Down Syndrome is a genetic condition that affects an individual’s development. It arises from changes in a person’s chromosomes. Understanding chromosomes helps clarify its genetic basis.
What Chromosomes Are
Chromosomes are thread-like structures found within the nucleus of nearly every human cell. They are organized packages of deoxyribonucleic acid (DNA), which contains the genetic instructions for an organism’s development and function. Each chromosome is essentially a long strand of DNA tightly coiled around proteins, allowing it to fit compactly inside the cell.
Humans typically have 23 pairs of chromosomes, totaling 46 in each cell. 22 pairs are known as autosomes, which are similar in both males and females. The remaining pair consists of sex chromosomes, designated as XX for females and XY for males, determining biological sex.
The Chromosomal Count in Down Syndrome
Individuals with Down Syndrome typically have 47 chromosomes in each cell, rather than the usual 46. This difference stems from an extra copy of chromosome 21, meaning they have three copies instead of the standard two.
This specific chromosomal alteration is known as Trisomy 21, meaning “three bodies” of chromosome 21. Trisomy 21 accounts for approximately 95% of all Down Syndrome cases. The additional genetic material influences how the brain and body develop, leading to the condition’s characteristic features.
Less Common Chromosomal Variations
While Trisomy 21 is the most frequent form, other less common chromosomal variations also lead to Down Syndrome. Robertsonian Translocation accounts for about 3% to 4% of cases. In this type, the total chromosome count remains 46, but an extra part or a full extra chromosome 21 becomes attached, or translocated, to another chromosome, often chromosome 14 or 22. This results in effectively having extra genetic material from chromosome 21.
Mosaic Down Syndrome is the rarest form, affecting about 1% to 2% of individuals. In this variation, some cells in the body have the typical 46 chromosomes, while other cells contain 47 due to an extra copy of chromosome 21. This mixture of cell types arises from an error in cell division that occurs after fertilization, during early fetal development.
Confirming the Chromosome Count
The specific number and arrangement of chromosomes in individuals with Down Syndrome are identified through a laboratory test called karyotyping. A karyotype is a visual representation of a person’s chromosomes, arranged in numerical order by size and centromere position.
To perform a karyotype, cells are collected, typically from a blood sample, and then processed to visualize their chromosomes under a microscope. Geneticists photograph the chromosomes and then organize them into pairs, allowing for a precise count and detection of any extra or missing chromosomes, or structural rearrangements. This detailed analysis confirms the diagnosis of Down Syndrome and determines its specific chromosomal type.