Sex determination is the process of determining sex of the organism, where the role of individuals is further characterized like whether they are donors or recipients, strong or weak, have longer or shorter life span etc. In Nature, the process of Sex determination is fascinating and there are many models described to explain the same. This article focuses on the major sex determination mechanisms from Insects to Mammals including Birds and Reptiles.

There are four major kinds of sex determination mechanism described such as autosome to X chromosome ratio, haploid / diploid mechanism, XY or ZW mechanism and climatic factor in determining the sex of the individuals.

Autosome to X chromosome ratio is the classical example of Drosophila (the common fruit fly) sex determination described in many text books. The ratio of Autosome to X chromosome determines the sex of individuals in Drosophila. The simple 1 ratio XX:AA (diploid number of autosomes to pair of X chromosomes) results in a fertile female and 0.5 ratio of X:AA results in male Drosophila. The Y chromosome is required to make male fertile as both XO and XY individuals are males with ratio of 0.5, but only XY individual with ratio 0.5 is fertile male. Any other ratio's like 0.33 or 0.67 or >1 results in intersexes called super males or super females.

The Honey Bees have a different mechanism of sex determination called Haploid to Diploid condition and they don't have any chromosomes differentiated as sex chromosomes or allosomes. The bees that gets diploid set of chromosomes becomes females and the bees that has haploid set of chromosomes develops into male bees or drones. In other words, the eggs that are fertilized with sperms will develop into females and the eggs that are not fertilized with sperms develop into males. The fertilized egg that is feed with special food called Royal Jelly during entire larval stages develops into Queen. Though all the worker bees are developed from the fertilized eggs, they develop into sterile individuals due the pheromones secreted by the Queen and food they are feed during the initial larval stages. Further worker bees shows phenomenon called Kin selection allowing only Queen to reproduce and sacrificing their reproductive capacity and thus their progeny continuation. Mealy bugs have different kind sex determination based on the diploid condition but inactivated sets of chromosomes from paternal side. Both the males and females have same diploid sets of chromosomes and during early development of males, the chromosome that are received from father called paternal side is deactivated by hetero-chromatinization and eliminated in gametogenesis.

In some Reptiles, environment temperature plays a key role in determining the sex of the individuals and is known as Temperature dependent Sex Determination (TSD). In TSD, many variations are seen and they are classified further. In few reptiles, if the eggs are incubated with low temperature, they develop into males while the higher incubation temperature results in females. In other reptiles, the reverse pattern is observed, if the eggs are incubated in low temperature, they develop into females and males are developed at higher temperature. In few other reptiles, a mixed pattern is observed, if the eggs are incubated at an optimum temperature, males are developed and any incubation temperature above or below optimal temperature develops the eggs into female individuals.

Bird and Mammals follow different pattern of sex determination and known as XY or ZW type of Sex determination. Mammals have XY type of sex determination; the zygote that has two XX chromosomes develop into female and zygote with XY chromosomes develop into male. The autosomes don't have any direct effect on the sex determination of individuals. The presence of a single Y chromosome is sufficient to shift the gender in humans and individuals with XXY chromosomes are males with reduced fertility called Klinefelter's syndrome or XXY males, alternatively the individuals with no Y chromosome are females known as Turner's syndrome or 45, XO females with no fertility. In XY sex determination, females are homozygous while the males are heterozygous, meaning females develop only one type of gametes called eggs with only X chromosomes and the males develop gametes called sperms with two different types of karyotypes one with X and other with Y chromosomes. Opposite to mammals, Birds have different kind of sex determination called ZW type of sex determination. In birds, the zygote that will have ZW chromosomes develop into female and the zygote that has ZZ chromosome turns male. The females are heterozygous in case of birds and males are homozygous.

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