[MUSIC] [MUSIC] [MUSIC] Hello and welcome back. Today I'm going to start by discussing the genetic significance of sexual reproduction. Two biological parents, your mother and your father, each contributed half of their DNA to produce you, or any offspring. You've probably noticed, unless you're an identical twin, that you don't look exactly the same as your siblings or your parents. So what sexual reproduction does, is it gives rise to variation. With all that variation, we have successes and failures. Sexual reproduction's what allows us to have two sets of homologous chromosomes. One set again from your mother, and one set from your father. So you have 46 total chromosomes. I know another way of saying that would be 23 pairs of homologous chromosomes. Alleles are different versions of the same gene and when we look at your chromosomes you have multiple alleles for different genes. You have at least two alleles for every gene, an example would be you have one allele for blood type from your mother and another allele for blood type from your dad. We have two versions because, again, you have one from your mother and one from your father. And these genes are located at very specific locations on each chromosome, and those specific locations on the chromosome are referred to as loci. These loci, or the location of the genes on the chromosome, are important because DNA is associated with other sequences which control and regulate gene expression, such as promoters or enhancers. When we consider the different forms of the genes, you may ask yourself. Where did these alleles come from? Those alleles, many of them actually arose through mutation. Remember, alleles are different versions of the same gene. Eukaryotes like us have multiple alleles from many of our genes. I gave you the example of blood type. Well, look at the variation in humans. Take eye color for example. There are many alleles for eye color. Different alleles can be beneficial, they can be harmful, and they can be neutral. But when it comes to eye color there is no real advantage to having brown eyes, blue eyes, versus hazel eyes, or green eyes. That makes this allele mostly neutral. There are different versions of alleles but they don't seem to give any major advantage for natural selection. Even if you have a preference for eye color, would you choose a partner solely based on his or her eye color? So, some of the definitions I want to recap before moving on. Alleles, that's a variation or different version of a gene. Phenotype is the physical characteristic that you see when a gene is being expressed. The genotype is your genetic makeup. And to summarize again, we have 23 pairs of chromosomes. These chromosomes are numbered 1 through 22 and sex chromosomes named, X and Y. You have a pair of each chromosome, 1 through 22, 1 from your mom and 1 from your dad. And you have two sex chromosomes, you're either XX, which makes you female. Or XY which makes you male. And each matching pair of chromosomes are called homologous. Now two neutrons I want to introduce. Homozygous, not to be confused with homologous, and heterozygous. Homozygous means you have two identical alleles for a specific gene. And heterozygous means you have non-identical alleles or two different versions of the allele. So, we look at these different alleles, we can see that they interact with each other, and some of the alleles will act in a dominant fashion and others are recessive. Dominant means that, that allele is present. When it is present the phenotype would be observed, it masks or blocks the recessive allele, and a recessive allele is only seen when a dominant allele is not present. And a good way to discuss this is with the dominant disease called human polydactyly. In the case of human polydactyly, a person who has this gene can express extra fingers, but what we see is that there's incomplete penetrance and variable expressivity. Penetrance is if you see the phenotype when a person has a particular genotype. Up until now, we've assumed if you had a genotype, you'd express the phenotype, but in some cases, there can be incomplete penetrance. Some people have the genotype for polydactyly actually show normal hand with no extra fingers. Also, we see something called variable expressivity, which means that you may have penetrance of this gene, but it can be expressed to different degrees. Where some people may have a complete extra finger form on their hand others just have a partial finger. So there's a higher degree of expressivity in some people with the entire extra finger versus others. So the two terms again were penetrance and expressivity. Penetrance refers to whether or not you're expressing the genotype. Yes, you have the extra finger. Or no you don't. Variable expressivity is the degree to which you're expressing that specific genotype. So you can see why simply stating one gene will result in one specific phenotype is rare. Additionally, phenotypes can be influenced by environment. So let's think of one example. Blood type, for instance. When we look at your blood type, it's solely determine by your genes. So if you're A positive blood type, or B negative, or O negative, that's determined by your genes. But your environment really doesn't impact that blood type. But what about how much blood? How many red blood cells, or how much hemoglobin you're synthesizing? Well that's determined by your environment. Say you want to go on vacation to Machu Picchu up, it's in a high altitude, to see the Incan ruins. Well, your body needs to adapt. You're going to secrete hormones to tell your body to make more hemoglobin and more red blood cells. After a couple of days you'll have more red blood cells because you're adapting to the environment. So you can see in this example how a change in the environment influenced gene expression. So just one gene just control one phenotype. Well, we're going to back to one of the first examples we talked about, the sickle cell trait, and what we see in the case here is Pleiotropy. This is the impact that one gene can have over multiple characteristics. Mutation in the DNA sequences that caused a mutated hemoglobin to be produced changes the function and ultimately changed the shape of the erythrocyte of red blood cell. But there's actually multiple manifestations or phenotypes to do this gene mutation, where you can see physical weakness in people, anemia. Also you can see some protection from being infected by the parasite that causes Malaria. So one mutation or one gene actually causes multiple physical manifestations. There are also examples where multiple genes influence a single phenotype; this is called Polygenic. A great example of this would be skin color. When you look at your skin color, that's not simple a dominate or receptive one gene, one genotype interaction. There's actually over a dozen genes involved in determining one visible trait, which is your skin color. And the last type of gene interaction we're going to talk about is epistasis. And that's where one gene product masks the expression of another genome. So gene products can interact with each other, and you can get phenotypes that aren't necessarily what you would expect with one specific genotype. So, imagine a mouse with a mutation. And that mutation forms an eyeless mouse. So, this mouse is born with no eyes. Well, that's going to mask the gene for eye color. So, you have one phenotype in one product that has been influenced by the expression or absorbance of another physical trait. In an eyeless mouse you are not going to know what the genes were for eye color. In our next lecture I will discuss how hormones can effect gene expression. See you soon. [SOUND]
