karyotypes

A karyotype is the complete set of chromosomes in a cell, usually displayed as a karyogram, a visual representation in which chromosomes are arranged in pairs by size, banding pattern, and centromere position. It reflects the number, form, and structure of chromosomes. In humans, the normal diploid karyotype consists of 46 chromosomes: 22 autosome pairs and 1 pair of sex chromosomes. The sex chromosome complement is typically 46,XX or 46,XY. Variants include monosomy or trisomy for whole chromosomes and structural rearrangements, which can alter phenotype.

To obtain a karyotype, cells are collected (often blood or amniotic fluid), cultured to obtain dividing cells, treated with a spindle inhibitor to arrest metaphase, fixed, stained (commonly with Giemsa to produce G-bands), photographed, and chromosomes are cut and paired to form a karyogram. Modern methods include fluorescence in situ hybridization (FISH) and spectral karyotyping, which can identify specific chromosomes and structural changes. Karyotyping detects numerical abnormalities (aneuploidies) and large structural rearrangements, but small deletions or duplications require higher-resolution methods such as microarray or sequencing.

Applications include prenatal diagnosis, cancer cytogenetics, and diagnosis of developmental disorders. Common aneuploidies include trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), trisomy 13 (Patau syndrome), and sex chromosome aneuploidies such as Turner syndrome (45,X), Klinefelter syndrome (47,XXY), Triple X syndrome (47,XXX), and XYY syndrome (47,XYY). Balanced translocations may be inherited without phenotypic effect but can cause infertility or recurrent miscarriages. Karyotyping remains a foundational tool in clinical cytogenetics and comparative genomics.