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Why Is Karyotyping a Major Activity in Medical Genetics?

Karyotyping is a major activity in medical genetics because it allows a quick view of common chromosomal abnormalities. While many genetic disorders require extensive testing to verify the presence and extent of the disease, karyotyping involves a picture of a patient's chromosomes to identify additional, missing or malformed chromosomes. Most karyotyping takes place while a child is still in the womb. However, doctors may recommend karyotyping for newborns, children or adults.

  1. Why a Doctor May Recommend Karyotyping

    • The human genome possess 22 autosomal chromosome pairs and 1 sex chromosome pair---XX for a girl and XY for a boy. One of the most well known chromosomal abnormalities, Down syndrome, also known as trisomy 21, is also the most common reason for having karyotyping performed. Women 35 years old and older have significantly higher risks of giving birth to a child with Down syndrome. Women or men who have previously been diagnosed with a sex chromosome-related abnormality such as triple X or XYY may want to know if their child has the same condition. A child may be born with one or more of the characteristic facial features or other symptoms related to a chromosomal abnormality.

    How Karyotyping is Performed

    • A sample of blood, bone marrow, amniotic fluid or placenta is treated in a laboratory to induce mitosis or cell division. Once the cell population reaches a predetermined size, an application of colchicine stops the cell division at metaphase, a phase of cell division where the chromosomes form and become visible under a microscope. A technician takes a picture of a cell with the most clearly visible chromosomes, enlarges the photograph, cuts out the individual chromosomes and arranges them according to size and pairs.

    What You Learn

    • After a karyotype is assembled, a technician, physician or genetic counselor identifies extra chromosomes, missing chromosomes, abnormally long or short chromosomes or other changes in chromosomal appearance. The type and location of a chromosomal abnormality predicts the disease. For example, a single X chromosome with no accompanying X or Y will result in girl with Turner syndrome. A child with two or more X chromosomes and one Y chromosome will be a boy with Klinefelter's syndrome. Children with a trisomy of the 13 chromosomes have Patau syndrome, and children with a trisomy of the 18 chromosomes have Edwards's syndrome.

    Considerations

    • The tests performed on unborn babies are called amniocentesis and chorionic villus sampling (CVS). Amniocentesis involves the insertion of a needle through the mother's abdomen and through the placenta in order to draw a sample of the amniotic fluid surrounding the baby. CVS also uses a needle inserted through the abdomen or cervix, but samples a small piece of the placenta. CVS produces accurate karyotyping earlier in the pregnancy but carries a higher risk of miscarriage. Side effects of both forms of sampling include mild vaginal bleeding, cramping and leaking of amniotic fluid. You should discuss your options and risks with your physician before participating in either test.

    Genetic Counseling

    • Genetic counselors are specially trained to help you interpret the results of a karyotyping test and make informed decisions regarding the future of your pregnancy or the care of yourself or your child. Like other diseases, those produced by chromosomal abnormalities range from mild to severe, not only among diseases but also among sufferers of the same disease. Visit the National Society of Genetic Counselors to locate one in your area (see resources).

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