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Microsoft word - karyotyping hw.doc
Complete the following questions after reading the article on the last two pages. The links in the
article are live and can be used for additional information.
a. Why, prior to the 1950’s, was it difficult to observe human chromosomes?
b. At what stage of mitosis do the colchicines stop a cell? Why do scientists choose this
c. Once cell division is stopped, dilute salt solutions are added to cell until they burst. This
i. Hypertonic Environment, Plasmolysis of the cell
ii. Hypotonic Environment, Cytolysis of the cell
iii. Isotonic Environment, Normal Cell Behavior
The failure of chromosome pairs to separate during cell division is known as non-
disjunction. If during meiosis of a female germ cell, non-disjunction of the 23rd pair of
chromosomes (sex chromosomes) occurred, what would be in the two daughter cells
c. Find two other genetic disorders caused by the non-disjunction of autosomal
chromosomes (non-sex!) List the name of the disorder and the chromosomal defect.
4. How are karyotypes made for unborn children?
Karyotyping Article – From howstuffworks.com Definition
Karyotyping is a test to identify chromosome abnormalities as the cause of malformation or disease. This test can:
Look for structural changes in chromosomes
The results may indicate genetic changes linked to a disease.
The direct observation of human chromosomes was for many years extremely difficult if not impossible.
Very few cells could be found in metaphase, when chromosomes are most distinct, and the large number
of chromosomes in human cells often caused them to lie on top of each other, making them difficult to
count. Indeed, up until 1956 it was believed that humans had 48 chromosomes. In the past 20 years
significant technological advances have been made: cells can be grown in glass vessels outside the
human body; a chemical agent, colchicine, stops mitosis at metaphase; and cells treated with dilute salt
solutions swell and burst, in effect spreading out the chromosomes. Once the chromosomes are spread
out, a picture is taken. The chromosome pictures are cut out and arranged in pairs according to size and
shape. The resulting karyotype can be used to identify a number of hereditary defects, as well as the sex
and genetic condition of an unborn child.
The test can be performed on a sample of blood, bone marrow, amniotic fluid, or placental tissue.
Chromosomes contain thousands of genes that are stored in DNA, the basic genetic material. The specimen is grown in tissue culture in the laboratory. Then, the cells are harvested, and the chromosomes are stained and viewed under the microscope. They are 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.
There is no special preparation for the blood test. To test amniotic fluid, an amniocentesis is performed. Testing on placental tissue is done after a chorionic villus sampling or after a miscarriage. A bone marrow specimen requires a bone marrow biopsy.
The preparation you can provide for this test depends on your child’s age, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics:
Infant test or procedure preparation (birth to 1 year)
Toddler test or procedure preparation (1 to 3 years)
Preschooler test or procedure preparation (3 to 6 years)
Schoolage test or procedure preparation (6 to 12 years)
Adolescent test or procedure preparation (12 to 18 years)
How the test will feel depends on whether the sample procedure is venipuncture (having blood drawn), amniocentesis, or bone marrow biopsy.
The blood test is usually performed to evaluate a couple with a history of miscarriages or to evaluate an abnormal appearance of the body that suggests a genetic abnormality. The bone marrow or blood test can be done to identify the Philadelphia chromosome that is present in 85% of those with chronic myelogenous leukemia (CML). The amniotic fluid test is done to evaluate a developing fetus for chromosome abnormalities.
Females: 44 autosomes and 2 sex chromosomes (XX), denoted 46 (X,X)
Males: 44 autosomes and 2 sex chromosomes (XY), denoted 46 (X,Y)
Abnormal results may show Down syndrome (Trisomy 21 = three copies of chromosome 21 instead of the normal two copies), Trisomy 18, Philadelphia chromosome, Klinefelter syndrome, Turner syndrome, or other abnormalities.
Additional conditions under which the test may be performed:
Chronic myelogenous leukemia (CML) or other leukemias
A baby born with genitals that are neither completely male or female
The risks are related to the procedure used to obtain the specimen. There is a specialized kind of risk in that an abnormal result may have occurred during growth of the cells after they left the body. For this reason it is often prudent to repeat the karyotype test to confirm that the abnormal chromosome constitution is in the body of the patient.
There is a rare difference between the apparent sex of the patient and their chromosomes. For example, a baby may look like it has a penis and be called a boy but turn out to have the chromosomes of a girl. This can raise issues of what gender to raise the child.
Chemotherapy may cause chromosome breaks interfering with normal results. Mixtures of 2 different populations of cells or chromosome constitutions are sometimes observed. This is called mosaicism and is more common in chorionic villus sampling or amniocentesis. It does not necessarily mean that the baby has abnormal chromosomes.
Your doctor may also order a test called telomeres that looks at the ends of the chromosomes. It is often ordered along with the karyotyping test.
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