Detecting chromosomal abnormalities is important for prenatal diagnosis, detection of carrier status, and for general diagnostic purposes. One of the ways to determine whether chromosomes are normal or abnormal is by obtaining an individual's karyotype. A karyotype is a technique that allows geneticists to visualize metaphase chromosomes under a microscope.
Karyotype analysis can be performed on virtually any population of rapidly dividing cells either grown in tissue culture or extracted from tumors. Chromosomes derived from peripheral blood lymphocytes are ideal because they can be analyzed three days after they are cultured. Lymphocytes can be induced to proliferate using a mitogen (a drug that induces mitosis) like phytohemagglutinin. Skin fibroblasts, bone marrow cells, chorionic villus cells, tumor cells, or amniocytes also can be used but require up to two weeks to obtain a sufficient amount of cells for analysis. The cultured cells are treated with colcemid, a drug that disrupts the mitotic spindle apparatus to prevent the completion of mitosis and arrests the cells in metaphase. The harvested cells are treated briefly with a hypotonic solution. This causes the nuclei to swell making it easier for technicians to identify each chromosome. The cells are fixed, dropped on a microscope slide, dried, and stained. The most common stain used is the Giemsa stain. The procedure is called G banding and the chromosomes that are visualized under a light microscope have 400 alternating dark and light staining bands. High-resolution banding procedures allow resolution of up to 900 bands. Other dyes, such as fluorescent dyes, can also be used to produce banding patterns. Typically, G bands are divided into one or more regions and then numbered from the centromere to the telomere. For example, the band closest to the centromere on the short arm of chromosome 18 might be designated 18p11 while the next band will be designated 18p12. High resolution banding might resolve sub-banding patterns within the 18p11 (i.e. 18p11.1 and 18p11.2).
Chromosome spreads can be photographed, cut out and assigned into the appropriate chromosome number or they can be digitally imaged using a computer. There are seven groups (A-G) that autosomal chromosomes are divided into based on size and position of the centromere. The standard nomenclature for describing a karyotype is based on the International System for Human Cytogenetic Nomenclature (ISCN). First, the total number of chromosomes are written followed by a comma, then the sex chromosome constitution and any abnormality written in parentheses. Many genetic abnormalities cannot be detected by karyotype analysis. These include small, esoteric aberrations such as point mutations, frameshift mutations, nonsense mutations, or single nucleotide polymorphism's.
Genetic counselors rely on karyotypes to diagnose abnormal pregnancies. Amniocentesis is a routine procedure used in prenatal screening that involves removing amniotic fluid for karyotype analysis. A karyotype can pick up aneuploidy (i.e. Trisomy 21 or Down Syndrome) and rearrangements such as deletions, duplications, and inversions that might be helpful in prenatal diagnosis. It also can be helpful in certain cases to obtain karyotypes from parents to determine carrier status, which can be relevant to recurrence risks in future pregnancies. Karyotypes also may help determine the cause of infertility in patients having reproductive difficulties.
Many sports organizations, including the Olympics, use karyotype analysis for "gender verification" purposes in order to prevent male athletes from competing in female sports events. To prevent an unfair competitive advantage by male imposters, the International Olympic Committee (IOC) in 1968 required that all female athletes undergo a controversial gender verification testing using buccal smears (cheek cells) to karyotype individuals. Unexpectedly, athletes that had genetic abnormalities were detected. Some of these individuals had part or all of a Y chromosome and appear phenotypically to be female due to different genetic conditions that results in ambiguous external secondary sex characteristics or degenerate internal genitalia. As a result of sex testing, many of these individuals suffered from public disgrace and humiliation, loss of titles and scholarships, and were banned from future competitive events. It wasn't until June 1999, over 30 years later, that the IOC Athletes' Commission discontinued gender verification on a trial basis. The proposal, similar to the International Amateur Athletic Federations plan adopted in 1992, allows for such testing by the appropriate medical personnel only if there is a question of gender identity. Most competitive sports organizations now require only individuals suspected of being male imposters to undergo sex testing. The IOC allows genetically abnormal individuals to compete only after confirmation of testing by medical professionals and the appropriate counseling has been completed.
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