Constructing a Karyotype to Diagnose Chromosomal Disorders
In each of our somatic cells, we have 46 chromosomes. These 46
chromosomes come in pairs, known has homologous chromosomes. Essentially
this means that we have two chromosomes of each type, with one from each pair
coming from each parent. Each gamete contains a complete set of 23
chromosomes and when they are combined after fertilization, the resulting zygote
has 46 chromosomes total (or 23 pairs).
Biologists can examine a person’s chromosomes by building a karyotype,
where chromosomes are identified by size and markings and matched up with
their homologous mates. Karyotypes allow biologists to identify gender since 2 X
chromosomes indicate a female and one X and one Y chromosome indicates a
male.
Problems can occur during meiosis (my-oh-sis: process that creates sex cells
that contain half the number of chromosomes as somatic cells) resulting in
gametes that contain the wrong number of chromosomes. Most of these cells will
not result in the viable embryos (embryos that can survive), but there are a few
exceptions. There are a handful of chromosomal disorders where individuals can
survive with cells that have one extra chromosome, for instance Down Syndrome
(extra chromosome #21), Edward Syndrome (extra chromosome #18), and Patau
Syndrome (extra chromosome #13). Each of these “trisomy” disorders create
many symptoms in the individuals such as mental retardation. There are also
three known sex chromosomal disorders. Klinefelter Syndrome occurs when a
male individual has 2 X and 1 Y chromosome. Double Y Syndrome, as you might
guess, is when male individuals have 2 Y chromosomes and 1 X chromosome.
Finally, Turner Syndrome is when a female individual has only one X chromosome
instead of two. Turner Syndrome is the only known viable chromosomal disorder
where an individual can survive with only 45 chromosomes.
Journal Questions #1: Each must be answered with a 3 point response.
1.
2.
3.
4.
How many chromosomes does a typical human cell contain?
How many chromosomes does a typical human gamete contain?
Why are some people born with an abnormal number of
chromosomes? What causes this?
For each karyotype below, state the gender of the individual, the name
of disorder the karyotype shows and how you made the diagnosis (the
evidence).
B
A
Directions for Constructing a Karyotype
Step 1:
Step 2:
Step 3:
Step 4:
Step 5:
On the attached sheet, you will find an enlarged image of the
chromosomes from a human cell. Upon your first inspection, are these
chromosomes from a human gamete or a zygote? How do you know?
Explain.
Cut out each chromosome.
Arrange the chromosomes in their pairs from largest to smallest. To do
this you will need to measure the length of each chromosome and
match each chromosome up with it’s homologous mate. Other
indicators to help match up chromosomes are banding patterns and
the centromere location.
Build your karyotype on a piece of paper in your journal by gluing
each homologous pair to the paper. Start by gluing the largest
chromosomes to the top left corner. Then continue adding your pairs
in descending size from left to right. The last pair of chromosomes
should be the X and/or Y chromosomes.
Add a title to your karyotype. Label each pair of chromosomes 1-22
and the last pair as XX or XY (as appropriate).
Journal Questions #2: Each must be answered with a 3 point response.
1.
2.
3.
What is the gender of this individual? How do you know?
Does your individual have a chromosomal disorder? How do you
know? If they do have one, which one do they have?
Write an explanation of how it might be possible to inherit a
chromosomal disorder.