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

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

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

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.

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