CP Biology
Name ______________________________
Date____________
LAB: Making a Karyotype for Genetic Diagnosis
Introduction--The Structure of Chromosomes:
Chromosomes, though present, are not usually visible in the nucleus of a non-dividing cell. The
DNA is usually in the form that we call chromatin. Chromatin is composed of loosely wound DNA plus
small protein molecules. Transcription and replication of the DNA (protein synthesis) only occurs when the
DNA is in the form of chromatin.
As a cell prepares to divide, the DNA in the nucleus is replicated. Each piece of chromatin can be as
long as 5 cm, and is not easily separated when the cell divides. So, before cell division, the thin strands of
chromatin begin to condense into the coiled structures we recognize as chromosomes. These condensed
structures are easily separated during cell division.
Chromosomes in most Eukaryotic organisms occur in pairs. One member of each chromosome pair
is derived from the female parent and the other from the male parent. The two members of a chromosome
pair are knows as homologues, and together they are referred to as a homologous pair.
1. What is the difference between chromatin and chromosomes?
Homologous pairs can be identified because they have the same: (use notes or PowerPoint)
1) _____________________________________________
2) _____________________________________________
3) _____________________________________________
4) _____________________________________________
5) BUT, ________________________________________
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2. There are two chromosomes in each homologous pair. Where do the two members of each pair come from?
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Chromosome number is constant within a species. Humans, for example, have 46 chromosomes (23
pairs) in each body cell. Any change in chromosome number will cause a change in the amounts of proteins
produced in a cell. These changes can have a considerable, detrimental effect on the individual.
The centromere is a region of DNA where the
two identical copies of DNA, formed during
replication, are attached. Each strand of replicated
DNA is called a chromatid. The chromatids on a
single chromosome are called sister chromatids.
Staining the chromosomes with a dye produces the
banding patterns on the chromatids.
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.
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Computer-assisted karyotype preparation is now commercially
available. In this system a television camera and a computer are
coupled to a microscope. As chromosomes in metaphase 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. Either way, 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.
3. What is a karyotype?
Notice that there are 22 pairs of homologous chromosomes. These are called autosomes. The 23rd pair
includes the sex chromosomes. If a Karyotype includes two X chromosomes (XX), the individual is a
female. Males carry an X and Y chromosome (XY).
4. Look at the karyotypes shown on the PREVIOUS page. Is the patient a male or female? How can you tell?
TWO METHODS OF PREPARING A KARYOTYPE DURING PREGNANCY:
A) AMNIOCENTESIS
This procedure, usually performed between about 14-20 weeks (possibly earlier or sometimes as late as
the 3rd trimester) 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 miscarries or loses 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.
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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 at the metaphase
stage. 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.
Amniocentesis:
Chorionic Villi Sampling:
5. Describe two similarities between the procedures of amniocentesis and CVS.
6. What is one major advantage that CVS has over amniocentesis?
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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 at the end of this lab as a guide to identify 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.
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:
46, XX or 46, XY
with one
chromosome #5 Cri-du-chat
upper arm
deletion
47, XY or 47, XX
with three copies
of chromosome #
21
Down
Syndrome
or Trisomy
21
47, XX or 47, XY
with three copies
of chromosome #
18
Edwards
Syndrome
or Trisomy
18
47, XYY with 2
copies of the Y
chromosome
Jacobs
Syndrome
47, XXY
Kleinfelter
Syndrome
47, XXX
XXX or
Metafemale
syndrome
45, XO
Only one X sex
chromosome
Turner
Syndrome
Description:
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.
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.)
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.
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.
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.
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.
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.
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Review Questions:
1) Look at the chromosomes on page #8. 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.
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2) What was the name of the chromosomal abnormality you identified in your sample karyotype?
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3) What are the general characteristics of an individual affected by the syndrome you identified?
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4) What is the probability or frequency that an individual will be affected by the abnormality you identified?
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5) Does your sample karyotype belong to a male or a female? How do you know?
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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:
_______ ________ ________
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2
3
________ ________
4
5
________ ________ ________ ________
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________ ________ ________
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________ ________ ________
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________ ________ ________
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________ ________
________ ________
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21
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What would be your diagnosis for this individual?
________
XX/XY
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.
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Name(s) ______________________________________________
Date_______________
Sample Human Karyotype Letter:________
Name of Genetic Abnormality:___________________________________________________
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