A karyotype is the number and appearance of chromosomes in the nucleus of a eukaryote cell.p28 The term is also used for the complete set of chromosomes in a species, or an individual organism.
Karyotypes describe the number of chromosomes, and what they look like under a light microscope. Attention is paid to their length, the position of the centromeres, any differences between the sex chromosomes, and any other physical characteristics.The preparation and study of karyotypes is part of cytogenetics.
Karyogram of human male using Giemsa staining.
The study of whole sets of chromosomes is sometimes known as karyology. The chromosomes are depicted (by rearranging a microphotograph) in a standard format known as a karyogram or idiogram: in pairs, ordered by size and position of centromere for chromosomes of the same size.
The basic number of chromosomes in the somatic cells of an individual or a species is called the somatic number and is designated 2n. Thus, in humans 2n = 46. In the germ-line (the sex cells) the chromosome number is n (humans: n = 23).p28
So, in normal diploid organisms, autosomal chromosomes are present in two copies. There may, or may not, be sex chromosomes. Polyploid cells have multiple copies of chromosomes and haploid cells have single copies.
Karyotypes can be used for many purposes; such as, to study chromosomal aberrations, cellular function, taxonomic relationships, and to gather information about past evolutionary events.
This test can : -
Count the number of chromosomes
Look for structural changes in chromosomes
How the Test is Performed ?
The test can be performed on almost any tissue, including : -
Amniotic fluid
Blood
Bone marrow
Tissue from the organ that develops during pregnancy to feed a growing baby (placenta)
To test amniotic fluid, an amniocentesisamniocentesis is done.
A bone marrow specimen requires a bone marrow biopsybone marrow biopsy.
The sample is placed into a special dish and allowed to grow in the laboratory. Cells are later taken from the growing sample and stained. The laboratory specialist uses a microscope to examine the size, shape, and number of chromosomes in the cell sample. The stained sample is photographed to provide a karyotype, which shows the arrangement of the chromosomes.
Certain abnormalities can be identified through the number or arrangement of the chromosomes. Chromosomes contain thousands of genes that are stored in DNA, the basic genetic material.
Normal Results
Females : - 44 autosomes and 2 sex chromosomes (XX), written as 46, XX
Males : - 44 autosomes and 2 sex chromosomes (XY), written as 46, XY
What Abnormal Results Mean ?
Abnormal results may be due to a genetic syndrome or condition, such as : -
Down syndrome
Klinefelter syndromeKlinefelter syndrome
Philadelphia chromosome
Trisomy 18
Turner syndromeTurner syndrome
This list is not all-inclusive.
Additional conditions under which the test may be performed : -
Ambiguous genitaliaAmbiguous genitalia
Chronic myelogenous leukemia (CML)Chronic myelogenous leukemia (CML) or other leukemias
Developmental delays
Multiple birth defects
Risks
The risks are related to the procedure used to obtain the specimen. See : -
