XX and XY and XXX

3. Five Facts about XX or XY
• If you are interested in gender selection for a boy baby or
gender selection for a girl baby, it helps to understand what is
going on in the first place. Here's five facts about XX and XY
that you may not know.
• Females have two of the same kind of sex chromosome (XX),
and are called the homogametic sex. Males have two distinct
sex chromosomes (XY), and are called the heterogametic sex.
• The XY sex determination system is found in humans, most
mammals, some insects and some plants.
• The XY sex determination system was not discovered until
1905 (previously it was thought that the sex of an infant was
determined by how much heat a man's sperm had during
insemination). Edmund Beecher Wilson and Nettie Stevens are
credited with the discovery.

4. • A man's semen contains approximately 50
percent female-carrying X sperm cells and 50
percent male carrying Y sperm cells. This is true
even in men with three or more children of a
single gender. That's why there is always a 50
percent chance of conceiving a girl and a 50
percent chance of conceiving a boy.
• If you want to choose the gender of your baby,
the only reliable way to do this is by choosing to
do in vitro fertilization (IVF) with a form of genetic
testing known as preimplantation genetic
screening (PGS). There is no other method that
can almost guarantee you will get the baby of the
gender you desire.

5. There is no other method that can almost guarantee you will get the baby of
the gender you desire.

6. Sex Determination
• At first gonads are sexually indifferent. In normal human males,
a “male-determining gene” on the Y chromosome called SRY (
sex-determining region Y ) organizes the developing gonad into
a testis instead of an ovary.
• Once formed, the testis secretes the steroid testosterone. This
hormone, and its metabolite, dihydrotestosterone (DHT),
masculinizes the fetus, causing the differentiation of penis,
scrotum, and the male ducts and glands.
• It also destroys the incipient breast primordia, but leaves
behind the nipples that are a reminder of the indifferent ground
plan from which both sexes develop.
• Testosterone is also responsible for the masculinization of the
brain, but it does so indirectly.
• Surprisingly, testosterone is enzymatically converted to
estrogen in the brain, and it is estrogen that determines the
organization of the brain for male-typical behavior.

7. • Biologists have often stated that in mammals the indifferent
gonad has an inherent tendency to become an ovary.
• Classic experiments performed in rabbits provide support for the
idea
• that the female is the default sex during development. Removal
of the fetal gonads before they have differentiated will invariably
produce a female with uterine tubes, uterus, and vagina, even if
the rabbit is a genetic male.
• Localization in 1994 of a region on the X chromosome named
DDS (dosage-sensitive sex reversal) orSRVX (sex-reversing X),
which promotes ovary formation, has challenged this view.
• In addition, the presence of such a region may help to explain
feminization in some XY males.

8. • It is clear, however, that absence of testosterone in a
genetic female embryo promotes development of female
sexual organs: vagina, clitoris,
• and uterus.
• The developing female brain does require special
protection from the effects of estrogen because, as
mentioned earlier, estrogen causes masculinization of the
brain.
• In rats, a blood protein (alpha-fetoprotein) binds to
estrogen and keeps the hormone from reaching the
developing female brain. This does not appear to be the
case in humans, however, and even though circulating
fetal estrogen levels can be quite high, the developing
female brain does not become masculinized.

9. Gametogenesis
•Mature gametes are produced by a
process called gametogenesis.
•Although the same essential
processes are involved in maturation
of both sperm and eggs in vertebrates,
there are some important differences.
•Gametogenesis in testes is called
spermatogenesis as figure(1), and in
ovaries is called oogenesis. figure(2)

10. figure(1)

11. Oogenesis in humans. Early germ cells (oogonia) increase by mitosis during embryonic development to
form diploid primary oocytes. After puberty, each menstrual month a diploid primary oocyte divides in the
first meiotic division into a haploid secondary oocyte and a haploid polar body. If the
secondary oocyte is fertilized, it enters the second meiotic division. The double-stranded chromosomes
separate into a large ootid and small second polar body. The ootid develops into an ovum. Both ovum and
second polar body now contain the N amount of DNA. Fusion of the haploid egg
nucleus with a haploid sperm nucleus produces a diploid (2N) zygote.
figure(2)

12. Mammalian sex determination
• Since the discovery of sex chromosomes it has been known
that possession of a Y chromosome is necessary to be male,
as human individuals of XO constitution are female while
those with one Y- and multiple X-chromosomes are male.
• But there is a very small minority of apparently XX individuals
who are male. They have experienced a translocation of the
sex-determining gene from the Y- to the X-chromosome.
Conversely there is a very small minority of apparent XY
females who have lost the sex determining gene from the Y
chromosome.
• The application of positional cloning techniques to the DNA of
these unusual cases resulted in the discovery of the sry gene.
• Studies on the mouse showed that the gene is expressed in
the correct location, and that the human gene could
masculinize genetically female mice.

13. CHROMOSOMAL SEX
DETERMINATION IN
MAMMALS
1- determination of the gonads
2- affects the bodily phenotype outside the
gonads

14. 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 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, 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. In mammalian primary sex
determination, there is no "default state." The formation of ovaries and testes are both
active, gene-directed processes. Moreover, as we shall see, both diverge from a common
precursor, the bipotential gonad.

15. 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 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 shown in (Figure3).
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.

16. figure(3)

17. The gonads embody a unique embryological situation.
All other organ rudiments can normally differentiate into only one type
of organ. A lung rudiment can become only a lung, and a liver rudiment
can develop only into a liver.
The gonadal rudiment, however, has two normal options. When it
differentiates, it can develop into either an ovary or a testis. The path of
differentiation taken by this rudiment determines the future sexual
development of the organism.
But, before this decision is made, the mammalian gonad first develops
through a bipotential (indifferent) stage, during which time it has
neither female nor male characteristics.
In humans, the gonadal rudiments appear in the intermediate
mesoderm during week 4 and remains sexually indifferent until week 7.
The gonadal rudiments are paired regions of the intermediate
mesoderm; they form adjacent to the developing kidneys.
The ventral portions of the gonadal rudiments are composed of the
genital ridge epithelium.
The developing gonads

18. During the indifferent stage, the genital ridge epithelium proliferates
into the loose connective mesenchymal tissue above it (Figure 4.A,B).
These epithelial layers form the sex cords.
The germ cells migrate into the gonad during week 6, and are
surrounded by the sex cords.
In both XY and XX gonads, the sex cords remain connected to the
surface epithelium.
If the fetus is XY, the sex cords continue to proliferate through the
eighth week, extending deeply into the connective tissue. These cords
fuse, forming a network of internal (medullary) sex cords and, at its
most distal end, the thinner rete testis (Figure 4.C,D).
Eventually, the sex cords now called testis cords lose contact with the
surface epithelium and become separated from it by a thick
extracellular matrix, the tunica albuginea.
Thus, the germ cells are found in the cords within the testes.
During fetal life and childhood, the testis cords remain solid.
At puberty, however, the cords will hollow out to form the seminiferous
tubules, and the germ cells will begin to differentiate into sperm.

19. figure(4)

20. • Several genes have been found whose function is
necessary for normal sexual differentiation.
• Unlike those that act in other developing organs, the genes
involved in sex determination differ extensively between
phyla, so one cannot look at Drosophila sex determining
genes and expect to see their homologues directing
mammalian sex determination.
• However, since the phenotype of mutations in sex-
determining genes is often sterility, clinical studies have
been used to identify those genes that are active in
determining whether humans become male or female.
• Experimental manipulations to confirm the functions of these
genes can be done in mice.
The mechanisms of mammalian
primary sex determination

21. • Other genetic disorders of sex determination can result
from errors of fertilization, i.e., the cells of an individual
may end up with an abnormal number of sex
chromosomes because non disjunction of sex
chromosomes can occur during meiosis; i.e., the two sex
chromosomes fail to separate during meiosis in the testis
or ovary (Fig. 5-11). An example is Turner’s syndrome, in
which the cells are 45:XO (i.e., only one sex chromosome
is present) and no Barr body is present.
• Individuals with Turner’s syndrome are sexually infantile
with female external genitalia. They have facial
abnormalities, a shield-like chest, and a neck that is short,
broad, and webbed..
Chromosomal Errors and Sex Determination

22. • They also tend to have cardiovascular and kidney disorders.
Their “ovaries” are sterile, consisting mostly of connective
tissue; no germ cells are present.
• A female can have cells that are XXX because of no disjunction
during meiosis in the ovary of her mother (Fig. 5). These
women are female but sterile.
• No disjunction of sex chromosomes can produce males with
cells of an XXY makeup (47:XXY).
• In this condition, Klinefelter’s syndrome (Fig. 5), males are
sterile and have female-like breasts.
• About 1 out of 600 males is born with this disorder. These
individuals have small external genitalia, cryptorchid testes,
and are mentally retarded. Their urethra may also fail to close
during development, resulting in an opening on the lower
surface of the penis, a condition called hypospadias

23. figure(5)

24. • the normal chromosome number in humans is 46 (2N, diploid).
Females have 22 pairs of autosomes and two X chromosomes.
• Males have 22 pairs of autosomes and an X and Y
chromosome.
• The genes for male sex determination are carried on the Y
chromosome. Thus, embryos without a Y chromosome are
female.
• As a result of meiosis in the adult testis, one diploid male germ
cell (spermatogonium) gives rise to four haploid spermatozoa.
Two of these spermatozoa will have 22 autosomes and a Y
chromosome, whereas the other two will have 22 autosomes
and an X chromosome.
• If a Y-bearing sperm (22Y) fertilizes an ovum (with 22
autosomes and an X chromosome), the embryo will be male; if
an X-bearing sperm (22X) fertilizes an ovum, the offspring will
Cause different in Sex Ratios

25. • However, the ratio of male to female embryos at
conception (the primary sex ratio) is about 120:100. This
ratio is based on the sexes of early aborted embryos.
• It is assumed that this means a greater fertilization rate
by Y sperm than X sperm, perhaps because Y sperm
are lighter and faster swimmers than X sperm.
• However, female embryos may die more frequently at an
earlier age than male embryos or more X sperm may die
in the female reproductive tract than Y sperm. The sex
ratio of male births to female births (the secondary sex
ratio) is 105:100. Thus, for reasons not yet understood,
male fetuses suffer a greater mortality than female
fetuses in the uterus.

26. The chromosomal basis for the existence of an equal number of X
and Y sperm, and thus a theoretical primary sex ratio of 100:100.
As discussed in the text, this theoretical ratio is not borne out, and
more embryos are male than female.

27. Sex ratio
• is the ratio of males to females in
a population.
• The human sex ratio is of particular interest
to anthropologists and demographers. In
human societies, however, sex ratios at
birth may be considerably skewed by
factors such as the age of mother at
birth, and by sex-selective abortion and
infanticide.

28. • In anthropology and demography, the human sex ratio is the sex
ratio for Homo sapiens (i.e., the ratio of males tofemales in
a population).
• Like most sexual species, the sex ratio is approximately 1:1.
• In humans the secondary sex ratio (i.e., at birth) is commonly
assumed to be 105 boys to 100 girls, an assumption that is a
subject of debate in the scientific community. The sex ratio for
the entire world population is 101 males to 100 females.
• Gender imbalance may arise as a consequence of various
factors ranging from natural factors and war casualties to
intentional gender control and deliberate gendercide.
• More data is available for humans than for any other species,
and the human sex ratio is more studied than that of any other
species, but interpreting these statistics can be difficult.
• .
Human sex ratio

29. • Human sex ratios, either at birth or in the population as a
whole, might be quoted in any of four ways:
• A. the ratio of males to females, b. the ratio of females to
males, c. the proportion of males, d. the proportion of females
• If there are 80,000 males and 100,000 females, the ratio would,
respectively, be quoted as 1.25, 0.8, 0.519 or 0.481.
• Sex ratio in scientific literature is often expressed as the
proportion of males. In contrast, sex ratio quoted in this article
is the ratio of males to females, unless specified otherwise.
• The CIA estimates that the current world wide sex ratio at
birth is 107 boys to 100 girls, though during the late 1990s there
was concern that the ratio of males to females was declining
too rapidly. In 2010, the global adult sex ratio was 986 females
per 1,000 males and trended to reduce to 984 in 2011.

32. References