A karyotype provides a visual representation of an individual’s complete set of chromosomes. It organizes and analyzes these thread-like structures found within the nucleus of cells. This display helps scientists identify genetic conditions linked to chromosomal variations.
Creating and Interpreting a Karyotype
The creation of a karyotype begins by collecting cells, often from a blood sample, amniotic fluid, or bone marrow. These cells are then grown in a laboratory culture to encourage division. Cell division is halted during metaphase, a stage where chromosomes are highly condensed and easily visible. The chromosomes are then stained, which produces distinct light and dark banding patterns.
Once stained, the chromosomes are photographed through a microscope. Individual chromosome images are then arranged into homologous pairs. These pairs are ordered by size, from the largest (chromosome 1) to the smallest (chromosome 22), followed by the sex chromosomes (X and Y). Scientists interpret a karyotype by examining the number of chromosomes, their overall structure, and unique banding patterns. This allows detection of deviations from the typical human chromosomal complement.
Chromosomal Number Abnormalities Visible on a Karyotype
A karyotype clearly reveals abnormalities in chromosome number, broadly categorized as aneuploidy. Aneuploidy describes a condition where an individual has an abnormal number of chromosomes, meaning extra or missing chromosomes compared to the usual 46. This deviation can have significant health implications, depending on which chromosome is affected.
Polysomy represents a specific type of aneuploidy characterized by the presence of extra copies of a particular chromosome. For instance, an individual might have more than the standard two copies of a single chromosome. This excess can lead to a range of developmental and physiological changes.
Trisomy is the most common form of polysomy, where an individual possesses three copies of a specific chromosome instead of the usual two. A well-known example is Trisomy 21, which results in Down syndrome, visibly showing an extra chromosome at position 21. Other trisomies, such as Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome), also involve an additional chromosome at their respective positions, leading to distinct clinical features. Sex chromosome aneuploidies, like XXY (Klinefelter syndrome) or XO (Turner syndrome), also appear as an extra or missing sex chromosome.
Limitations of Karyotyping
Despite its utility, a karyotype cannot detect all genetic variations. For instance, histones, proteins that help package DNA into chromosomes, are not individually visible or identifiable as abnormalities on a standard karyotype. Karyotyping focuses on larger chromosomal structures and their numerical or gross structural integrity, not their molecular components.
Furthermore, a karyotype is unable to detect very small genetic changes. Single gene mutations, involving changes to a single nucleotide base within a gene, are too minute to be observed. Similarly, small deletions or insertions of DNA segments, often called microdeletions or microduplications, are beyond the resolution of standard karyotype analysis. These molecular alterations require more advanced genetic testing techniques that analyze DNA sequence directly.