What is Sex Determination System? Definition and Examples

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Sex Determination Overview

By nature, we all look every living organism in this environment as male or female, considering from a plant, pets and we the humans; because these characters are externally shown and they can be easily differentiated into separate individuals.

We will be discussing here about how each sexes are determined, either through gene or through other external features.

Usually, all organisms in this living environment are classified as two sexes namely male and females depending upon their gametes and sexual characters and differentiation among them.

Accordingly, the Latin term sex means separation or section. Some organisms of lowest forms contain more than two sexes, where the species of protozoa which is known as Paramecium bursaria has eight sexes.

The organisms which usually posses two sex organs in a same individual are referred as hermaphrodite individuals.

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Earthworms are hermaphrodites: each carries male and female sex organs.

Where as in flowers the staminate or anther is seen in males and pistillate is being seen in females but they are present in the same plant hence the term monoecious is used here.

But most of the flowers has both male and female organs in a same plant and it is called as the original or a perfect flower.

As such for animals the organisms which produce male and female gametes individually are referred by the term dioecious.

However, these sex cells and reproductive organs and their respective hormones play an important role in determining their primary and secondary sexual characters.

Mechanism of Sex Determination

Mostly our sexes are determined in a genetical manner and they are classified into four categories as follows,

1. Heterogamesis or sex chromosome mechanism

2. Genic balance mechanism

3. Male haploidy or haplodiploidy mechanism

4. Single gene effects.

1. Heterogamesis

Generally, in dioecious organisms two types of chromosomes are present such as autosomes and sex chromosomes.

The sex chromosomes form two types of sex determination depending on two systems namely heterogametic females and heterogametic males.

In heterogametic sex, females have two x chromosomes where as the males have one x and one y chromosomes with 50 percent of the gametes.

The heterogametic male contains XX-XO type in plants and XX-XY type in mammals.

Where as the heterogametic female contains ZZ-ZO types in moths and ZW-ZZ type in fishes and certain insects

2. Genic Balance

This mechanism explains that some genes are being carried by sex chromosome which is responsible for the determination of sex in individuals.

There will be mostly an inherited mechanism of sexes in both individuals considering from masculine features in male to feminine characters in female.

This was discovered by Wilson, Bridges and Goldschmidt in different organisms.

If an individual is found intermediate between both sexes they are referred to as intersexes.

Sex Determination in Drosophila

The Y chromosomes present in a drosophila are very much important for maintaining the fertility in male flies but they do not have much importance in determining the sex.

The sex of the flies are determined polygenically, where the genes are situated in a distributive manner such that autosomes determine the male character and where as in female it is determined by X chromosome.

Bridges crossed the polyploid flies of experimentally produced female drosophila which has triploid set of chromosomes (3AA:3XX) to that of the diploid male chromosomes.

Which resulted in the formation of intersexes, superfemales and super males and also a normal diploid male along with the triploid females.

From this experiment he concluded that autosomes also carry the genes which are responsible for sex determination.

The intersexes obtained were sterile with both male and female phenotypic characters. This proves the genic balance mechanism.

Sex Determination in Human

Like Drosophila, sex determination can also be seen in humans, where male has XX-XY pair of chromosomes, here Y trait has a potential to bring all male determining characters, where as X chromosome is considered as a feminine character determiner.

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Whereas the male having one X and one Y chromosomes can be a normal male by overcoming the feminine characters of X chromosome until there is no chances of chromosomal abnormalities or abbretions, which results in syndromes such as Turner and Klinefelter’s which causes feminine characters in male; like inhibiting sperm production, lack of masculine features; such as bread growth and body hairs, etc. which leads to many physical and mental abnormalities.

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Where in Turners syndrome the lack of female characters tends to occur, such as shield chest, short stature, congenital malformations and webbing of neck etc,.

The individual who has addition of both type of chromosomes that is extra X and Y chromosomes, shows true hermaphroditism which have both ovarian and testicular tissues and genitalia appears to be intersexual.

3. Male Haploidy or Haplodiploidy

This is considered as one of the types of parthenogenesis mechanism which is commonly seen in hymenopterous insects.

These types of insects include ants, bees, wasps and sawflies.

Where fertilise eggs develop into diploid females and also as unfertilised eggs are developed into haploid males.

So, this is also considered as a form of sex determination. Considering an example of honey bee, where they can lay two varieties of eggs.

Such that it produces a fertilised egg by controlling a splinter of her sperm receptacle and an unfertilized one. Here the fertilised egg has diploid zygote and develops into a female and an unfertilised egg has only 16 zygotes and develops into a male.

Later the females are grown up as a honey bee or worker bee depending upon the food they consume or diet they undertake and the one which is developed from unfertilised eggs are males.

Single Gene Control of Sex

In certain organisms like Neurospora, Chlamydomonas, asparagus, yeast, maize etc, the single gene plays a very important role in determination of sexes and also in expressing the genes hence it is known as single gene control of sex.

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