Hereditary Determination of Sex
Primary sex determination- Concerns the determination of the gonads. In
mammals, determination strictly chromosomal; not influenced by the
environment. Most cases- female = XX; male= XY
Every individual organism has atleast one X Chromosome. Since the female has
2 X chromosomes, each of her eggs posses one X chromosome. The male posses an
X and a Y, so therefore the male can produce 2 kinds of sperm, one with an X
chromosome and one with a Y chromosome. If an offspring receives an X and a Y, then it
will be a male.
TheY chromosome carries a gene that encodes a testis determining factor. If a
person had an innumerable number of x chromosomes and one y chromosome, they
would be male. If a person is born with only a single x chromosome and no
second x or y, then they develop as a female, but are infertile.(not able to
maintain ovarian follicles)
More Primary Sex Determination- In the is absence of the Y chromosome , the primordial
gonad body develops into ovaries. the ovary then produces the estrogenic hormones,
which contains estrogen and other such hormones, enabling the development of the
Mullerian duct into the uterus, fallopian tubes, and upper end of the vagina.
In the presence of Y chromosome, the testes form. The testes secrete two major
enzymes. The first hormone, AMH(anti-Mulllerian duct hormone), destroys to Mullerian
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duct. The second, testosterone, stimulates the masculinization of the fetus. During this
process the penis, scrotum, and other male anatomical structures form. The development
of the primordial breast is inhibited. The body, therefore, has the female phenotype unless
it is altered by two hormones created in the fetal testes.
The development of gonads is the only fetal organ development process that has
the chance of developing into more than one organ (under normal circumstances and
barring mutations).
The primordial gonad can develop into either an ovary or a testis.
Before the gonad develops into the testes or ovary, it first goes through an indifferent
stage, also known as a bipotential stage, during which time it has neither male or female
characteristics. In humans, the primordial gonad first develops in the 4th week and remains
indifferent until the 7th week.
Sex Determinant genes- In humans, the major genes for the testis determining factor reside
on the short arm of the Y chromosome. Individuals born with the short arm of the Y
chromosome, but not the long are males. Those born with the long end but not the short
are actually female. Through scientific research on XX males and XY females, the position
of the testis-determining gene has been narrowed down to a small region. On the short
arm of the Y chromosome there is believed to be an area called the HMG box, which
stands for high-mobility group box. This HMG box is believed to contain the genetic
information to establish masculinity. There are two known major genes in this HMG box
that are believed to have an effect on the determining of sex, SRY and SOX9.
SRY (sex-determining region of the Y) is found in XY males, is absent from XX
females, is found in the rare XX males, and is absent in the XY females. Many XY women
were found to have a point mutation in the SRY gene, which would prevent the SRY
protein from binding to the DNA. Since humans are difficult to study, Scientists found a
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different way to study this gene.
In mice, there is a gene homologous to SRY, which is named Sry. the mouse gene
also correlates with the presence of testes; it is present in XX males and absent in XY
females. To further test this theory of Sry being the testes determining gene, scientists
injected the Sry sequence into XX fertilized mice zygotes. In most instances the mice
developed testes and the rest of the male accessory organs, but weren’t fertile(the
presence of two X chromosomes prevents sperm formation in both mice and men).
This is
the majority of the evidence supporting this gene as the one that determines whether you
are male or female.
The function of SOX9 is unclear. If a male is born without a functional copy of
SOX9, then a syndrome called campomelic dysplasia develops. It involves numerous
skeletal and organ systems. If born without SOX9, the male child dies soon there after
from distress arising from defective bronchia and tracheas. However, 3/4 of those males
born without SOX9 phenotypicaly appear to be females or hermaphrodites. Since SOX9 is
on the Y Chromosome, almost all women are born without it.
Secondary sex determination- concerns the bodily phenotype outside the
gonads. Secondary sex determination concerns the development of the female and male
phenotypes form the hormones secreted by the ovaries and testes. In the absence of
gonads, the female phenotype is generated. A male mammal has a penis, seminal vesicles,
a prostate gland, and often sex specific size, vocal cartilage, and musculature. A female
mammal has a vagina, uterus, oviducts, mammary glands, and often sex specific size, vocal
cartilage, and musculature.
(1953- scientist named Jost removed fetal rabbit gonads before they had differentiated.
The rabbits that resulted were all female, regardless if they had a pair of XX
chromosomes, or a pair of XY chromosomes. They were all infertile, but developed a
uterus, a vagina, and fallopian tubes.)
Hermaphrodites are named after the son of Hermes and Aphrodite. Having inherited the
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beauty of both parents, he excited the love of the nymph of the Salmacis fountain. As he
bathed in this fountain, she embraced him and prayed to the gods that they might forever
be united. She got her wish.
Hermaphroditism in Fishes- The most common vertebrate hermaphrodite. There are 3
kinds of hermaphrodites in fish.
The first are synchronous hermaphrodites. These have ovaries and testes at the
same time. Fish in this group form spawning pairs. Each fish takes turns spawning the
other fish from the pair’s eggs.
The second are protogynous hermaphrodites. These fish are females at the start of
their lives but later become males.
The third group is protandrous hermaphrodites. These fish are males first, and then
become females at the end of their lives.
