Transcript Sex Determination in Man

Sex determination
Lec/6
Sex Determination in Man
• In man XX-XY type sex determining
mechanism occurs but here the Y chromosome
contains potent male sex-determining genes
which can almost completely overcome the
feminizing action of the rest of the genotype.
The conclusive evidences that Y chromosome
is a determiner of fertility and sex of male
individual came from certain abnormal
conditions (called syndromes) which contained
aneuploidic sex-chromosomal abnormalities.
• For instance, Turner’s syndromes (XO) are
sterile female individuals. Similarly, Klinefelter’s
syndromes (XX Y) are males, despite the
presence of two X chromosomes. A person with
extra one X and Y chromosome display true
hermaphroditism having both ovarian and
testicular tissues and variable degrees of
intersexual development of the genitalia.
Sex differentiation
• 1. Genetic Sex
•
Normal females ordinarily have two X chromosomes;
normal males have one X and one Y. The genes on these sex
chromosomes determine femaleness or maleness.
• Further, since the X-chromosome carries much more genetic
information in striking contrast to Y chromosome, one might
wonder how it is that the female can carry a double dose of
many vital X-linked genes, whereas the male has only a single
dose of these X-linked genes. Such inequality in fact cannot be
tolerated and so female seem to have developed their
owntypes of dosage compensation mechanisms.
Dosage Compensation of Genes
• (i) X-chromosome inactivation in mammals.
• It has been demonstrated that in
homogametic XX female individuals, one X
chromosome gets characteristically condensed
and inactivated.
• Since it becomes inactive in certain part of the
life cycle and resumes activity before entering
the germ line
• The phenomenon of inactivation of X
chromosome was confirmed by the observation
of the Barr body. It has also been observed in
most of the body cells (e.g., skin, oral
epithelium and blood cells) of man .
• The sex chromatin appears in the interphase nucleus as a
small chromocentre, heavily stained
• with basic dyes.It can be found in four position:
(i) attached to the nucleus as in nerve cells of certain species;
(ii)attached to the nuclear membrane as in cells of epidermis or
of the oral mucosa;
(iii)free in the nucleoplasm as in neurons after electric
stimulation,
(iv)as a nuclear expansion.
• The best known example of nuclear expansion is that of the
neutrophil leukocyte of female in which the sex chromatin
(Barr body) appears as a small rod called the drumstick.
•
Lyon’s hypothesis.
• The number of X chromosomes was two or
more than two, the number of Barr bodies was
one less than the number of X chromosomes
(nX-1; i.e., one Barr body in XX females and
XXY males; two Barr bodies in XXXY males
and XXX metafemales). Thus, in normal
female only one active X chromosome is
present. Which of the two X chromosomes
remains active in female individuals, is
determined at the early stages of development.
• In other words, the inactivation of X
chromosome is a random phenomenon. This
fact has been demonstrated in human diseases
linked to X chromosome. The Lesch-Nyhan
syndrome, in which a deficiency of one
enzyme of the purine metabolism (i.e.,
hypoxanthineguanine
phosphoribosyl
transferase) produces mental retardation and
• increased uric acid levels results, from a
recessive mutation in the X chromosome.
• if fibroblasts of these patients are cultured in
vitro, two types of cell clones are obtained. Half
the clones contain the enzyme, whereas the other
half (in which the X carrying the normal gene is
condensed) lack the enzyme.
• In the human embryo X chromosome inactivation
starts in the late blastocyst about the 16th day of
life(100 cell). Once the inactivation is established,
it is irreversibly maintained in somatic cells,
however, in germ cell line reactivation occurs at a
specific stage of germ cell development .
• The good illustration of X chromosome inactivation is
seen with calico cats, where
• the coat is a mosaic patchwork of black and yellow
hair. Black hair is produced by the dominant allele B,
and yellow by its recessive allele b. This gene is Xlinked, so if one X chromosome contains the dominant
allele B and other X chromosome the recessive allele b,
random inactivation will allow both coat colours to be
expressed. Male calico cats are understandably rare,
since it has only one X chromosome.
2. Gonadal Sex
• In the human embryos until the six weeks the
gonads and primordia of the urinogenital tract
are identical in males and females. At this
stage (time) the gonad has already been
invaded by the primary XX or XY cells. At
this point, a gene or set of genes, called testis
determining factor or TDF, present in the Y
chromosome causes the undifferentiated gonad
to differentiate into a testis and the absence of
this gene allows the gonad to become an ovary
• . The development into a testis starts as soon as
the gonocytes (i.e., primordial germ cells) from
the yolk sac have finished their migration into
the gonadal ridge. Gonocytes of the male (XY)
migrate deeper into the gonadal blastema
forming the medulla and female gonocytes
(XX) remain at the periphery, forming a thick
cortical layer. Hence, the XX genetic sex is
ordinarily associated with ovarian gonadal
sex, and XY is associated with testicular
gonadal sex.
Sex determination
• In human beings, the presence of Y chromosome
determines maleness and its absence determines
femaleness. So, males are XY and females are XX in
human beings. However, in 1986, certain peculiar cases
have been reported which were found to be males with
XX chromosomes and females with XY chromosomes.
These can be due to any one of the following two
reasons : (i) A sex reversal gene SRY located on the Y
chromosome leads to XX males and XY females. (ii)
Translocation of a small segment of the Y chromosome
to an X chromosome in XX males and its deletion from
the Y chromosome results in the XY females.
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