Karyotyping

In 1961 an international meeting was held at the University of Colorado Medical School in Denver, Colorado to standardize the format for a normal human karyotype. The format that evolved is known as the "Denver System."

Each chromosome has its own individuality as shown by its size, shape, and position of its kinetochore. Using the "Denver System," the chromosomes are put into similar groups designated by letters. Then numbers are used to subdivide the chromosomes within the groups designated by numbers based on the position of the kinetochore and the length of the chromatids. The homologous chromosomes are paired based on their banding.

The student will obtain a normal male or female karyotype. Next, using scissors, carefully cut out the individual chromosomes and arrange them on a blank karyotype form with the kinetochore on the dotted line using the Denver System Table and the sample karyotype as a guide. It is much easier if done in the following order-group A, group B, group D, group F, group G, group E, and group C last. Attach chromosomes to the karyotype form as directed.

After the normal karyotype is completed the student will be given an abnormal karyotype. First, count the number of chromosomes on the sheet of paper. There will be either 45, 46, or 47 chromosomes. Knowing the number of chromosomes will be a big help in identifying the unknown karyotype.

Using scissors, cut out the chromosomes and arrange them on a blank karyotype form with the kinetochore on the dotted line using the Denver System. It is much easier if done in the following order-group A, group B, group D, group F, group G, group E, and group C last. Attach chromosomes to the karyotype form as directed

		Denver System Table
		HUMAN KARYOTYPE
Group Chromosome	Number
A	1	Longest with median kinetochore.
	2	Second longest, kinetochore slightly off center.
	3	Third longest with median kinetochore.
B	4-5	Four long chromosomes with kinetochore close to one end.
C	6-12	Fourteen medium length chromosomes with slightly sub-median
D	13-15	kinetochores. Six chromosomes slightly smaller than the C group with the kinetochores in a near terminal position.
E	16-18	Six chromosomes; short with median kinetochores in 16 and submedian in 17 and 18.
F	19-20	Four chromosomes, shorter than E group with median kinetochores.
G	21-22	Four shortest chromosomes with sub-median kinetochores. It is difficult to see any short arms.
X	23	Similar to C group but often the largest in this group.
Y		Similar to G group but the chromatids tend to remain close together.

14/21 Translocation Carrier. A 14 chromosome and a 21 chromosome will appear to be missing and there will seem to be an extra C. This is because one of the 21 chromosomes is attached to the short arm of one of the 14 chromosomes. Put the 14/21 chromosome with pair number 14 of the D group.

21/21 Translocation Carrier. The two 21 chromosomes will appear to be missing and there will seem to be an extra F chromosome. This is because the two 21 chromosomes are attached to each other at their kinetochores. Put this attachment on the 21 space of the karyotype form.

46 CHROMOSOMES

14/21 Translocation Down Syndrome. A 14 chromosome will appear to be missing because a 21 chromosome is attached to the short arm of a 14 chromosome. Put the 14/21 chromosome with the D group.

21/21 Translocation Down Syndrome. One of the 21 chromosomes will appear to be missing because a 21 chromosome is attached to the sub median kinetochore of a 21 chromosome. Put this chromosome in the space of the missing 21 chromosome.

5 B Deletion (Cat Cry Syndrome). The karyotype will appear to be normal except one of the 5 B chromosomes will have lost part of its short arm.

XYY (Super Male). Place this extra Y chromosome with the other Y chromosome. XXY (Klinefelter). Place the extra X chromosome with the other X chromosome. XXX (Super Female). Place the extra X chromosome with the other X chromosomes. 21/21/21 (G Trisomy). Place the three 21 chromosomes together.

1. Give a brief description of the two techniques for obtaining a human chromosome smear.
2. Name the two characteristics of chromosomes that determine how they will be grouped.
3. Trisomy of a large chromosome is usually lethal. Name one exception (not G trisomy).
4. Give a brief description of the abnormality you were given. Describe the mental and physical conditions of an individual with this karyotype.