Genetic Inheritance of Human Sexuality
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Genetic Inheritance of Human Sexuality

Hi! Welcome to BIO 301. My name is Cynthia Anderson Sanchez, and I will be I will be your instructor for this course. Today we’re going to start by talking a little bit about genetics and what that has to do with your gender. So, first let’s talk a little bit about universal questions that we probably all have. For example, “Why do children tend to look a little bit like their mom and a little bit like their dad?’ Also, some deeper questions like, “What is DNA?”. We hear a lot about it we hear a lot about chromosomes and a lot about genes. When you to look very closely at yourself, you’ll notice that you are made up of trillions of cells. The latest estimate by National Geographic actually estimates that we have about 40 trillion cells in our bodies. So, that’s pretty impressive. If you were able to uncoil all of your DNA and just stretch it out as far as you could in all of your cells, it actually is long enough to go all the way to Pluto and back! That’s 10 billion miles, so that’s a lot! So, let’s find out what this DNA is. DNA stands for deoxyribonucleic acid. That’s why most people just stick with calling it DNA! Each cell in your body contains DNA and it lives in the central part of your cells called the nucleus. What DNA does, is it’s basically an instruction manual for the cell. It carries genetic information that’s going to tell the cell information on what type of cell it is to become, what functions the cell will have within the body and also what functions will take place inside of that cell. In textbooks sometimes you’ll see a picture of DNA that looks something like this top picture here. This is the DNA double helix and this shows how DNA would look if you stretched it out as much as possible. In this long, super-thin form, it’s actually not visible even under a microscope. This form looks somewhat similar to a telephone cord. If any of you has ever had the pleasure, or should I say “torture”, of having to use one of these corded, wired phones, the phone cord will usually end up tangled into what is called a supercoil. We see the super coil formed here. Now this is not just a fun fact. The reason that I’m telling you this is the DNA that is within our bodies will actually supercoil in on itself during a particular part of the cell cycle and it’s going to end up forming these nice ‘X’ shapes that we see here. Now we always have chromosomes in our body but the chromosomes are a lot easier to visualize when they’re in this condensed to form. This is what your chromosomes actually look like. Each of your cells has 46 chromosomes total. You have one set of 23 chromosomes from your mother and one set of 23 chromosomes from your father. This is an actual photograph of human chromosomes taken using an electron microscope. This particular photo was taken from SCIBLOGS and I have the link here if you would like to see this and other pictures as well as more information on chromosomes. Now we’ll just take a moment to look at a short youtube video to show you how the DNA gets packaged into these chromosomes. Enjoy. What we’re seeing here is an illustration of a single cell and we’re slowly traveling towards its nucleus; the hub of genetic material. Inside you can see chromosomes; the familiar x-shaped blob of DNA. Chromosomes don’t really exist like this in most cells in fact this is only a fleeting structure. Most of the time chromosomes look like this. These are for typical cell nuclei in which we’ve stained all of the DNA with the blue dye and one of the chromosomes in green. You can see it looks nothing like the familiar x-shape cartoon structure of chromosome. Using a new technique that we’ve developed, we’ve created a much more accurate picture of how the DNA folds within the chromosome. The DNA is represented here by the single strand that weaves through the 3d structure. This is how DNA exist within a chromosome in its usual state; the state in which all the important functions of the genome are operating and controlled. 3D models of chromosomes allow us to map specific genes and other important genomic features under the structures. Putting DNA into its proper context like this is important because the folding of DNA and the positions of genes on a chromosome, contribute to genome control. Great! So, I hope that you enjoyed that short clip. Let’s talk a little bit more about our chromosomes and what they look like when they are in their condensed state. Here we are looking at a male human karyotype. Remember that there are 46 chromosomes total in your cells but you have 23 from one parent and 23 from the other parent. So, what we do in a karyotype, is we will pair up the number one chromosome with mom with the number one chromosome from dad and then we do the same thing with a chromosome to chromosome 3 chromosome for all the way up to chromosome number 23. Now let’s take a closer look at chromosome number 23. in this particular karyotype, which is a male karyotype, we see that the number 23 chromosomes look different from one another between the male and female versus everything else. All the other chromosomes look similar as they’re paired up. Let’s look at why that is. The 23rd chromosome has a special function. It does other things too, but one of the things that it does is it determines what gender you are going to be. For this reason to 23rd chromosomes are also called sex chromosomes. A female is ‘XX’. What this means is, for the 23rd chromosome, the female would have inherited to sex chromosomes that look like an ‘X’, as you see here. One X chromosome must have come from her mother and the other X chromosome came from her father. So, females are ‘XX’. A male will have inherited two sex chromosomes that look different from one another. One chromosome appears like an ‘X’ and the other appears more like a ‘Y’ shape. For this reason, we consider males ‘XY’ for their 23rd chromosomes. A male inherits the X chromosome from his mother and the Y chromosome from his father. Let’s look at this in a little bit more detail . Let’s think about which parent then is the one that is dictating what sex (gender) the offspring will be. We know that you get half from each, but your mom was female so since she was female she had XX for her 23rd chromosomes. So that means that you MUST have received one X from your mother. Your father, on the other hand, you could have received his X chromosome or his Y chromosome, depending on which spermatozoon fertilized the egg that made you. An egg must always carry an X chromosome for the sex chromosome. The male would have had a 50/50 chance. every baby has a 50/50 chance of becoming male or female. Keep in mind that your chromosomes do a whole lot more besides just determining your gender. Today we are just going to focus on that alone. When we talk about DNA too, a lot of times we talk about genetics and genetic traits that are inherited from your parents. Chromosomes carry genes. What we mean by a ‘gene’ is a gene is a segment of DNA within the chromosome that carries out a very specific function. So we have different genes for different traits. For example you may have inherited the genes for red hair freckles and green eyes like this lovely stick figure here. Now when the egg and sperm join together to create the fertilized egg that we call a ZYGOTE, the result is that you end up with 46 chromosomes; 23 from each parent. But, in order for that to happen, that means that the egg and the sperm have to have only HALF the number of chromosomes as all of your other cells in your body. Let’s take a moment to see why that is. When we talk about our sex cells, they have a specific name so that we don’t always have to say “sex cells” or “eggs and sperm”. We can also say GAMETES. So when we say “gametes”, that means all of your sex cells; the sperm and the eggs. Everything (all cells) that is not a gamete is considered a ‘body’ cell, otherwise known as a SOMATIC CELL. The word ‘somatic’ actually comes from the greek word for “body”, so that makes sense as to why it would be called the somatic cells. “Gametes”, on the other hand, comes from the greek word for “to marry”, so that kind of makes sense as well, because when you are joining the egg and sperm together you are able to get a whole new organism. All of your ‘body’ (somatic) cells divide, no matter what type of cell they are. So your somatic cells or your ‘body’ cells are always dividing. You have to do this in order to grow, to repair wounds, to replace old worn-out cells… remember you started as just a zygote and somehow you went from thatsi zygote to 40 trillion cells! That’s pretty impressive. Now somatic cells or body cells divide and when they divide they do it by a process that we call mitosis. And you can see here that one parent cell will end up having two daughter cells. When we talk about mitosis we start off with a parent cell that just like all your other cells has two sets of 23 chromosomes, 46 total. And what happens here is the genetic material is going to double. When it doubles, it’s then going to line up line up at the center of the cell and then these spindles are going to pull the genetic information apart. When they are pulled apart like that they’re called sister chromatids. So you have one sister chromatid going to one side and one sister chromatid going to the other side. Eventually, that cell is going to undergo cytokinesis where it’s going to completely separate into two individual daughter cells. The two daughter cells are genetically identical to the parent and they’re genetically identical to one another. Now your sex cells, or gametes, undergo a different process. This process is called meiosis. You can think of it loosely as two consecutive rounds of mitosis. So let’s learn a little bit about this process. You start off with a normal cell that has 46 chromosomes; 23 from mom and 23 from dad. We call this the parent cell or sometimes you’ll hear it called the ‘progenitor’ cell. What happens here is you also get this doubling of genetic information. You end up with new with a chromosome number that iss going to be the same. You’ll still have the 46 but they are going to double so that they create this ‘X’ shape. Now once they’ve created this X shape with double the amount of genetic material, they’re then going to “pair up”, kind of like we saw what we saw with the karyotype. So you’re going to have chromosome number one from mom and chromosome number one from dad join together and they’re going to basically lay on top of each other like this (showing one hand overlaying the other hand). I’m showing them side-by-side here to try to get you not to be too confused. But they lay on top of each other like this (showing one hand overlaying the other) and then they’re going to shuffle their genetic material. You can think of it as shuffling a card deck. so at this point the individual chromosomes are no longer going to look the same. Chromosome one from mom and chromosome one from dad are now all mixed up where each of those chromosomes are going to have different genes from different parents. At this point, the homologous chromosomes (that’s what we call the number one for mom and number one from dad so they’re similar) now they’re going to line up down the middle of the cell. This time you’re going to have each side of the cell only have a spindle that’s attached to 23 individual chromosomes so when it is pulled apart you’re going to have 23 chromosomes that looks like an X on in one cell and 23 chromosomes that look like an X in the other cell. We see here then that that cell splits into two separate cells. You’ll notice here, as well, that they’re still in that x-shape, so they still have double the amount of genetic material. What they’re going to do again is they’re going to take those 23 chromosomes which are now a mixture between each of the parents and they’re going to line those up down the middle and then the sister chromatids of the chromosome are going to be separated using the spindle; one chromatid going to one side and one chromatid going to the other side, to eventually pinch off and create a total of four (daughter) cells at the end of this process. So, what you need to know for this process is one parent cell or one progenitor cell is going to end up creating four daughter cells. Each of these daughter cells are genetically different from one another and they are genetically different from the progenitor cell. This is because each of those chromosomes have been mixed in that crossing-over event where the DNA is then shuffled. So the end result is you get a gamete (an egg or sperm cell) that has 23 chromosomes. But, those 23 chromosomes are a mixture from the mom and dad from the owner of the egg and the host of the spermatozoa.

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