Question

Developmental Biology class: Compare and Contrast Sex Determination in Mammals to that of Drosophila

Answer 1

Sex determination

• Both mammals and Drosophila produce XX females and XY males.
• But the sex-determining mechanisms in mammals and Drosophila are very different.

In mammals -

• In mammals, the Y chromosome plays a important role in determining the male sex. Thus, XX/XO mammals are females.
• The primary sex determination is strictly chromosomal (It is not usually influenced by environment).
• In most cases, the female is XX and the male is XY.
• 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 and half the Y.
• If the egg mates with another X chromosome from the sperm, the resulting individual is XX, forms ovaries, and it will be female.
• if the egg receives a Y chromosome from the sperm, then the individual is XY, forms testes and it will be male.
• The Y chromosome carries a gene that encodes a testis-determining factor. This organizes the gonad into a testis. It is a crucial factor for determining sex in mammals.
• The gene present in the short arm of Y chromosome is called SRY (sex-determining region of the Y chromosome)
• A person with more than one X chromosomes and one Y chromosome (XXXXXY) would be male.
• 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).

• 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 them, the secondary sex characteristics are usually determined by hormones secreted from the gonads.

• The ovaries produce estrogen, a hormone that enables the development of the Mullerian duct into the uterus, oviducts and upper end of vagina.

• If the Y chromosome is present, testes form and secretes two major hormones. The first hormone—anti-Mullerian duct hormone that destroys the Müllerian duct.

• The second hormone, testosterone masculinizes the fetus and stimulates male anatomy.

In Drosophila -​​​​​​

• In Drosophila, sex determination is achieved by a balance of female determinants on the X chromosome and male determinants on the autosomes.
• Normally, flies have either one or two X chromosomes and two sets of autosomes.
• If there is one X chromosome in a diploid cell (1X:2A), then the fly is male.
• If there are two X chromosomes in a diploid cell (2X:2A), then the fly is female.
• Thus, XO Drosophila are sterile males.
• Flies are unable to survive with more than two copies of an X chromosome because of the mechanism that they use for dosage compensation.
• All of the Drosophila X chromosomes remain active, and they adjust the levels of X-linked gene products by doubling the expression from the X chromosome in males.
• An extra copy of the X chromosome, which contains close to one-third of fly genes creates an aneuploid condition that disrupts the cell equilibrium.
• In flies, the Y chromosome is not involved in determining sex. It contains genes that activate sperm in adults.
• In Drosophila, we can observe gynandromorphs i.e., animals in which certain regions of the body are male and other regions are female.

Thank you.