5. Characteristics of chromosome groups: Karyotyping
The rule of karyotyping
is to arrange 22 autosomes following the size and sex chromosomes,
X and Y, at the end. Chromosomes are classified into seven groups,
A to G, by the length and centromere position. Characteristics
of each chromosome group will be explained below.
Figure 5 shows an ideogram or idiogram of human chromosomes, and
Figure 6, metaphases and karyotypes of a male and a female. They
may help you to understand the characteristics that I am going
to describe. The information shown in Table 2 was taken from ISCN
(1985), but I omitted standard deviations (on the order of 5 to
10%) associated with the observation. Because group-C chromosomes
are the most difficult to identify, I will explain them at the
|Group A (chromosomes 1 to 3)
This group is comprised of the three largest pairs of chromosomes
with centromeres located in the middle regions. Chromosomes 1 and
2 have similar lengths, but their centromere positions are different.
Chromosome 1 has its centromere in the middle, while it is slightly
shifted toward the short arm (or CI = 40%) in chromosome 2. Chromosome
1 bears a secondary constriction, an "h", and can be easily distinguished
from chromosome 2. Chromosome 3 has a centromere in the middle and
is shaped similarly to chromosome 1 (CI = 48.6% in chromosome 1
and CI = 47.3% in chromosome 3), but chromosome 3 is about 20% shorter
than chromosome 1. Thus, chromosome 3 can be easily distinguished
One possible problem is "h" in chromosome 1 because there are individual
variations. Some may carry an "h" in both number 1 chromosomes,
in only one number 1 chromosome, or in neither number 1 chromosome.
The "h" is located at the upper portion of the long arm immediately
below the centromere, like a round shoulder below a long neck of
a stork (a long-necked bird). Please refer to chromosome 1 on the
right in Figure 6 (left). As the heterochromatin content increases,
the "h" region becomes longer. This is not regarded as a structural
abnormality but is called a normal variant or heteromorphic chromosome.
If one of the two parents bears the chromosome, one half of the
progeny inherits the chromosome according to the Mendelian law.
Group B (chromosomes 4 and 5)
Group B is comprised of two pairs of large chromosomes with centromeres
in the sub-terminal regions. Group B chromosomes are the second
longest (about 75% of group A chromosomes) and CI = 27 to 28%, or
the short arm comprises nearly one-fourth of the chromosome. The
long arm is nearly the same as the long arm of chromosome 1. Number
4 and number 5 chromosomes are very similar, so it is not possible
to distinguish them by the conventional method.
C (to be described later)
Group D (chromosomes 13
Group D is comprised of three pairs of middle-sized chromosomes
with centromeres located terminally (CI = 17-18%). The whole chromosome
length corresponds to the short arm of chromosome 3. All of these
chromosomes bear an "s" (satellite). These chromosomes are rather
easy to distinguish.
The "s" may not be visible in every cell. Satellites of group-D
and group-G chromosomes form a focus, termed satellite association,
which is the site for ribosomal RNA synthesis.
As with the "h" of chromosome 1 (heteromorphic chromosome), the
shape and size of the "s"'s are known to vary with individuals,
and the variation is genetically determined. Deletion of "s"'s is
also observed in rare cases but does not relate to any hereditary
Group E (chromosomes 16 to 18)
Group E is comprised of three pairs of chromosomes that are slightly
smaller than group-D chromosomes. Chromosome 16 has the centromere
near the middle (CI = 42%), whereas chromosomes 17 and 18 have centromeres
in more distal regions (CI = 32% and 27%, respectively). Hence,
their p-arms tend to be clearly shorter than that of chromosome
16. Chromosomes 17 and 18 are very similar but can be distinguished
with experience since the short arm of chromosome 18 is slightly
shorter than that of chromosome 17.
Chromosome 16 has an "h" as does chromosome 1 and considerable variation
exists in its size and shape.
Group F (chromosomes 19 to 20)
Group F is comprised of two pairs of short chromosomes with
their centromeres located in the middle regions. Chromosomes 19
and 20 cannot be distinguished.
Group G (chromosomes
21, 22, and Y)
Group G is comprised of the smallest two pairs of autosomes
with an "s" at the end of every short arm. As the Y chromosome belongs
to this group, the total number is five in males and four in females,
but the Y chromosome does not contain an "s". Although chromosome
21 was found later to be smaller than chromosome 20, the original
chromosome order was kept in karyograms.
The Y chromosome belongs to this group and is slightly longer than
chromosomes 21 and 22. As the lower half of q-arm is heterochromatic,
the Y chromosome has a normal variation in its total length. It
is easy to distinguish it from other group-G chromosomes by it lack
of "s" and presence of "h" that is stained lighter, and the sister
chromatids of "h" tend to be attached.
Group C (chromosomes
6 to 12, and X)
Group C is composed of middle-sized chromosomes, comprising sevem
pairs of autosomes and one or two X chromosomes. Thus, males carry
a total of 15 group-C chromosomes, and females carry 16. Centromeres
are located either in the middle or sub-terminal regions. The X
chromosome is one of the large chromosomes in this group. Because
every group-C chromosome looks similar, chromosomes of this group
are the most difficult to distinguish. Nonetheless, I will try to
describe the basic characteristics.
When chromosomes of this group are arranged in order, you may find
differences in centromere positions. Specifically, CIs are large
for chromosomes 6, 7, 11, and X; slightly smaller for chromosome
9; and smaller for chromosomes 8, 10, and 12. In other words, the
trend of centromere position from chromosomes 6 to 12 is large -
large - small - slightly small (with "h") - small - large - small,
and is actually bumpy.
Chromosome 6 is the largest among the group-C chromosomes, and you
will recognize it is larger than chromosome 7 if you put the chromosomes
in order. However, it will take some time to recognize the small
difference between the two chromosomes under the microscope (i.e.,
without arranging the chromosomes in order). Quite often, the middle
part of chromosome 6p is lightly stained.
Chromosomes 7 and X are almost impossible to distinguish. Thus,
the three (in males) or four (in females) second largest chromosomes
after chromosome 6 with large CI's are regarded as chromosomes 7
I should mention here that cells from women aged over 50 years occasionally
show one of the two X chromosomes with no centromeric constriction,
and the chromosome looks like one large acentric fragment. These
chromosomes are termed "premature centromere division (PCD)" chromosomes
and represent one inactive X chromosome in female cells. The frequency
tends to increase with donor age.
Chromosomes 8, 10, and 12 show centromeres at slightly higher positions;
no other unique characteristics are present. Chromosomes 10 and
12 look very similar and are difficult to distinguish. In practice,
the smallest pair of chromosomes is assigned as chromosome 12.
Chromosome 9 may be distinguished from other group C chromosomes
with relative ease. This is because it carries a secondary constriction,
an "h", as do chromosomes 1 and 16 with individual variations. As
mentioned earlier, if we consider the centromere as a head of a
stork, the "h" corresponds to a round shoulder toward the wings.
Such a variation is observed in families with varying frequencies.
Chromosome 9 also shows pericentric inversion at a frequency of
about 1% in a population. In this case, 100% of the metaphases should
show the same characteristics. The inversion does not accompany
any clinical abnormalities, and, hence, this is regarded as one
of the normal variations.
These are the characteristics of each group. The basic attitude
to train yourself is to enumerate the exact number of chromosomes
for each group, namely, females should have A = 6, B = 4, C = 16,
D = 6, E = 6, F = 4, and G = 4 and males should have A = 6, B =
4, C = 15, D = 6, E = 6, F = 4, and G = 5. This approach can also
tell you the gender of the blood samples.
In summary, if you don't find anything wrong in the number of chromosomes
in each group, you may consider the cell is normal. However, if
the number is increased in one group and decreased in another group,
an aberration may be present.
Although I attempted to describe chromosomes from groups A to G
here, you don't need to follow this order. I personally start with
groups D and G, followed by F, E (chromosomes 16, and 17 plus 18),
A, B, and C at the end but most carefully.
I would recommend drawing metaphase sketches. Although it might
take some time at the beginning, I assure you that you can do it
very quickly after some experience. Drawing sketches is useful as
proof of the aberrations.
||In section 3, I presented
one example of an inappropriate metaphase that showed partial distortion
(Figure 3B). As you see in the karyogram shown in Figure 7, several
chromosomes at the right are abnormally extended, as even spiral
structure can be seen. Those marked by arrows are especially extended
compared with their homologues. I judge this is a normal cell, but
I should say that you must discard this metaphase before starting