Name ______Making a Karyotype for Genetic Diagnosis Activity
Background Information
In order to study the chromosomes of a patient, a sample of cells must first be obtained from them. Chromosome analysis can be performed using mitotic (dividing) cells from a number of sources, including white blood cells or skin cells. Then, chemicals are added to stop the cells at a particular point in the process of cell reproduction when the double armed chromosomes can be easily seen, counted and organized into numbered groups. The scientist then attempts to find all the homologous pairs and organize them into a picture called a karyotype. Observing a karyotype can help us understand the nature of several genetic disorders. One of the most common times for a karyotype to be performed is during pregnancy.
Computer-assisted karyotype preparation is now commercially available. As chromosomes are located, the television camera records the microscopic image, and the image is transmitted to the computer, where it can be analyzed and processed into a karyotype. The scientist can look at the completed picture to see whether any of the chromosomes are missing or damaged or if there are any extra chromosomes.
Homologous pairs of chromosomes can be identified because they have the same size, shape, banding patterns, and gene locations. However, the DNA sequence at each gene location might be different. For example, both chromosomes in a pair may have the gene location for eye color, but one might have the DNA sequence for brown eyes and the other blue eyes. In a karyotype, chromosomes are paired by matching their banding patterns (light and dark lines). In a karyotype homologous pairs are arranged from largest to smallest. Humans have 46 chromosomes, so they have 23 pairs of chromosomes. One chromosome of each pair comes from the father and one chromosome comes from the mother. In that way, every organism has DNA from both of its parents. The first 22 pairs of chromosomes are called autosomes. Pair 23, the last pair, determines sex (male or female). Human females have two X chromosomes (XX), and males have one X and one Y chromosome (XY).
Part 1: Review questions. Use the background information to help you.
1. What is a karyotype? ______
2. When can the chromosomes be easily seen? ______
3. How are homologous chromosomes matched when creating a karyotype? ______
4. Look at the karyotype in the background information. Is the patient male of female? How can you tell? ______
5. Is this a normal or abnormal karyotype? How can you tell? ______
Part 2: PREPARATION OF A SAMPLE KARYOTYPE
1) You will be given a sample chromosome spread. It will have a letter (A-F) printed at the top of the page.
Record the letter of your karyotype on the page titled “Sample Karyotype.”
2) Cut out each chromosome. It doesn’t have to be exact; a rectangular cut is best.
3) Use the normal karyotype found in this lab as a guide to identify the number of each cutout chromosome.
4) Carefully glue the homologous pairs of chromosomes next to each other, above the correct number of the chromosomes on the “Sample Karyotype” page. It helps to glue as you cut, so that no pieces get lost.
5) Use the information in the next section to determine the genetic syndrome that your sample karyotype indicates.
Table of Genetic Abnormalities
Abnormal Karyotype: / Syndrome: / Description:46, XX or 46, XY with one chromosome #5 upper arm deletion / Cri-du-chat / Babies with the “cry of the cat” syndrome have a cry that sounds like that of a cat in distress, because the infant’s larynx or voice box is improperly developed. The cause of this condition is a deletion of about half of the short arm of chromosome number five. Cri-du-chat babies are severely mentally retarded and have a small cranium. The incidence of this syndrome is 1/100,000 live births.
47, XY or 47, XX with three copies of chromosome # 21 / Down Syndrome or Trisomy 21 / This syndrome is one of the most common causes of mental retardation. Down syndrome is marked by a number of characteristic features, such as short stature, broad hands, stubby fingers and toes, a wide rounded face, a large protruding tongue that makes speech difficult and mental retardation. Individuals with this syndrome have a high incidence of respiratory infections, heart defects and leukemia. The average risk of having a child with trisomy 21 is 1/750 live births. (Mothers in their early twenties have a risk of 1/1,500 and women over 35 have a risk factor of 1/70, which jumps to 1/25 for women 45 or older.)
47, XX or 47, XY with three copies of chromosome # 18 / Edwards Syndrome or Trisomy 18 / This syndrome produces severe mental retardation and a highly characteristic pattern of malformations such as elongated skull, a very narrow pelvis, rocker bottom feet, and a grasping of the two central fingers by the thumb and middle finger. In addition, the ears are often low set and the mouth and teeth are small. Nearly all babies born with this condition die in early infancy. The frequency of this syndrome is 1/5,000 live births.
47, XYY with 2 copies of the Y chromosome / Jacobs Syndrome / A chromosome aberration in which the individuals are not markedly affected. Although these males are tall and have a slightly higher risk for behavioral problems, many individuals with this syndrome live normal healthy lives. The incidence is about 1/1000 live male births.
47, XXY / Kleinfelter Syndrome / Characteristics in this syndrome do not develop until puberty, and many of the symptoms seem to be related to low testosterone levels. Affected males are generally infertile, display poor sexual development and have some degree of subnormal intelligence. Most men with this syndrome appear relatively normal in other ways (in fact some cases have no obvious signs) but a degree of breast development occurs in about half of the reported cases. May display problems with learning and behavior. Occurs in about 1/1000 male births.
47, XXX / XXX or
Metafemale syndrome / Approximately 1 in 1000 females are born with three copies of the X chromosome. In most cases these females are physically and mentally normal, although there is a slight increase in sterility and mental retardation compared with the population at large. In rare cases, XXXX and XXXXX karyotypes have been reported, and problems of mental retardation are severe.
45, XO
Only one X sex chromosome / Turner Syndrome / Individuals are visibly female. As girls, they appear normal although they are shorter and have a chunky build. At birth, the distinguishable characteristics include a thick fold of skin on either side of the neck. At sexual maturity, the secondary sex characteristics do not develop and no eggs are produced. There is no menstruation or breast development. The frequency is 1 / 2,500 live female births.
Part 3: Conclusion Questions
1) Look at the chromosomes on page 4. Could you determine the difference between chromosome #4 and chromosome #5 without the banding patterns on them? Explain why banding patterns are so important in determining if two chromosomes are truly homologous. ______
2) What was the name of the chromosomal abnormality you identified in your sample karyotype?
______
3) What are the general characteristics of an individual affected by the syndrome you identified? ______
4) What is the probability or frequency that an individual will be affected by the abnormality you identified? ______
5) Does your sample karyotype belong to a male or a female? How do you know? ______
Part 4: Challenge Questions
6) Imagine that you are a genetic counselor. You have a 28-year old male patient who is trying to figure out the cause of his infertility. Chromosomes were obtained from nucleated cells in the patient’s blood and the following karyotype was prepared:
______
1 /
______
2 /
______
3 /
______
4 /
______
5
______
6 /
______
7 /
______
8 /
______
9 /
______
10 /
______
11 /
______
12
______
13 /
______
14 /
______
15 /
______
16 /
______
17 /
______
18
______
19 /
______
20 /
______
21 /
______
22 /
______
XX/XY
What would be your diagnosis for this individual?
What symptoms would you tell your patient to expect? ______
7) It is unlikely to find a living individual who is missing both copies of a particular chromosome in their cells. They don’t survive past the embryo stage and are usually aborted naturally. Explain why a fetus might not survive if an entire chromosome pair (for example, both copies of chromosome #5) is missing. Focus on the function of DNA in the chromosomes and how this would affect the cells of the embryo.
______
Part 5: Two methods for preparing a karyotype during pregnancy
A) AMNIOCENTESIS
This procedure, usually performed between about 14-20 weeks of pregnancy, requires the removal of a small amount of amniotic fluid which contains cells from the growing fetus. After culturing the cells for several weeks, a karyotype can be prepared to check whether the chromosomes of the developing embryo are normal.
Although useful, amniocentesis takes time and cannot be performed early in pregnancy. In addition, one in 200 mothers miscarry or lose her baby as a result of the procedure. Doctors now often use a blood test to help determine whether an amniocentesis is necessary (though unfortunately the blood test is not 100% accurate) so as to avoid unnecessary risks to the mother and child.
B) CHORIONIC VILLUS SAMPLING
Another technique, chorionic villus sampling or CVS can be performed as early as one month into a pregnancy. Shown below, a narrow tube is used to collect fetal cells in a tissue layer called the chorion. An additional advantage in CVS is that a karyotype can be produced in a number of hours vs. weeks.
In each procedure, embryonic cells are treated with a chemical that stops cell division. The cells are broken open by placing them in a hypotonic solution and the contents stained with dye. The resulting mixture is observed under a microscope and photographed. A karyotype is then prepared to check whether the chromosomes of the developing embryo are normal. If there are problems, genetic counselors work with the parents to make the best decision as to the welfare of their baby.
1. Describe two similarities between the procedures of amniocentesis and CVS. ______
2. What is the major concern regarding amniocentesis? ______
3. What are the advantages of CVS over amniocentesis? ______
4. Why might a pregnant women want to have an amniocentesis or CVS? ______
2