Sex Determination: Role of X and Y Chromosomes

Biology is the sub-branch of science or more precisely natural science. It is the study of life. Sex determination is an essential part of reproduction that differentiate most precisely between male and female offsprings at the time of the fusion of male and female gametes, known as Fecundation, Fertilization, or Reproduction. In simple terms, it means the determination of the development of sex organs into testicular or ovarian type. An integral part of this system is that it characterizes the sexual parts of a living body. It can be determined by the presence of chromosomes. Environmental factors, like cool or warm temperature, have a greater impact on sex determination. Such an impact can be greatly seen in species like crocodiles and turtles. 

Read Also: How do Organisms Reproduce?

This system affects the growth of living beings as it supervises cellular assimilation and produces hormones that activate sexual organs. It is then that they have an identity and one can know their gender. If a hormone imbalance occurs during fertilization due to more or less number of hormones, then the offspring might show a mixed growth, which will mean possessing characteristics of one sex but behaving the exact opposite, in simpler terms, the offsprings exhibits other gender's behavior. In a few cases, it involves only one living species called hermaphrodites, which is the species that has both testicular and ovarian i.e. male and female reproductive organs, and has the ability to produce both gametes which later fuse to produce offspring. For Example - Earthworms, slugs, etc are these types of species. 

The living beings possessing different sex organs are characterized as male and female. These species, opposite to hermaphrodites, are called gonochoric species and in such species, few may have both male and female characteristics, such beings are known as Intersex. In some species due to the process of parthenogenesis, a form of asexual reproduction which doesn't involve fusion of male and female gametes, females produce offsprings without any act of Reproduction. 

There is some kind of species which do not have secured sex organs and change their sex organs based on the biological situation throughout their life cycles For example Shrimps and Sea horses.

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Discovery

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Sex determination was first discovered in a mealworm, which belongs to the family of Tenebrionidae beetles commonly known as Darkling beetle, in the year 1903 by Nettie Maria Stevens, an American geneticist. She is greatly known for XY sex-determination system. Stevens observed male mealworms possess two different sized chromosomes, one large and another small whereas a female mealworm has two large chromosomes. When a male gamete of a large chromosome fuses with a female gamete, a fertilized egg or zygote is the female offspring. Similarly when the small chromosome of the male gamete fuses with the female gamete then the zygote formed is the male offspring. These large and small chromosomes are commonly known as X and Y chromosomes. Usually, male sex organs have a heterogeneous type of XY chromosome that is a pair of X (larger chromosome) and Y (smaller chromosome) whereas female sex organs have homogenous type chromosomes that is a pair of two larger chromosomes that are XX.

This "XY" sex-determination system is used to determine sex organs in mammals. The zygote

formed will be female offspring.

Read More About Rules for the Inheritance of Traits - Mendel's Contributions

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Origin

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Prior to Nettie Stevens’ discovery, Aristotle, who was honored as the Father of Biology for his contribution, put forward that heat produced by males during reproduction is responsible for sex determination i.e. intensity of males during sexual intercourse depends on sex determination. According to him, when male heat intensity is higher than female coldness then the zygote formed will be male offspring similarly when female coldness overcomes male's heat intensity or male heat intensity is low then the zygote formed will be female offspring. His theories were popular until the 20th century when sex chromosomes were discovered in mealworm by Nettie Stevens in 1903. 

One thing to keep in mind is that in any species male is the determinant of sex in the offsprings that means the Y chromosome is responsible for sex determination. 

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Sex determination in humans 

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In animals and human beings, sex determination is usually escorted by hormonal differences, the combination of different chromosomes during reproduction. In human beings, males and females generally possess different gene variants and sex determination is hereditary. Usually, in human beings, the father(male chromosome) is responsible for sex determination. 

We already know that the male partner (Father) is responsible for the child's sex (gender). Sex differences include the growth of a human's inner and outer sex organs and genitalia which plays a pivotal role in Sex determination into masculinization and feminization that is by the growth of Sertoli cells in male child and granulosa cells in a female child. As the zygote cell matures into adulthood certain things take place in sexual differentiation, that is the growth of different sex hormones, genes, long DNA molecules (chromosomes), reproductive glands. The initial stages of sex differentiation in humans are somewhat similar to those of any other mammalian species biological processes. In humans, males have a pair of XY chromosomes whereas females have a pair of XX chromosomes and the Y chromosome of the male parent arouses testicular development like testis formation whereas X chromosomes of male parent arouse ovarian development and forms ovaries. Hence it is obvious that Father is the sex determinant.