Chromosomal sex determination BY Pardeep kaushal

1. BTY-107
BASIC GENITICS
CASE STUDY
TOPIC:
chromosomal sex
determination in mammals.
--------------------------------------------------------------------------------------------------------------
1· History bhind the sex determination.

2. 2· Introduction.
3· Chromosomal sex determination in
mammals.
4· The sex chromosomes abnormalities.
5· Conclusion.
6· References
-------------------------------------------------------------------
History:- Aristole principle:-
sex was determined
by the basis of heat of male partner during
intercourse.The infant how much heat a man's
sperm had during insemination.
Aristole wrote that:- The semen of the male
differs from the corresponding secretion of female
in that it contains a principle within itself of such a
kind as to set up movements also in the embryo
and to concoct thoroughly the ultimate
nourishment,whereas the secretion of the female
contains material alone. If, then, the male element
prevails it draws the female element into itself,but

3. if it is prevailed over it changes into the opposite
or is destroyed.
Around the year 200 BC, Galen
viewed that woman were but poorly developed
men and that their genitalia were like men's, only
turned inside out, was a very popular one for over
a thousand years.
As late as 1543, Andreas vesalius
Paduan anatomist, showed that he saw the
female genitalia as internal representaions of the
male genitalia..
During the 1600s and 1700s the females were
seen as producing eggs that could transmit traits.
-----------------------------------------------------------------

4. Introduction to chromosomal sex
Determination
The hermaphrodite is a modified female that in
the fourth larval stage makes and stores sperm to
be used later to fertilize oocytes produced within
the gonad of the same animal after
spermatogenesis is finished. The embryos
produced by self-fertilization are encased in an
egg shell and initiate development within the
uterus of the hermaphrodite. When they reach
about the 30-cell stage, the egg-embryos are laid
by the hermaphrodite through a vulva. A useful
consequence of this mode of reproduction is that a
single hermaphrodite heterozygous for a recessive
gene automatically generates one-quarter
recessive homozyotes in its brood of self progeny
—a feature, shared with Mendel's peas, that
helped attract Sydney Brenner to the worm in the
first place. At the same time, Brenner saw that
males, which can mate with and transfer their
sperm to hermaphrodites to produce cross
progeny, are useful to the experimentalist for
making new combinations of genes. Presumably
this is also why C. elegans has retained the male

5. sex, which in the short term at least is completely
dispensable for reproduction.
Hermaphrodites are normally diploid, with five
pairs of autosomes and two X chromosomes.
Males have the same five pairs of autosomes but
only a single X chromosome. Nearly all gametes—
sperm and eggs—produced by hermaphrodites are
haplo-X and thus give rise to XX hermaphrodite
self progeny, but rare males are generated
through spontaneous X chromosome loss. Males
produce equal numbers of haplo-X and nullo-X
sperm, so that half of the cross progeny they sire
will also be male.
-------------------------------------------------------------------
Chromosomal sex determination in
mammals :-
Primary sex determination is the determination of
the gonads. In mammals, primary sex
determination is strictly chromosomal and is not
usually influenced by the environment. In most
cases, the female is XX and the male is XY. Every
individual must have at least one X chromosome.
Since the female is XX, each of her eggs has a
single X chromosome. The male, being XY, can

6. generate two types of sperm: half bear the X
chromosome, half the Y. If the egg receives
another X chromosome from the sperm, the
resulting individual is XX, forms ovaries, and is
female; if the egg receives a Y chromosome from
the sperm, the individual is XY, forms testes, and is
male. The Y chromosome carries a gene that
encodes a testis-determining factor. This factor
organizes the gonad into a testis rather than an
ovary. Unlike the situation in Drosophila (discussed
below), the mammalian Y chromosome is a crucial
factor for determining sex in mammals. A person
with five X chromosomes and one Y chromosome
(XXXXXY) would be male. Furthermore, an
individual with only a single X chromosome and no
second X or Y (i.e., XO) develops as a female and
begins making ovaries, although the ovarian
follicles cannot be maintained. For a complete
ovary, a second X chromosome is needed.
Secondary sex determination affects the bodily
phenotype outside the gonads. A male mammal
has a penis, seminal vesicles, and prostate gland.
A female mammal has a vagina, cervix, uterus,
oviducts, and mammary glands. In many species,
each sex has a sex-specific size, vocal cartilage,
and musculature. These secondary sex

7. characteristics are usually determined by
hormones secreted from the gonads. However, in
the absence of gonads, the female phenotype is
generated. When Jost (1953) removed fetal rabbit
gonads before they had differentiated, the
resulting rabbits had a female phenotype,
regardless of whether they were XX or XY. They
each had oviducts, a uterus, and a vagina, and
each lacked a penis and male accessory structures.
The general scheme of mammalian sex
determination is If the Y chromosome is absent,
the gonadal primordia develop into ovaries. The
ovaries produce estrogen, a hormone that enables
the development of the Müllerian duct into the
uterus, oviducts, and upper end of the vagina. If
the Y chromosome is present, testes form and
secrete two major hormones. The first hormone—
anti-Müllerian duct hormone (AMH; also referred
to as Müllerian-inhibiting substance, MIS)—
destroys the Müllerian duct. The second hormone
—testosterone—masculinizes the fetus,
stimulating the formation of the penis, scrotum,
and other portions of the male anatomy, as well
as inhibiting the development of the breast
primordia. Thus, the body has the female
phenotype unless it is changed by the two

8. hormones secreted by the fetal testes. We will
now take a more detailed look at these events.
Diagram:-
-------------------------------------------------------
The sex chromosomes abnormalities:-
The majority of known types of chromosomal
abnormalities involve sex chromosomes. In
frequency of occurrence, they are only slightly less
common than autosomal abnormalities. However,
they are usually much less severe in their effects.
The high frequency of people with sex
chromosome aberrations is partly due to the fact
that they are rarely lethal conditions. Like Down
syndrome and other autosomal problems, sex

9. chromosome gross abnormalities can be
diagnosed before birth by amniocentesis and
chorionic villi sampling.
Female Sex Chromosome Abnormalities:-
Turner syndrome the preceding term pronounced
occurs when females inherit only one X
chromosome--their genotype is X0 (i.e., monosomy
X). If they survive to birth, these girls have
abnormal growth patterns. They are short in
stature, averaging 4 foot 7 inches as adults, and
often have distinctive webbed necks (i.e., extra
folds of skin), small jaws, and high arched palates.
They generally lack prominent female secondary
sexual characteristics. They have exceptionally
small, widely spaced breasts, broad shield-shaped
chests, and turned-out elbows. Their ovaries do
not develop normally and they do not ovulate.
The few oöcytes that they produce are destroyed
by the time they are two. They are in a sense
postmenopausal the preceding term pronounced
from early childhood and are sterile. However,
they can become pregnant and give birth if
fertilized eggs from a donor are implanted.
Women with Turner syndrome have a higher than
average incidence of thyroid disease, vision and

10. hearing problems, heart defects, diabetes, and
other autoimmune disorders. In a few individuals,
there is slight mental retardation. Turner
syndrome is rare. Current estimates of its
frequency range from 1 in 2,000 to 1 in 5,000
female infants. If diagnosed in early childhood,
regular injections of human growth hormones can
increase their stature by a few inches. Beginning
around the normal age of puberty, estrogen
replacement therapy can result in some breast
development and menstruation. These treatments
allow Turner syndrome women to appear
relatively normal.
Triple-X syndrome:-
occurs in women who inherit three X
chromosomes--their genotype is XXX or more
rarely XXXX or XXXXX. As adults, these "super-
females" or "metafemales" the preceding term
pronounced, as they are sometimes known,
generally are an inch or so taller than average
with unusually long legs and slender torsos but
otherwise appear normal. They usually have
normal development of sexual characteristics and
are fertile but tend to have some ovary
abnormalities that can lead to premature ovarian

11. failure. They may have slight learning difficulties,
especially in speech and language skills, and are
usually in the low range of normal intelligence
(especially the XXXX and XXXXX individuals). They
frequently are very tall in childhood and tend to be
emotionally immature for their size. This
sometimes results in teachers and other adults
labeling them as troublemakers because they
expect more maturity from bigger girls. However,
they are usually as emotionally mature as other
girls of their age. None of these traits prevent
them from being socially accepted as ordinary
adult women. Individuals who are genetic mosaics
(XX/XXX) have less noticeable symptoms. Triple-X
syndrome is less rare than Turner syndrome, but
little is known about it. The frequency is
approximately 1 in 1,000 female infants and it
occurs more commonly when the mother is older.
-------------------------------------------------------------------
Male Sex Chromosome Abnormalities:-
Klinefelter syndrome males inherit one or more
extra X chromosomes their genotype is XXY or
more rarely XXXY or XY/XXY mosaic. In severe
cases, they have relatively high-pitched voices,
asexual to feminine body contours as well as

12. breast enlargement, and comparatively little facial
and body hair. They are sterile or nearly so, and
their testes and prostate gland are small. As a
result, they produce relatively small amounts of
testosterone. The feminizing effects of this
hormonal imbalance can be significantly
diminished if Klinefelter syndrome boys are
regularly given testosterone from the age of
puberty on. Like triple-X females (described
above), many Klinefelter syndrome men are an
inch or so above average height. They also are
likely to be overweight. They usually have learning
difficulties as children, especially with language
and short-term memory. If not given extra help in
early childhood, this often leads to poor school
grades and a subsequent low self esteem.
However, most men who have Klinefelter
syndrome are sufficiently ordinary in appearance
and mental ability to live in society without notice.
It is not unusual for Klinefelter syndrome adults
with slight symptoms to be unaware that they
have it until they are tested for infertility. They are
usually capable of normal sexual function,
including erection and ejaculation, but many, if
not most, are unable to produce sufficient
amounts of sperm for conception. Klinefelter

13. syndrome males with more than two X
chromosomes usually have extreme symptoms
and are often slightly retarded mentally. Men
who are mosaic (XY/XXY) generally have the least
problems. There is no evidence that Klinefelter
syndrome boys and men are more inclined to be
homosexual, but they are more likely to be less
interested in sex. They have a higher than average
risk of developing osteoporosis, diabetes, and
other autoimmune disorders that are more
common in women. This may be connected to low
testosterone production. Subsequently, regular
testosterone therapy is often prescribed. Both
syndromes are more likely to occur in babies of
older mothers.
XYY syndrome males inherit an extra Y
chromosome--their genotype is XYY. As adults,
these "super-males" are usually tall (above 6 feet)
and generally appear and act normal. However,
they produce high levels of testosterone. During
adolescence, they often are slender, have severe
facial acne, and are poorly coordinated. They are
usually fertile and lead ordinary lives as adults.
Many, if not most, are unaware that they have a
chromosomal abnormality. The frequency of XYY
syndrome is not certain due to statistical

14. differences between different studies. It may be
as common as 1 in 900 male births to as rare as 1
in 1500 or even 1 in 2,000. XYY syndrome is also
referred to as Jacobs syndrome.
conclusion:-
7· Although the same essential processes are
involved in maturation of both sperm and
egges in vertebrates, there are some
improtent differences
8· Gametogenesis in testies is calles
Spermatogenesis and in ovaries is calles
oogenesis.
9· Mature gametes are produced by a process
called gametogenesis.
References:-
Hickman C.P,L.S. Roberts,A.Larsolh.I'Anson.(2004)
Integrated Principles of zoology edition, McGrew-
Hill companies,inc.80-81-141
Gilbert.Developmental Biology. 6th edition,463 to
465
Jones,Richard E.(2006).Human Reproductive
Biology. Acadmic Press in an imprint of

15. elsevier.US.14
Slack,J.M.W.(2006).Essential developmental
Biology. blackwall publishing Ltd,second edition.
(2004).
Brown, S. Entomological contributions to genetics:
Studies on insect germ cells linked genes to
chromosomes and chromosomes to Mendelian
inheritance. Archives of Insect Biochemistry and
Physiology 53, 115–118 (2003)
Henking, H. Über Spermatogenese und deren
Beziehung zur Entwicklung bei Pyrrhocoris apterus
L. Zeitschrift für wissenschaftliche Zoologie 51,
685–736 (1891).