584 Chapter 17 NEL
Karyotype Charts
One tool for detecting the results of abnormal meiosis is a chart of the chromosomes
called a karyotype. Technicians obtain a karyotype chartby mixing a small sample of tissue
with a solution that stimulates mitotic division.A different solution is added which stops
division at metaphase. Since chromosomes are in their most condensed form during
metaphase—their size, length, and centromere location are most discernible—it is the
best phase in which to obtain a karyotype. The metaphase cells are placed onto a slide and
then stained, so that distinctive bands appear. A photograph of the chromosomes is taken.
The image is enlarged, and each chromosome is cut out and paired up with its homologue.
Homologous chromosomes are similar in size, length, centromere location, and banding
pattern. Finally, all the pairs are aligned at their centromeres in decreasing size order. The
sex chromosomes are always placed last.
Figure 4shows karyotypes of a normal male and of a female with Down syndrome.
In about 95 % of cases, a child with Down syndrome has an extra chromosome in chro-
mosome number 21. This trisomic disorder is produced by nondisjunction; the person
has too much genetic information. Compare the chromosomes of a male shown in
Figure 4 (a), with the chromosomes of a female who has Down syndrome, shown in
Figure 4 (b). Notice how the chromosomes are arranged in pairs.Figure 4
(a)Karyotype chart of a male with 46 chromosomes. Notice that the chromosome pair
number 23 is not homologous. Males contain an X and a Y chromosome. They act as a
homologous pair in meisois, but they are not similar in size and shape as are the other
chromosome pairs.
(b)Karyotype of a female with Down syndrome. Note the trisomy of number 21. Down
syndrome affects both males and females.(a) (b)karyotype chart a picture of
chromosomes arranged in
homologous pairs
Karyotype Preparation
This animation depicts the steps
involved in preparing a karyotype
chart. You can also see
representative karyotypes from
individuals with nondisjunction
disorders.http://www.science.nelson.com GO