, leads to an increased expression of these genes. This overexpression can disrupt the balance of normal developmental pathways and cellular processes.
The altered gene dosage impacts various systems throughout the body. While the exact mechanisms are complex, the cumulative effect of these overexpressed genes contributes to the characteristic features and health considerations associated with Down syndrome. This affects how the body and brain develop.