Sex determination• A Brief History on how to Determine Sex of an

Individual• Chromosomal Sex Determination in Mammals• Chromosomal Sex Determination in

Drosophila• Environmental Sex Determination

Aristotelian view of Sex Determination(335 B.C.)

sex was determined by the heat of the male partner during intercourse. The more heated the passion, the greater the probability of male offspring. Aristotle counseled elderly men to conceive in the summer if they wished to have male heirs.

Around the year 200 BC, Galen

viewed that women 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

the Paduan anatomist, showed that he saw the female genitalia as internal representations of the male genitalia

During the 1600s and 1700s

females were seen as producing eggs that could transmit parental traits.

20th century

The environment temperature and nutrition, in particular was believed to be important in determining sex.

This environmental view of sex determination remained the only major scientific theory, the environment temperature and nutrition, in particular was believed to be important in determining sex. Rediscovery of Mendel's work in 1900 and the rediscovery of the sex chromosome by McClung in 1902 further explained sex determination.

• In 1905, a correlation (in insects) of the female sex with XX sex chromosomes and the male sex with XY or XO chromosomes was established. This finding suggested strongly that a specific nuclear component was responsible for directing the development of the sexual phenotype. Thus, evidence accumulated that sex determination occurs by nuclear inheritance rather than by environmental happenstance.

Sex determination

Mammals

Primary sex determination- chromosomal and is not usually

influenced by the environment- female is XX . Each of her eggs

has a single X chromosome. - male is XY. 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.

DrosophilaPrimary sex determination- sex is determined by the ratio

of X chromosomes to autosomes, and the Y chromosome does not play a role in sex determination. There are no sex hormones, so each cell makes a sex determination decision.

- Normally, flies have either one or two X chromosomes and two sets of autosomes. If there is but one X chromosome in a diploid cell,

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the fly is male. Thus, XO Drosophila are sterile males. In flies, the Y chromosome is not involved in determining sex. Rather, it contains genes active in forming sperm in adults.

Gene sex determining factor

humans

• SRY gene is the testis-determining factor on the Y chromosome. It synthesizes a DNA-binding protein that is thought to compete with the DAX1 protein. It is thought that if SRY is produced at a high enough level, it activates (either directly or indirectly) the SF1 gene and inhibits the WNT4 gene.

Drosophila

The Drosophila Sxl gene is activated in females (by proteins encoded on the X chromosomes) and is repressed in males (by factors encoded on the autosomes). Sxl protein acts as an RNA splicing factor to splice an inhibitory exon from the tra transcript.

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humans

• The SF1 product is believed to activate the SOX9 gene, as well as several other genes involved in synthesizing steroid hormones and anti-Müllerian duct hormone (AMH). SOX9 may organize the genital ridge epithelium to form testes, but the corresponding ovary-forming genes have not yet been found, although the WNT-4 gene may be important in this regard.

drosophila

Therefore, female flies have an active Tra protein, while males do not.

The Tra protein also acts as an RNA splicing factor to splice exons of the double sex transcript. The double sex gene is transcribed in both XX and XY cells, but its pre-mRNA is processed to form different mRNAs, depending on whether Tra is present. The proteins translated from both messages are active, and they activate or inhibit transcription of a set of genes involved in producing the sexually dimorphic traits of the fly.

Secondary sex determination

humansSecondary sex determination in

mammals involves the hormones produced by the developing gonads. Under estrogenic stimulation, the Müllerian duct differentiates into the oviducts, uterus, cervix, and upper portion of the vagina. In male mammals, the Müllerian duct is destroyed by the AMH produced by the Sertoli cells, while the testosterone produced by the Leydig cells enables the Wolffian duct to differentiate into the vas deferens and seminal vesicle. In female mammals, the Wolffian duct degenerates because of the lack of testosterone.

drosophilaThere are no sex hormones, so

each cell makes a sex determination decision.

Continued… The conversion of testosterone to dihydrotestosterone in the genital rudiment and

prostate gland precursor enables the differentiation of the penis, scrotum, and prostate gland

Environmental Sex DeterminationTemperature-dependent sex determination in

reptiles-While the sex of most snakes and most lizards is

determined by sex chromosomes at the time of fertilization, the sex of most turtles and all species of crocodilians is determined by the environment after fertilization. In these reptiles, the temperature of the eggs during a certain period of development is the deciding factor in determining sex, and small changes in temperature can cause dramatic changes in the sex ratio (Bull 1980).

Often, eggs incubated at low temperatures (22 27°C) produce one sex, whereas eggsincubated at higher temperatures (30°C and above) produce the other. There is only a small range of temperatures that permits both males and females to hatch from the same brood of eggs.

shows the abrupt temperature-induced change in sex ratios for the red-eared slider turtle. If eggs are incubated below 28°C, all the turtles hatching from them will be male. Above 31°C, every egg gives rise to a female. At temperatures in between, the broods will give rise to individuals of both sexes. Variations on this theme also exist. The eggs of the snapping turtleMacroclemys, for instance, become female at either cool (22°C or lower) or hot (28°C or above) temperatures. Between these extremes, males predominate.

Location-dependent sex determination in Bonellia and Crepidula

the sex of the echiuroid worm Bonellia depends on where a larva settles. If a Bonellia larva lands on the ocean floor, it develops into a 10-cm-long female. If the larva is attracted to a female's proboscis, it travels along the tube until it enters the female's body. There in it differentiates into a minute (1- 3mmlong) male that is essentially a sperm producing symbiont of the female

Young individuals are always male. This phase is followed by the degeneration of the male reproductive system and a period of lability. The next phase can be either male or female, depending on the animal's position in the mound.If the snail is attached to a female, it will become male.

Another example in which sex determination isaffected by the location of the organism is the case of the slipper snail Crepidula fornicata. In this species, individuals pile up on top of one another to form a mound.

If such a snail is removed from its attachment, it will become female. Similarly, the presence of large numbers of males will cause some of the males to become females. However, once an individual becomes female, it willnot revert to being male.