Preparation of Karyotype

A karyotype is the graphic representation of the physical appearance of the chromosomes of a particular organism.

It is the chromosome complement of a cell or organism depicting the number, size and form of the chromosomes as seen in mitosis.

Karyotypes are now prepared routinely for genetic Counseling of couples at risk for having children with Down’s Syndrome or other defects associated with chromosome abnormalities. The chromosomes show in a human karyotype are characteristically metaphase consisting of two sister chromatids held together at their centromere. The prepare a karyotype, white blood cells in the process of dividing are interrupted at metaphase by the addition of colchicine. The prevents the subsequent steps of mitosis from taking place. By other words, division of the human cells in culture is arrested at the metaphase with the help of colchicine. After treating and staining such cells, when fixed with alcochol and stained, the chromosomes are photographed, enlarged, cut out, and arranged to size. The photographs or diagrams of chromosomes are arterially arranged in homologous pairs, which are arranged in order of their size, except the sex chromosomes that are placed at the end.