Studying the Genetics of Development - Online Tutor, Practice Problems & Exam Prep

Genetics of Development

Hi. In this video, we're going to be talking about studying the genetics of development. So, scientists use many different types of techniques to study development, and they generally ask four main questions when studying the genetics of development. The first question is, what genes control development? Assuming that genes do, which they do, as a lot of the times genes control development, we need to figure out which genes are important for proper development. And also, when in development are they expressed? Are they expressed when it's just a single cell? Are they expressed when it's 8 cells? Are they expressed when there are thousands of cells? There are many different stages of development, and we need to know when those genes are expressed. There's also where, so when we ask about where, we can talk about it in two different ways. We can talk about in a single cell. Is it located in a certain region in a single cell? Or, when discussing multicellular organisms, we can say, you know, in which cells is it expressed? So these types of when, what, where questions are really important. How are these genes regulated? This is a big one because much of development involves all the cells having the same genes. Right? So differences in development have to depend on how the genes are regulated, when certain genes are turned on, and when they're turned off. The major question that developmental scientists use to study these questions is to ask, what happens if these genes are defective? If we introduce a mutation, or if there's a mutation, what happens to development? Is something not developed correctly, and if so, what is it? To do this, scientists mainly use model organisms. These are organisms that have been studied for a very long time and are used to represent the genetics of development. Whatever answers we get from these organisms, we hope to be able to say that these exist in other organisms and even humans as well. Some examples of model organisms include yeast, plants, and a big one for genetics and development are fruit flies, which we've talked about a lot already. Here's a picture of those. There's also the worm C. Elegans, which is really important in development, because actually every single cell that this organism has can be followed. We know exactly what it will divide into and when it will divide. It's really interesting. And then, of course, mice. We always use mice. Model organisms are chosen because they grow easily, rapidly reproduce, and are genetically similar. So here's the fruit fly. Here's the worm, C. Elegans. Like I said before, C. Elegans is really interesting because from the moment it's a single cell until it's a full grown adult organism, we know every single cell, what it will do, what it turns into, when in development it does that, and how that's all regulated in C. Elegans. So C. Elegans is a super important organism when studying development. Now, development begins with a zygote, and this is going to be just a fertilized egg. You know, we've talked about this before, females have eggs, males have sperm, at least for humans. And so, a fertilized egg is called a zygote. Zygotes have a certain characteristic, which is called totipotent. And what that means is that that zygote can develop into any cell type. So when that zygote divides, it can produce any cell type. It eventually produces brain cells, skin cells, kidney cells, any type of cells in the body. Right? Because it's the single cell, It starts out, and it can become anything. And so, the process through which a cell becomes something, like, becomes a brain cell or a kidney cell, is called determination. And this determination is the process through which a cell becomes committed to its fate. So what cell type it's gonna be? Is it going to survive or die, etcetera, etcetera. Anything that this cell is going to do, the process of which it becomes committed to that, so it says, yes, I'm gonna die, or no, I'm gonna become a kidney cell, or maybe I'm gonna become a nurse cell, or any of these cell types, it's called determination. Now there are two types, there's mosaic and regulative, and mosaic is what I talked about before with C. Elegans. So this is when every single cell in the organism has a predetermined fate. So you start out with a zygote, and that's going to divide, and every single time in every single organism under mosaic determination, this cell will become something, say a kidney cell, and this cell will become a brain cell. And this is a very generalized explanation of this, but this is say, and then these two divide and they become something, and these two divide and they become something, and so on and so forth throughout all the cells in the organism. Now, in every single one of them, these cell divisions will happen this way. These cells will become that, no matter the organism, no matter what's going on in the environment, it's committed. It is saying, if this cell is dividing, I'm becoming this and this. And if something goes wrong, and say one of the cells, it dies, then there's nothing to replace it. It's either it happens or it doesn't, and it happens in every organism, and there's no kind of room for mistakes. Regulative determination, on the other hand, is cells can regulate their fates according to the environment. So in this case, if you have a cell and they divide into a kidney and a brain, and let's say something happens to this cell and it dies. Well, when this cell divides, it can compensate for that brain cell. It can make an extra copy. It can turn one of its cell divisions into a brain cell. And kidney cells can actually turn into brain cells, but I'm just using this as an example. It's not determinate. If something messes up, that's okay, it can fix it. If the environment if something is screwy in the environment, it can regulate that development based on what's in the environment. So mosaic determination is very fixed, this is gonna happen, it's gonna happen in every organism, and nothing can fix it if something goes wrong. Regulative determination is much more, sort of flowy, you know, it says, well, this is what we want, this is ideal, but if something goes wrong, we can fix it, and it's not the end of the world. And so this regulative is actually what happens in humans. If some kind of cell division thing goes wrong, some other cells can compensate for it. Now, like I said before, development and changes in development really depends on the genes that are expressed. And so that is given a hypothesis called the Variable Gene Activity Hypothesis, and this states that determination, so that process of committing those cells to something, like cell death or cell type, is controlled through activating or inactivating genes. So that says that genes, whatever genes are expressed and whatever genes are repressed, is what's controlling that development. And so the reason that different cells become different things is because gene activation or inactivation occurs at different times and in different places in different cell types. So gene regulation is really what's controlling development. So we're going to talk about what these genes are, when they're expressed, how they're expressed, and how that controls development, in some of the later videos. But first, I want to make sure you understand this mosaic development and regulated development. So let's say we start off with two embryonic cells, so these cells, are both totipotent, they can turn into anything, and one cell is destroyed. So now you only have one cell. In mosaic development, that's going to result in abnormal development because you've lost half of the cells that you started with, and that's going to mean that half of the cells that that organism needs aren't going to be replaced, and so that's going to result in severely abnormal development. Whereas in regulated development, this has a high potential of just being normal. This cell will divide or other cells will take its place, and development will most likely be, unharmed because, in regulated development, those cells can actually make up for that missing cell. So that is the difference between mosaic and regulated. Let's now, move on.

During early development, human cells determine their differentiated fates based on the environment. What term describes this phenomenon?

• A. Totipotent
• B. Mosaic determination
• C. Regulative determination
• D. Variable gene hypothesis

The variable gene activity hypothesis states that cell differentiation and determination is controlled through differences in gene activation or inactivation.

• A. True
• B. False

If an embryonic cell is damaged in an organism that undergoes mosaic determination, will the offspring produced develop normally or abnormally?

• A. Normal development
• B. Abnormal development