Demystifying trans-sexuality and homosexuality

This post is a collection of questions arising from my curiosity about trans-sexuality and homosexuality. Some of the answers found are listed along with references. Sometimes the content is simply a summary or a quote from my interactions with people, who may or may not be actively working on these fields. Although not meant to be an article written in the tone of peer-reviewed scientific journals, the scientific spirit of questioning and answering in an objective manner is maintained, and this post strictly does not attempt to discuss social/legal/political/personal issues related to these topics. Some of the references are semi-informal instead of being papers published in scientific journals. Discussions to improve the content and/or answer the unanswered questions are strongly encouraged in the comments below, but the scientific spirit is to be maintained and due references will be appreciated. Feel free to contact me if you want the conversation to be personal instead of public and/or are not able to comment. This blog is dynamic, i.e. it will be updated from time-to-time as better answers and/or references are found, or more questions are raised from my interactions with people online/offline.

Disclaimer: I cannot stress enough that this blog is not meant to discuss any social/legal/political/personal issues pertaining to these topics, but is purely meant to satisfy scientific curiosity and demystify the mysteries.

For people on Quora: You may be wondering why I am not doing this on Quora. It is simply because the website is turning me off recently, and I want this to be a permanent blog, even if I quit Quora in the future.

Question 1:

If the pairing of one sex chromosome from each parent determines the gender of the off-spring in humans, how come there are people who have traits from both genders? Specifically, the pairing can either be X/Y or X/X (egg from mother always has X but sperm from father can have either X or Y). Then, the sex determination is a yes/no question (is the baby female/male?), and there doesn’t seem to be a way out to have a mixture of traits.


Trans-sexuality is sometimes acquired during birth, because individuals are born with 47 chromosomes rather than 46, due to the presence of both the sex chromosomes (from the father, both X & Y) in the winning sperm, see an informal reference. In a normal child birth, both the reproductive organs, the penis and the vagina, grow simultaneously till roughly the 35th week inside the mother’s womb, after which the symmetry is broken by the sex chromosome pair (XX/XY) in the DNA. There are signatures of these growths in both the adult male (the scrotum on the testis is stitched in the middle, it started out as the vagina) and the adult female (the clitoris above the vagina). For trans-gendered individuals, both keep growing.

Trans-sexuality, however, is not determined at birth in all cases (Q: what is the rough ratio?). In fact, the age at which a person appreciates trans-sexuality can vary between 4 to 23. The likely reasons are:

  • Hormonal imbalances caused by one of the other 22 pairs of genes (called “autosomes” as opposed to “sex chromosomes”), at any age from infancy to full development of organs, which is around 21 years for an average human.
  • During the development of a baby inside the mother’s womb, wrong hormones may be secreted by the mother which affects the development of the brain of the child. The question is still open whether such changes can give rise to trans-sexuality in the baby, but indications from recent studies are in the direction that there are subtle differences in the male and female brains, and these pre-natal (i.e. before birth) growth differences caused by the mother’s hormones are enough to cause trans-sexuality. For example, even if the off-spring develops male sex organs and has regular hormonal discharges during its growth outside the womb, the subtle differences in the brain can lead the person to feel like a female trapped inside a male body. For details, one can read the following:
    1. (“FtM” means Female to Male) Our results show that the white matter microstructure pattern in untreated FtM transsexuals is closer to the pattern of subjects who share their gender identity (males) than those who share their biological sex (females). Our results provide evidence for an inherent difference in the brain structure of FtM transsexuals.”
    2. (“Perinatal” means roughly during the birth, or a few days before.) “Gender-dependent differentiation of the brain has been detected at every level of organization — morphological, neurochemical, and functional — and has been shown to be primarily controlled by sex differences in gonadal steroid hormone levels during perinatal development.”
    3. Transsexual differences caught on brain scan.”


Question 2:

How is it possible that all different organisms have different numbers (of pairs) of chromosomes, there being so many species?
It is counter-intuitive to think that the space of the number of chromosomes has a one-to-one to mapping with the space of the number of species: if orangutan, homo erectus and homo sapiens were so similar, and the change along the evolutionary track was a continuous rather than a discrete one, then it is hard to believe that the number of chromosomes is different for each of these species. If that was the case, one could tell the difference between the different species by counting the number of chromosomes (assuming that is possible via palaeontology).


The number of pairs itself does not necessarily separate the species. There are details related to the types of chromosomes and their functionalities that make different species different. For example, guppy (poecilia reticulata) are a type of fish that share the same number of chromosomes as humans (46). Hence, only counting number of pairs of chromosomes is not enough. Mutations at the level of the DNA (which make up the chromosomes) and its structure can distinguish between different species, and the change is indeed continuous.

Question 3:

What makes some individuals homosexual? We know that there is a tendency of animals to fall in love or stay committed, which are basically driven by the urge to reproduce and keep the offspring safe. What, if any, such ulterior motive drives people to be attracted to people of the same sex?

Partial answer:

Quoting a friend from email conversation: “There is nothing wrong in having a range of sexualities. Just like humans have heights ranging from 4 feet to 7 feet, the sexual tendencies of a man can range from violent attraction to beautiful women, to gentle liking for women and tolerating men, to liking towards men, to sexually getting attracted to men.” Summary: The scale of sexual attraction of individuals towards others of the same gender to very heavily for those of the opposite gender, is a continuous one.

Rephrasing my question:

What exactly sets the continuity, if so, on this scale? The answer could be pH level of a certain substance in some component of the DNA/RNA/whatever or the amount by which a certain hormone is secreted (very uneducated guesses!). If that is indeed so, then is it dependent on the levels of this determining factor at a certain age (if not then the sexual orientation of a certain individual can change over time) or is it determined during birth/puberty/certain-age once and for all? On the other hand, it could be something discrete: for example, the number of certain protein molecules in the DNA: if the number lies within a range, people are “straight”, whereas if not, they are homosexuals.

Satisfactory answers have not been found.


Question 4:

Is homosexuality limited only to humans? If so, why? Is it then an evolutionary trend? If that is the case, the question of homosexuals surviving in the long run seems to be nil, by natural selection.

Answer attempts / opinions :

  • No, homosexuality is common across species. For example, see Wikipedia article containing further references. In fact, that homosexuality is not uncommon in the animal world and hence not “unnatural”, was cited as one of the reasons for making it legal in certain nations/states across the world.
  • Summarizing a conversation with a student of Biology: Current research is not being able to answer much simpler questions like — What exactly causes sexuality in zebra-fishes? (The reason zebra-fishes are interesting is that they are in certain ways similar to humans, and Biologists hope to answer the questions of human nature through the study of zebra-fishes.) No study until now has been able to identify any sex chromosome in them, but they certainly have sexual reproduction. If only the autosomes define the gender, then at what stage does the sex get determined? Currently, the male/female ratio in a zebra fish community seems to strongly respond to parameters like temperature, food, etc. However, patterns, if any, seem to be eluding scientists.
  • Quoting a friend from email conversation: “Evolution may be moving in a totally different direction! Instead of an individual fighting for the survival of its genes, the fight could be the survival of the society as a whole. Instead of having a man fighting to death to protect his genes making a deal with a woman to take care of their off-springs, we may be moving towards a collective responsibility. Let there be two societies. Society A kills all homosexuals, every individual fights to protect his/her genes. Society B, instead, develops a collective trend. Instead of every individual fighting for his/her survival, every individual fights for the collective survival. Suppose homosexuals are endowed with empathy and tendency to take care of children, invalids and old, they will be valuable members of this society. There could be a situation where society B survives and that society rewards/encourages homosexuality to the extent of having a good balance in the society for its survival as a whole. In such a case, homosexuality will survive for a long time.” Summary: Whatever be the reason behind homosexuality, the trait can survive by the survival of that community which encourages this trait over that which doesn’t. Thus, even though individuals who are homosexuals do not reproduce and pass on their genes, the trait survives.

Satisfactory answers have not been found.

This blog is dynamic. It will be updated as answers (to the unanswered questions) and/or references (to statements already made) are found, or more questions are raised.


4 thoughts on “Demystifying trans-sexuality and homosexuality

  1. This is really profound, and a clever way of addressing the LGBT issues, Debdutta. I’d never thought about the aforementioned in a biological manner and after reading this, it all makes (near) perfect sense. I already had a fair idea of how these things work but now having understood them wonderfully in layman’s terms, I’d say your article makes for an interesting read. Of course, you already know I am bisexual (halfway to homo?) so it really delights me that this sexuality is not being looked down upon (especially in a country like ours), and heterosexuals, like yourself, are actually trying to comprehend it better! On a side note, I doubt that homosexuality is a ‘trait’ that can be passed on, it’s very much a psychological thing than it is a biological one; but that’s just my gut feeling.

    Liked by 1 person

    1. May-hek! Thanks for the appreciation. 🙂

      Just clarifying, I am in no way trying to address LGBT issues, that’s not the point at all. This is coming from curiosity, which I have seen in many around me, specially while growing up, but mostly addressed in a hush-hush tone and satiated by misinformation. If anything, I am trying to address ignorance and/or misconceptions, to bring out the science behind it.

      Also, about homosexuality being a trait: Of course it is a trait, if you look at it from the point of view of evolution. Individuals are devised by genes, they function on hormones: there is a cause of homosexuality. The fact that homosexual individuals are found across generations, is equivalent to saying that this is a trait that is passing on (even if each homosexual individual is due to some mutation, that kind of mutation is itself a trait).


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