“A biological process by which the sex of an individual is decided under the influence of genetic or environmental factors known as sex determination.“
Sex determination is a complex biological process in nature. Different species have different types of sex determination mechanisms. It isn’t limited to chromosomes, genetics or environment only.
A few diverse mechanisms show that the process significantly differs. But how?
XX-XY type of sex determination
Humans are dioecious, both males and females have different sexes. A German biologist, H. Henkings, first discovered the 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.
The discovery of the X chromosome favor’s that the X chromosome is responsible for femaleness. Two X chromosomes are present in females and one X along with a short Y chromosome is present in the males. For males, Genes like SRY present on the Y chromosome do the job to give maleness with masculinity and fertility.
However, evidence demonstrates that the presence or absence of X or Y chromosomes or numerical changes may lead to chromosomal abnormalities related to sex, for example, Turner syndrome or Klinefelter syndrome.
In some organisms the change in the ratio from sex to autosomes also creates problems.
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 an individual fly because the Y chromosome is not present in Drosophila.
Related article: What are Autosomes? Definition, Meaning, Genes, Inheritance and Disorders.
C.B Bridge showed that the X chromosome determines females while the autosome determines males. 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 chromosomes, the ratio of 1X: 2A becomes 0.5 and develops into a male, while the 2X: 2A ratio becomes 1 and develops into a female.
Interestingly, a metafemale also exists in Drosophila with a 1.5 X/A ratio. The ratio of the X chromosome to a pair of autosomal chromosomes decides the sex of the fetus (Calculate the number of X and autosome depending upon the ratio by yourself).
Conclusively, the X chromosome is involved in the sex determination of Drosophila but it depends on the number of autosomal pairs versus the number of X chromosomes. Non-disjunction during cell division is a reason why the different combinations of X/A originated.
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 the female is heterogametic (Hetero means two different types 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 the 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 unfertilized eggs and have haploid numbers of a chromosome. In the honey bees, haploid males are termed as a drone, while queen bees 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.
Environmental factors and sex determination
Yet another type of sex determination mechanism in which the 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 females 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 turtles 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 the female worm. A single isolated egg is released into the water and becomes a female, if it is attached to a 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 depends 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 of what to develop, a male or a female.
Besides the involvement of chromosomes in sex determination, genes play a crucial role in the same. SRY (sex determining region on the 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.
Additionally, hormones play a crucial for human sex determination. FSH, LH and estrogen are periodically released differently in males and females. 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.