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Human genetics
HUMAN SEX CHROMATIN
Sex chromosomes
In humans, the sex chromosomes are labeled X and Y. Females have two X
chromosomes and males have one X and one Y chromosome. All the eggs produced
during meiosis have an X chromosome. Half of the sperm produced by a male
contain an X chromosome and the other half have a Y chromosome. Thus, sperm
determine the sex of the offspring. If the egg is fertilized by a sperm with an X
chromosome, the zygote develops into a female. If the sperm contains a Y
chromosome, the offspring is a male. There is a difference between the sex
chromosomes. While the X chromosome is relatively large (approximately 6% of
nuclear DNA), the Y chromosome is quite small, and only a few genes have been
assigned to it.
X-chromosome inactivation
In 1961, the British geneticist
Mary Lyon proposed that the X
chromosomes in somatic cells
of mature females are of two
types. One is active and
expresses its full complement
of genetic information; the
other is inactivated in some
manner and does not serve as a
source of genetic information.
The biological meaning of
suppression of the functional
activity of one of two X
chromosomes is the dose
compensation, as in male karyotypes there is only one X chromosome present, and in female two. Thus the genotypic possibilities of male and female karyotype are equalized. It is important
that this inactivation occurs randomly, so that in early embryonic life (after 16 days) different cell
may have alternative X chromosomes inactivated. So in some cells the X-linked genes inherited
from the mother are expressed, whereas in other cells, the X-linked genes inherited from the
father are active. Thus, the somatic tissues of females are said to be 

mosaic
because they
represent the contribution of genes from different X chromosomes. In which somatic cell will
express the genes in only one X chromosome, but the X chromosome that is genetically active
will differ from cell to cell. This mosaicism has been observed directly in women who are
heterozygous for an X-linked recessive mutation resulting in the absence of sweat glands; these
women exhibit patches of skin in which sweat glands are present (these patches are derived from
embryonic cells in which the normal X chromosome remained active and the mutant was
inactivated), and other patches of skin in which sweat glands are absent (these patches are
derived from embryonic cells in which the normal X chromosome was inactivated and the
mutant X remained active).
HUMAN SEX CHROMATIN
The molecular basis for X chromosome inactivation is not completely understood. The process
begins with activation of a gene called X-inactivation-specific transcript (XIST) on the long arm
of the X chromosome. XIST is expressed only in the inactive X chromosome and produces an
RNA molecule (not translated into protein) that transmits the inactivation signal throughout the
chromosome. The process involves physical reorganization of the DNA within the chromatin and
also the addition of methyl groups to the DNA bases of substantial regions of the inactivated X
chromosome.
Sex chromatin (Barr Bodies)
Sex chromatin (or Barr bodies) is normally
one X chromosome, which in interphase
nucleus is completely or partially coiled. One
of the two X chromosomes in female cells is
facultatively
heterochromatic
and
is
condensed during interphase forming the
Barr body. A Barr body is about 1
micrometer in diameter and is located at the
periphery of the nuclear membrane. The
number of Barr bodies is one less than the
number of X chromosomes. Although cells
of normal females have one Barr body, cells
of normal males have none.
Individuals with two or more X chromosome have a number of Barr bodies equal to the number
of X chromosomes minus one (that is, equal to the number of inactivated X chromosomes). For
example, XXX individuals have two Barr bodies, XXXX individuals have three, and XXXXX
individuals have four. Thus, an XYY male has no Barr bodies, and XXY or XXYY males have
one Barr body.
Determination of X-chromatin in mucosa cells
Barr bodies can be determined most easily in bucal mucosa, hair roots and fibroblast cells. The
normal positive range for sex chromatin bodies is 20-60 percent.
Determination of sex chromatin in mucosa cells includes the following stages:
1) making the preparation;
2) microscope analysis of the preparation;
3) making of conclusions.
Epithelium cells of the mucosa on the inside of cheek serves as a material.
 Before taking the scrape of the cells, the mouse should be rinsed with clean water.
 Using a sterile scalpel a scrape is made on the inside of the cheek.
 The scrape is spread over the object-plate, which is dipped into methyl alcohol for fixation.
 10-15 minutes later the preparation is taken out of the alcohol, air-dried.
 Then one or two drops of acetoorceine are dropped on the preparation, the preparation is
covered with a cover-glass and the excess of dye is removed by blot.
 Microscopical examination of preparation.
When the preparation is studied by microscope only those nuclei are taken into account, in which
the chromatin appears as oval or kidney-shaped bodies, They are usually localized by the inner
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HUMAN SEX CHROMATIN
surface of nuclear envelope. About 100-150 cells are analyzed, those containing Barr bodies are
counted. The frequency of cells containing Barr bodies is calculated in %.
Examination of Barr bodies is a rapid and convenient test. It is useful to:
 determine the sex in prenatal stage;
 determine the sex of a person with 

ambiguous genitalia

,
 detect abnormal karyotypes such as Turners syndrome and Klinefelter syndrome.
Y-chromatin
Y-chromatin or F body is formed by 2/3 of q arm of the Y chromosome and can be observed
microscopically as a intensive fluorescent staining body in the nucleus of interphase cells. Its size
is about 0,25 microns and it is situated apart (separately) or being attached to nuclear membrane.
The frequency of cells containing F bodies varies in different tissues of male organism. For
example, it is 70-85% in fibroblatst and it is about 45% in sperm cells.
The number of Y-chromatin bodies is equal with the number of Y-chromosomes.
Thus, cells of normal females have no F bodies, cells of normal males have one F body, XYY
males have two F bodies.
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