Consider a species with an XY chromosomal
sex determination system. In a diploid somatic cell,
the female (XX) karyotype [top
right] comprises a pair of metacentric sex
chromosomes, plus one pair of acrocentric
and one pair of telocentric autosomes.
There are thus six chromosomes with a total of ten
arms [the telocentrics
have only one arm each], and the karyotype is
described as 2N=6, XX. The DNA content
is 2C,
where C is
defined as the mass of DNA present in
a haploid chromosome set. The male (XY)
karyotype [top left] comprises a pair of sex
chromosomes, one metacentric and one telocentric
(with a single arm), along with the same
autosome complement as the female. The numbers
of chromosomes is the DNA content are
the same as in the female, however their
are only 9 arms (the C value is
effectively the same). The karyotype is
described as 2N=6, XY. At this stage,
the number of chromosomes equals the number of
chromatids, and these are the same between males
and females.
When this cell undergoes mitotic division, metaphase chromosomes
are maximally compact and have duplicated their
DNA (4C),
however the two DNA strands (chromatids)
remain attached to a single centromere.
In a standard metaphase spread [middle set of
diagrams], each chromosome appears as a
single "X" shape with two chromatids joined
at the centromere, therefore twice the
number of arms, but the same number of
centromeres as in the diploid cell. The
convention in a metaphase karyotype is to count the chromosome number
as the number of centromeres,
even though chromatids and their arms are
doubled.
[If the
chromosomes could be visualized during anaphase,
we would see the centromeres separate
as they moved towards the cell poles, so that
the cell nucleus would temporarily have twice
the usual number of chromosomes (4N).
At the completion of telophase, the two
sets have separated to either pole, karyokinesis
is complete, and there are two daughter
cells each with a 2N chromosome
complement].
In haploid
gametes [bottom set], only one of
each pair of chromosomes is present, so the
number of chromosomes and arms is half that
of the diploid cell, and the DNA content
C. The heterogametic
males produce two kinds of
gametes, with either an X or a Y chromosome,
with 5 or 4 arms respectively, whereas the homogametic females
produce only one kind, with an X
chromosome and 5 arms. [In some groups,
notably birds, females are heterogametic and
males are homogametic].
"X"
and "Y" are just names. They do not refer
to the shapes of the sex chromosomes:
all chromosomes are X- or V-shaped
in a metaphase karyotype, and the "Y" chromosome is not an X
chromosome with "one leg missing."