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 end.
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 in
chromosome 2. . Chromosome 3 has a centromere in the middle and is shaped
similarly to chromosome 1, but it is about 20% shorter than chromosome 1.
Thus, chromosome 3 can be easily distinguished with experience.
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) , 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.
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.
Group D (chromosomes 13 to 15)
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..
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 whereas chromosomes 17 and 18 have centromeres in more distal
regions .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.
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" .