Sex Determination

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Sex Determination is the process by which two sexes, male and female, become into being. It determines the development of the sexual organs of the organism.

Protenor Mode:

Otherwise known as the XX/XO mode of sex determination.

The presence or absence of the X chromosome in male gametes determines sex. Females are the homogametic sex (XX). Males only have one chromosome (X) and are represented by XO - they act as the heterogametic sex. Examples include: some insects such as cockroaches and grasshoppers[1].

Lygaeus Mode:

The more common mode of sex determination: XX/XY.

There must be the presence of a Y chromosome in order to determine the male sex. In humans there is a area, found on the Y chromosome, called the Sex Determining Reg determines maleness. This stands for sex-determining region Y gene. When this is present it activates the formation of testes thanks to a conformational change in the DNA which therefore changes the level of gene expression in order to form testes[2]. Individuals that have a Y chromosome will become males independent of the number of X chromosomes[3]; genetics disorders such as Klinefelter syndrome are caused due to the presence of extra X chromosomes. In this mode the females are homogametic (produce one type of ovum), and the males are heterogametic (producing two types of gametes)[4]. Examples of organisms using this form of sex determination include: most mammals, including humans, as well as some insects.

Although in mammals, etc, the heterogametic sex is male, in birds it is the female sex that is heterogametic and the male that is homogametic. 


  3. Alberts. B, Johnson. A, Lewis. J, Raff. M, Roberts. K, Walter. P, (2008), Molecular Biology of the cell, fifth edition, New York, Garland Science.
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