Sex chromosomes: Genetic Determination of Biological Females and Males

Widya Mulyasasmita, a geneticist from Stanford University, says that every living organism has a ‘recipe book’ which gives instruction on how to build it. In humans, this recipe book is split into 46 chapters, known as chromosomes. In each of these chapters, there are recipes for eye colour, how to make fingers, and how to build a brain; each of these chapters has a near identical partner chapter, one where the content is inherited from your father and the other from your mother. The important chapters, when it comes to determining whether you will be biologically female or male, are chapters 45 and 46, known as the sex chromosomes.

There are two types of sex chromosomes: X and Y. Biological females have two X chromosomes (XX), and biological males have one X chromosome and one Y chromosome (XY). As the mother can only pass on an X chromosome, it is therefore the father who determines the child’s sex, as the dad can either pass on an X chromosome (which would make the resultant child XX: biologically female) or pass on the Y chromosome (which would make the offspring XY: Biologically male). Biological males need to inherit an X chromosome, because the Y chromosome is much smaller than the X chromosome, and therefore doesn’t hold all the genes required to live. Ultimately, biological males need the X chromosome to survive, whilst the Y chromosome determines their sex.

The size of the Y chromosome also explains why biological males are more prone to sex-linked disorders like red/green colour blindness. Let’s go back to the ‘recipe book’ analogy of chromosomes. As mentioned, each chapter has a near identical partner; by this, I mean that whilst both chapters have the recipe for a particular trait, for example, for eye colour, one of the chapters may have the recipe for particular eye colour (e.g. blue), and the other chapter has a recipe for a different eye colour (e.g. brown). So which recipe gets used? Well, in a lot of traits, we have something called recessive and dominant genes. In the case of the recipe book, it’s like the recipe for blue eyes has been written in size 20 bold and underlined (dominant), whereas the recipe for brown eyes has been written in size 8 (recessive). In this case, the recipe for blue eyes is much easier to read, so will be the one that’s used, resulting in that child having blue eyes. This child will only have brown eyes if both recipes were written in size 8, because the ease of reading is the same for both, and there is no huge, bold, underlined writing to distract.

As the Y chromosome is much smaller than the X chromosome, for the chapters which determine biological sex, the Y chapter has a lot fewer recipes in it compared to the X chapter. Because of this, some of the recipes in the X chapter have no matching recipe in the Y chapter. This is important, because if the X chapter has a recipe for a recessive disorder, it will still be made because it has no partner recipe to cancel it out, meaning that biological males are more likely to suffer from recessive sex-linked disorders, compared to biological females.

The investigation of differences in biological females and males have been studied for centuries. What began as noting the physical differences has evolved into investigating differences at the cellular level, including genes. Specifically, scientists have found that the X chromosome is responsible for several bodily processes, such as ovary development in biological females, coloured vision, and mental abilities, whereas the Y chromosome is primarily responsible for testis development. The investigation of sex differences has shed light on a wide range of phenomena, including sex-linked disorders, life longevity, mental health and more, making sex-linked similarities and differences a fascinating and vast area of science.

References

https://pubmed.ncbi.nlm.nih.gov/30294719/

https://www.dailymail.co.uk/sciencetech/article-4475252/There-6-500-genetic-differences-men-women.html

https://www.vedantu.com/biology/sex-determination

https://colors-newyork.com/why-are-chromosomes-arranged-in-pairs/

Disclaimer

The information in this blog is for information and entertainment purposes only. I am not a medical professional, so I have never and will never give medical advice in this blog. You should always speak to a healthcare professional about your unique health needs. My opinions are entirely my own and do not reflect the organisations or people I work for. I only discuss published literature in this blog which are referenced with links.

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