X and Y – An Overview of Mammalian Genetics (Part 1)

Welcome back to the blog! In the previous post, there was a brief introduction and discussion regarding some of the molecular components within the general genome – using humans as an example. This post will continue to expand on the topic.

Since all of this post’s readers are human, it would not be a terrible idea to lay the foundation with the sex determination system operating within mammals – the XY system. This, of course, refers to the two sex chromosomes, or final pair of chromosomes in the set of 23 pairs in the genome. For those unaware, the X chromosome (named according to its shape when duplicated) is the larger one, two of which are present in biological females. In contrast, males only have one X (not to be confused with “ex”) with the other sex chromosome in the pair being the Y chromosome (refer to the previous blog post for an additional figure).

After reaching proper sexual maturity, an individual’s genome is split, or segregated, into exactly two halves that are non-identical to each other’s makeup but still equal in the number of chromosomes. This occurs during gametogenesis, or the production of gametes – the egg and sperm sex cells that combine through fertilization to form a zygote/fertilized egg. These cells are referred to as haploid cells as opposed to diploid cells due to the fact that they only have half (23 in this case) of the total number of chromosomes (46). In all mammals, one chromosome in any given pair of chromosomes is inherited from the mother and the other in the pair is inherited from the father, so genetically speaking, humans are technically a genetic combination of one half of each biological parent; after all, 23 + 23 = 46! Because females only hold X chromosomes, it is impossible for them to give anything else while a male can “give” either his X or Y chromosome to the child through a sperm cell, as illustrated in the punnett square to the left. Fun fact: this is why the male is actually the one to unintentionally determine the sex of a child. This is also why King Henry VIII of England, lacking this knowledge, blamed his wives for not bearing any sons. Nice going, Clown Henry, and thanks dad, I guess!

What’s more is that although rare, it is possible to have a viable trisomy disorder with sex chromosomes – viable indicating that the individual can live with the genetic disorder and trisomy indicating that there are two copies of a single chromosome, leading to three in the intended “pair” and a total of 47 chromosomes in the entire genome rather than 46. In this case, the following trisomy cases involving sex chromosomes could exist: XXX, XXY, and XYY. A set including YYY, even if it exists, will not give rise to viable offspring. Neither will YY. Notice how all viable individuals’ combinations must have at least one X chromosome! Thanks, mom! Often times, it only takes a small genetic change to observe a significant phenotypical (physical) change, as evidenced here! Lastly, although not the primary topic of this blog post, it is important to note that particular conditions known as sex-linked traits like colorblindness are influenced by such differences resulting from humans’ sex chromosomes.

In addition to what has been discussed here, there are many more biochemical mechanisms and “reproductive oddities” out there in the world. However, this blog post will not dive any deeper into this topic than it already has, at least for the moment. As always, thanks for reading, stay tuned, and take care until next time!