Pox genes are a special type of what are called toolkit genes, and these toolkit genes are a small subset of genes that control an organism's development, also known as the genes we've been talking about this whole time. Right? Those genes important to development. So, to get a little more specific, a subset of toolkit genes is these homeotic genes, which are genes that control the development of specific anatomical structures. Hox genes, which come from "homeotic box," hence "Hox" for short, are a type of homeotic gene. They are highly conserved genes, meaning that they've been around for a long time through the course of evolution and they help control development along the anterior to posterior axis. They are activated after segments form. So you might remember those segment genes that we just mentioned on the previous page. After all that, they come towards the end of development and help determine the specific structures that will form in a segment.
Here we have a nice example: you can see this fruit fly, a very common model organism used in biology. You can observe how the fruit fly has been divided into different segments, represented by different colors here. Each segment is associated with a particular gene. These are all Hox genes and they lead to the development of the specific anatomical structures you see at those segments. Initially, like you saw on the previous page, this embryo just looks like a segmented blob, but through the activation of these Hox genes, specific anatomical structures will develop.
Development in general is a highly conserved process and is directly linked to evolution which is why when you look at the embryo formation of different animals, for example, we tend to all look the same in the beginning and then slowly branch out and become different. It's because the developmental process has been passed along through evolution. So even though, for instance, fish are quite different from us, during our development we actually have gills. Yes, you and I had gills at one point in our life when we were a fetus. We developed gills and then we lost them. This is because development is so heavily conserved and is directly linked to evolution, and famously, there were fish before there were people. So we continue to carry some of those traits to this day, but they only appear during our development.
Another facet of the conservation of the developmental process is that many animals use the same genes and chemical signals to govern body plan development. See, Hox genes are very important and highly conserved genes. One last interesting thing to note about development is that many of the same chemical signals are used repeatedly during the course of development, but depending on when and how they're used, they actually will elicit different effects. This is just another case in point of how everything in biology is conserved. Biology is not wasteful; rather, it takes things it already has and repurposes them to meet new needs. So, development is a highly conserved process with a direct link to evolution, and you can see here in this image how these developing embryos all look very similar. This is another reason we use organisms like chickens or sea urchins, for example, to learn more about our own human development because there is so much crossover between the development of a chick or even a sea urchin and a person. Alright, that's all I have for this video. I'll see you guys next time.