Welcome back to the blog! This post will begin to examine how one of the most significant defining traits of any animal – sex – is determined on a molecular level. Although it may seem better simply to observe physical features and/or behavioral characteristics of an animal in question, it is easier said than done because there can be some grey area and inconsistencies that may not be immediately apparent among different types of animals, especially for ones other than humans and avian species exhibiting explicit differences between sexes (also known as sexual dimorphism) in particular. After all, every organism on Earth shares the same exact genetic language (the ACGT of DNA) and has the same fundamental cellular processes (DNA replication/synthesis in addition to cell growth and division), so as long as the means are available, examining the genome of an animal is arguably more effective and reliable for sex identification in many cases.
Before taking a look at anything in particular, there is a pinch of basic, fundamental background terminology that should first be gone over. For the sake of visualization, the karyotype of an “average” (in this case – no detectable genetic mutations present that result in a chromosomal disorder) human male is provided to the left. A karyotype refers both to a depiction of an individual’s collection of chromosomes and the method by which the depiction can be obtained. The numbering of the chromosomes is generally dependent upon their relative size in comparison to the rest of the chromosomes. This karyotype is stained in such a way that it produces color for contrasting purposes. Additionally, most people know what a chromosome is – a structure that DNA forms when it is condensed for certain specific processes that are out of the scope of this post at the moment. Humans naturally have 46 total chromosomes, or 23 pairs. The final two chromosomes in humans are known as sex chromosomes (as opposed to the other 44 autosomal ones) and are of particular interest for this (and the next) post; as their name implies, together, they determine the biological sex of a person. The differentiation between male and female is significant, so it does make sense that there is a clear name dedicated to this pair.
This post only scratches the surface as not all of the intended content would fit, so this topic will be split into an overflow post following this one! As a preview, the next post will touch on the intriguing and complex sex-determination systems found in humans and other types of animals. Until next time, feel free to theorize about it all. Thanks for reading, and take care!
This was really interesting! I took AP biology in high school and we talked extensively about this. The biology is quite fascinating. I think it is so interesting that each species just knows it’s respective mating traits. The biodiversity that results from this helps differentiate each species as well. Genetics are extremely interesting to me, and I really enjoyed this!
I guess that I always took human genetics for granted. Your post, and especially the video you linked, really demonstrated the greater complexity that comes with sex-determination in other species. I also felt it was a nice review of some of the concepts related to chromosomes. One thing that the beginning of your post reminded me of, which I think a lot about at this time of year, is the color variation between male and female robins, since they have some distinguishing features in their plumage. This was an insightful post, and I look forward to reading the follow-up in the future! Good job!