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 Region Y, which determines maleness. When this is present it activates the formation 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 become males independent of the number of X chromosomes[3]; genetic disorders such as Klinefelter syndrome are caused by to the presence of extra X chromosomes in an individual with a Y chromosome. 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.

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

Additional X chromosomes lead to Barr bodies being present. These are inactive forms of the X chromosome. For example a sufferer of Klinefelter Syndrome with chromosomes XXY would have one Barr body, and if they have XXXY they would have 2. The number of Barr bodies is determined by number of X chromosomes (n) – 1. (Journal of Forensic Dental sciences online, 2013).

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 eostrogen is affected by temperature.

R Amirthaa Priyadharscini, T.S
Barr bodies in sex determination 2013.
Journal of Forensic Dental sciences [online] p.64 [Accessed 12 Nov 2014]


  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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