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.

Although the X chromosome is larger than the Y chromosome and carries over 1300 genes, there must be the presence of a Y chromosome in order for the male phenotype to be expressed. In humans, there is a gene found on the Y chromosome, called the Sex Determining Region Y (SRY), which determines maleness. When this is present it activates the formation SRY proteins which in turn activates the formations of testes, thanks to a conformational change in the DNA due to a change in the level of gene expression[2]. Individuals that have a Y chromosome will appear to have the male phenotype, independent of the number of X chromosomes in their genotype[3].; A genetic disorder such as Klinefelter syndrome is caused by the presence of an extra X chromosome in an individual with a Y chromosome, hence having the genotype XXY with a male phenotype. Also, the addition of an extra Y chromosome in males would cause a genetic disorder called, "Super Male Syndrome".

Often in this mode, the females are homogametic (having two X chromosomes and consequently producing only one type of gamete, X), and the males are heterogametic (having one X and one Y chromosome and consequently producing two types of gametes, X or Y)[4]. Examples of organisms using this form of sex determination include: most mammals, including humans, as well as some insects.

Males being the heterogametic sex does not occur in every mammal. For example, in birds, it is the female sex that is heterogametic and the male that is homogametic.

Additional X chromosomes lead to Barr bodies being present. These are inactive forms of the X chromosome formed through the process called X Chromosome Inactivation, where one of the X chromosomes will be randomly switched off. For example, a sufferer of Klinefelter Syndrome with genotype XXY would have one Barr body while in individuals with genotype XXXY, they would have 2 Barr bodies. The number of Barr bodies is determined by number of X chromosomes (n) – 1.

Temperature and Sex determination

The environment affects sex determination in a number of ways. In reptiles, incubation temperature during the critical period of embryonic development can affect the sex of the offspring. This is because the activity of enzymes and inhibitors controlling the production of steroid hormones such as oestrogen is affected by temperature.


  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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