How sex determination varies from species to species?

“A biological process by which the sex of an individual is decided under the influence of genetic or environmental factor”.

Sex determination is a complex biological process in nature. Different species have a different type of sex determination mechanism.

Male produces male gametes and female produces female gametes however some organism exhibits production of both gametes which become hermaphrodites. Different type of sex determination mechanisms are discussed here,

XX-XY type of sex determination

Humans are dioecious, both male and female have different sexes. A German biologist, H. Henkings, first discovered X chromosome in 1891. However, he was not sure what the nuclei body is, hence he termed it randomly as an “ X body”. Later on, C.E. McClung verified his observation by cytological analysis in 1902 and denoted it as an X chromosome.

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The discovery of X chromosome favour’s that X chromosome is responsible for femaleness. Two X chromosomes are present in the female and one X chromosome along with a short Y chromosome present in the male.

Interestingly, Genes play important role in sex differentiation. TDF and SRY genes, present on Y chromosome strongly associated with masculine, fertile maleness.

It is also evident that if XX chromosome leads to female gamete it does not mean, only one X chromosome become male. XO is a cytological abnormality in which an individual is a genetically abnormal female, called as a Turner syndrome.

Sex chromosome to autosomal chromosome ratio and sex determination

Sex determination in Drosophila melanogaster is similar to humans but the role of a sex chromosome is restricted in Drosophila. A ratio from autosome to sex chromosome decides the sex of individual fly because Y chromosome is not present in Drosophila.

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C.B Bridge showed that X chromosome determines female while autosome determines male. Normal Drosophila flies have 4 chromosomes, three autosomes and one X chromosome. If one X chromosome is present in a combination of two autosomal pairs of chromosome, the ratio of 1X: 2A become 0.5 and develops into a male, while 2X: 2A ratio becomes 1 and it develops into a female.

Interestingly, a metafemale also exists in Drosophila with 1.5 X/A ratio. The ratio of X chromosome to a pair of autosomal chromosome decides the sex of fetus (Calculate number of X and autosome depending upon the ratio by yourself).

Conclusively, X chromosome is involved in sex determination of Drosophila but it depends on the number of autosomal pairs versus a number of X chromosome. Non-disjunction during cell division is a reason why the different combination of X/A is originated.

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ZW and ZZ type of sex determination

In birds, some moths and fishes a unique type of sex determination mechanism developed, and it is similar to humans. Here female is heterogametic (Hetero means two different type and gametic means gametes). Hence female is ZW (in humans, two X chromosomes are present in female-homogametic).

In birds, a female egg has two different chromosomes Z and W and male sperm has two identical Z chromosomes. The chromosomal composition of sperm determines sex in the human while the chromosomal composition of an egg determines the sex in ZW-ZZ type of sex determination.

Haplodiploidy and sex determination

Another interesting type of sex determination mechanism is haplodiploid. In humans, Diploid- somatic cells have 46 numbers of the chromosome (23 pairs ) while haploid cells- germ cells have only 23 numbers of the chromosome. Depending upon the haploid and diploid number of chromosomes the type of sex is decided in bees, sawflies and wasps.

Males are developed from an unfertilized egg and have haploid numbers of a chromosome. In honey bee, haploid males are termed as a drone, while queen bee and male worker bees have a diploid number of chromosomes.

Interestingly, normal males are haploid and normal female is diploid while diploid male bees (worker bees) are sterile and spent their entire life for nurturing and serving queen and drones. The only drone can do sex with the queen.

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Environmental factor and sex determination

Yet another type of sex determination mechanism in which sex of an individual is not decided by chromosomes or genes. Elaborately, genetic factors do not have any specific role in deciding the sex of several organisms. Male and female have the same genotypes but external environmental stimuli decide the fate of an organism.

In the turtle chrysems picta, temperature decides the sex of fertilized egg. Higher temperature during the incubation period of egg produces mostly female turtle and vice versa. While in lizard agama agama, high temperature during incubation mostly develops into a male lizard.

In bonellia worm, males are smaller and live in the body (specifically reproductive track) of female worm. A single isolated egg released into the water and become female, if it is attached to female and migrates inside the female body, it becomes a parasitic male worm.

Sex determination is mostly based on genetic factors in almost all organisms however nature gives power to some organisms by which they can decide the sex of their progeny by their own decisions.

It depending upon the external environment but ultimately the decision is taken by the parent organism. Though non-genetic sex determination is a choice base decision, we can assume that these organisms do not have a sense what to develop, a male or a female?

Besides the involvement of chromosome in sex determination, genes play a crucial role in same. SRY (sex determining region on Y chromosome) is a candidate gene in sex determination and differentiation. Notably, several autosomal genes DAX1, SOX3, SOX9 etc are an integral part of the genetic pathway in sex differentiation and sex determination.

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Additionally, hormones play a crucial for human sex determination. FSH, LH and estrogen periodically released differently in male and female. Though it is a part of sexual differentiation, it can not decides the sexual orientation of a fetus.

Sexual orientation is a sexual identity of an individual which may not be in co-relation with his or her gender. we will discuss sexual orientation in other articles.

Article written by- Tushar Chauhan

Article reviewed by- Tushar Kachhadiya