PROCEDURE
BACKGROUND
In this lab, you will use a Map of the
Human Chromosomes to determine genotypes and phenotypes for
two "parents" and the "baby" they create. Characteristics
such as blood type, hair color and eye color can
be determined by the
presence of a gene on a specific chromosome. Many times the paternal gene
can
contribute a different quality trait than the maternal gene. For example,
the paternal gene may indicate
brown eyes while the maternal gene indicates blue eyes. One of these gene
traits must take precedence
over the other and be expressed in the baby. The gene trait which takes
precedence is called the dominant
trait; the other is recessive. The dominant gene is denoted by a capital
letter, and the recessive gene is
denoted by a tower case letter. Although a baby may outwardly display
the dominant trait, it can still
genotypically contain the recessive gene as well. However, for the recessive
gene to be expressed, it is
necessary to have both parents give recessive genes.
Refer to your Map of the Human Chromosomes
(on another page). A key to the symbols included
on this map is listed below. (These same symbols will apply to the
chromosomes you will
receive as part of this lab, in which you and your partner will join to
produce a "baby.")
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On your map, you will
note that the gene for brown eyes (B) is not listed. While it is known
that brown eyes
are dominant over blue and green eyes, the locus of the gene for brown
eyes has not been found. Due to
this fact, in the following lab, when an indication of eye color is absent
in the chromosomes, it can be
assumed that the dominant brown eye gene is present. Additionally, if
the indication of eye color is absent
in one parent, but present in the other, the dominant trait of brown eyes
will take precedence. The gene for brown hair (B) is also dominant over
the gene for red (R) hair. Red or blond hair is only expressed when
brown hair is not coded on Chromosome #19.
As for many of the other traits noted on
your map, keep in mind that all people have genes for all traits.
However, for the sake of simplicity, only the abnormal genes for diseases
are shown. Some normal genes,
for traits such as eye color, hair color and blood type, are also shown.
In fact, each chromosome contains
between 900 and 4,000 genes. This map shows only a few of the genes on
each chromosome.
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PART I: MATCHING
CHROMOSOMES DURING FERTILIZATION
In this lab, you and your partner will
join chromosomes, to produce a "baby." For each baby "produced"
during the lab you will need to determine eye color, hair color, and blood
type. Each baby will also have and
carry a genetically-linked disease, which you will need to determine.
Keep in mind that this is done for study
purposes only. In reality, very few people actually have a genetic disease.
This is because even though
most people carry at least one recessive gene that, if expressed, would
cause a serious genetic disease,
the recessive gene is masked by a normal dominant gene. Also, remember
that genes contain information
for "putting together a person." It is only when a gene malfunctions
that you get a genetic disease.
PROCEDURE
To better understand "What makes you
unique?", you will assume the role of mother or father and
contribute one set of chromosomes to your "offspring." Your
partner will contribute a second set of
chromosomes to your "offspring." In this way, you will simulate
the events that contributed to the formation
of the unique-individual that is you!
1. The envelope that you received contains paternal (male) or maternal
(female) chromosomes. If
your chromosomes are pink, you are the mother. If your
chromosomes are blue, you are the father.
2. To begin karyotyping, spread out the contents of your envelope. Your
partner should do the same
with the contents of his/her envelope. Mix the contents
of the two envelopes together. Now, match
up the sets of chromosomes by number, size, banding,
etc., until all the chromosomes have been
paired off. (NOTE: Occasionally, there may be a missing
or an extra chromosome; in this instance,
you will have a chromosome that will not join with
another to create a matching set.) Next, place
the chromosomes that are marked with an X or a Y together.
(Again, note that in some instances
there may be an extra X or Y chromosome, or one of
these chromosomes may be missing.) Then
arrange the remaining sets of chromosomes in order
by number, size and banding. Place the
X and Y chromosomes at the end the set.
3. Answer questions 1 - 9 in PART I on the Data Sheet.
PART II: KARYOTYPING CHROMOSOMES
For the last 25 years karyotyping has been
used to detect chromosome abnormalities in humans. The
chromosomes are usually obtained from blood samples. Since the white blood
cells contain large nuclei,
these cells usually contain the best source of human chromosomes. Through
a series of staining
procedures, the cells with the largest visible chromosomes are isolated,
photographically enlarged, and
then cut out and arranged into chromosome pairs. Highly trained scientists
and doctors analyze the results
to determine if there are any variations in the shape, length, or number
of chromosomes. Over the years,
this technique has been used to detect, and in some cases correct or treat,
an increasing number of
genetic diseases.
A related technique called amniocentesis
now permits the detection of abnormal chromosome numbers
in a fetus as early as the sixteenth week of pregnancy. In this procedure,
the tip of a syringe is inserted
through the mother's abdominal wall and uterus into the fluid-filled amniotic
sac surrounding the fetus. Dead
skin cells and cells from the respiratory passage that enter the amniotic
fluid are sucked into the syringe.
These cells can then be karyotyped and analyzed for chromosomal abnormalities.
A simplified diagram of
the entire process using amniocentesis is shown below.
Specific groups of chromosomal abnormalities
are accounted for by the occurrence of non-disjunction.
Non-disjunction is the failure of chromosomes to appropriately separate
during meiosis at Anaphase I or
Anaphase II. Non-disjunction during the first meiotic division results
in the abnormality of all gametes
formed. Half will contain the paternal and maternal chromosomal haploids,
giving rise to trisomic zygotes;
and half will contain no chromosomes at all, producing monosomic zygotes.
Non-disjunction in the second
meiotic division can produce half of the gametes normally. In the other
half, however, there will be some
gametes lacking chromosomes, again creating monosomic zygotes, as well
as gametes which contain
two copies of a chromosome, either paternal or maternal, producing trisomic
zygotes. The consequences
of non-disjunction are serious, resulting in Down's Syndrome, Turner's
Syndrome, and Klinefelter's Syndrome,
among others.
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In
this lab, you and your partner will obtain raw drawings representing unsorted
chromosomes.
These chromosomes must be cut out of the sheet, matched with their “like”
pair, and taped onto a
final form in the Data Sheet. To complete this process correctly the structure
of the chromosome must
be understood, and the order in which the chromosomes are displayed must
be explained. The next
diagram will help explain how to arrange the chromosomes on the Data Sheet.
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PROCEDURE
You and your lab partner are to work together to complete two karyotypes.
The first karyotype is
labeled “CHROMATID CHROMOSOMES”
and consists of chromosomes photographed during the
Chiasmata stage (metaphase) of mitosis. The second karyotype is labeled
“SINGLE CHROMOSOMES"
and consists of chromosomes photographed while not duplicated (not paired
as chromatids). In both
cases you should follow the steps listed below.
1. Obtain the diagram of the chromosomes, pairs of scissors, and transparent
tape.
2. ROUGHLY cut out around each chromosome.
3. Pair up all like chromosomes, remembering that not all like chromosomes
must be IDENTICAL.
Note-> If you have a male not ALL chromosomes
will have an identical pair (ie. the X and Y
chromosomes do not look alike).
4. Use the diagram above to begin grouping the paired chromosomes according
to their lengths
and positions of their centromeres.
5. When all chromosomes of a group are located, tape them onto the appropriate
form on the Data Sheet.
(NOTE -> On the “Single Chromosomes” Data Sheet you need
to indicate if you
karyotyped Person #1, #2, or #3.
When you have each of the karyotypes completed answer
the questions associated with them on the
Data Sheet.
If time permits try to do a third karyotype called “REAL
CHROMOSOMES ” that represents a
photograph of a real persons chromosomes. (Warning -> This is much
harder than the two you have
already completed). If you choose to do this one for Extra Credit, you
need to obtain another blank form
to tape the chromosomes onto, and again you need to indicate if you karyotyped
Person #1, #2, or #3.