Download Chromosomes and Karyotyping Instructions

Chromosomes and Karyotyping
Instructions
This week, you will gain experience in constructing and interpreting karyotypes. Unlike “old-fashioned” karyotypes
that were generated from black-and-white photos, these karyotypes were prepared using a technique called FISH
(fluorescence in situ hybridization). In FISH, fluorescently-labeled DNA molecules are allowed bind to specific
chromosomes. These “probes” allow you to identify homologous pairs and to distinguish between pairs by looking at
their patterns of colors. An example of a FISH karyotype is shown in Fig. 1. Note the banding patterns of the
chromosomes, usually several per chromosome. Each band represents regions covering several hundred genes.
Fig. 1. A normal human karyotype, prepared using FISH.
Activity 1. Constructing normal human karyotypes
Procedure:
1.
Begin by cutting out the chromosomes on the sheet marked Karyotype 1.
Once the chromosomes have been cut out, I suggest placing them on a sheet of white paper so you don’t lose
them on the dark benchtop.
2.
Using Fig. 1 as a guide, arrange each chromosome on the blank karyotype form. Do not fasten the
chromosomes to the karyotype form until your instructor has checked it. Keep all paper scraps until you have
identified each chromosome.
The chromosomes can be arranged in seven groups (A-G) according to length (see Table 1).
Group
Characteristics
A
Group A consists of the six longest chromosomes.
B
The B group consists of four long chromosomes with the centromeres very close to one end.
C
The C group consists of 14 medium-length chromosomes with the centromeres slightly off-center.
The female sex chromosome (X chromosome) also falls in this group. Therefore, a male will have 15
C-Iength chromosomes, and a female will have 16.
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Group
Characteristics
D
The D group consists of six chromosomes slightly smaller than the C's with the centromeres very
near to one end.
E
The E group resembles the C group. but the chromosomes are much smaller.
F
The F-group chromosomes are very small with the centromeres in the middle.
G
The G group includes the four smallest chromosomes with the centromeres so close to the ends
that it is difficult to see any short arms at all. The male sex chromosome (Y chromosome) falls into
the G group. Therefore a male will have five G-Iength chromosomes and a female four.
3.
Once you are sure that Karyotype 1 is complete and correct, use the tape to secure the chromosomes to the
Karyotype Form and complete the form.
4.
Repeat this process with Karyotype 2. When you are finished, you will have two normal human karyotypes
(male and female) that you will use as guides when completing your case studies.
Activity 2. Using karyotypes to diagnose genetic disorders
Now that you have established normal, baseline karyotypes, you will solve two cases that involve chromosomal errors.
Loss or gain of chromosomal material is frequently but not always, associated with mental retardation. In the United
States, approximately 20,000 infants are born with chromosomal abnormalities; each year this is about 1 out of every
200 live births.
For each of these, I suggest counting the chromosomes before you begin cutting them out. This will give you an idea
of what type of disorder (if any) you are studying.
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•
The normal number of chromosomes is 46.
•
If there are more than 46, there is extra chromosomal material and there are three possibilities: Down
syndrome (three copies of chromosome 21), Klinefelter syndrome (a male with two X chromosomes) or an
XYY male.
•
If there are 45 chromosomes, the condition is Turner syndrome. When you prepare the karyotype of
Turner syndrome, there will be only one X chromosome and no Y chromosome. Such people are females
(45,X).
•
If there are 46 chromosomes, the condition is fragile X syndrome. The karyotype of the fragile X syndrome
is prepared by pairing chromosomes of the same size and shape. The staining technique used to show fragile
X produces opaque chromosomes. This is done to show the changed gene location on the end of the long
arm (q) of the X chromosome. Be careful not to cut off the fragile site on the end of the X chromosome.
Case 1.
Mr. and Mrs. Garcia are concerned about their three-year old daughter, Ana. Like all babies, Ana was chubby as she
put on weight after she was born. However, Ana did not seem to lose her baby fat. Her hands and feet were
especially fat. The older Ana got, the more unusual she looked. She was unusually short, had low-set ears, drooping
eyelids, and small, spoon-shaped nails. While each of these caused concern, but it wasn’t until Ana showed signs of
hearing problems that the Garcias were spurred into action.
Last week, the Garcias saw a daytime talk show that covered genetic illnesses and thought to themselves that Ana
may have such a problem. The Garcias had always planned on having more children but are afraid that they may be
carriers of a genetic disorder that they will pass on to other children.
Case 2.
Carolyn is a 44-year old woman who works at a pharmaceutical firm as a drug developer. She has recently found out
that she is pregnant, despite the fact that doctors had diagnosed her as infertile while she was in her early twenties.
Because of her age, her obstetrician has strongly urged her to undergo amniocentesis, a procedure in which amniotic
fluid is collected from the sac that surrounds the fetus. Carolyn is understandably nervous; her marriage is strained
and she fears that it won’t survive a difficult diagnosis. In addition, her job, though comfortable, is demanding. She
was already concerned that a child would complicate her life. Now, the idea of having a child with an incurable
illness seems to be at odds with her career.
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Chromosomes and Karyotyping
Lab Report
Name ________________________________________
Activity 1. Constructing normal human karyotypes
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Activity 2. Using karyotypes to diagnose genetic disorders
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Case 1.
1.
Use the laptop to explore the internet to provide the Garcias with the name of a support group that they can
consult to get more information.
2.
List two or three positive outcomes of this diagnosis that should make the Garcias feel better.
3.
List two or three medical issues that her parents should keep an eye out for as Ana gets older.
4.
Should the Garcias be concerned about having another daughter with this condition? Why or why not?
Case 2.
5.
Is Carolyn’s fetus a boy or a girl?
6.
What are some of the symptoms that Carolyn should expect to encounter with her baby?
7.
Carolyn believes that some of the genetic problems in her husband’s family are to blame for this diagnosis. Do
you agree? Why or why not?
8.
Use the laptop to explore the internet to provide Carolyn with the name of a support group that she and her
husband can consult to get more information.
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