Genetics Foundations and Mendel's Laws

Welcome back everyone. Welcome back everyone. In this video we're going to talk about genetics. In this video we're going to talk about genetics. More specifically, More specifically, we're going to discuss Mendel's three laws. we're going to discuss Mendel's three laws. This will be the first of three videos about genetics. This will be the first of three videos about genetics. As for the previous videos, As for the previous videos, I recommend that you watch lesson number five. I recommend that you watch lesson number 5, where we talk about the DNA nucleus. where we talk about the DNA nucleus. It will be very important for understanding Mendelian genetics, It will be very important for understanding Mendelian genetics, as will the video in lesson 10, as will the video in lesson 10, which deals with meiosis. which deals with meiosis. This is also essential for understanding how Mendel's laws of genetics work. This is also essential for understanding how Mendel's laws of genetics work. So, So, first of all, first of all, we must obviously start by asking ourselves who Mendel was. we must obviously start by asking ourselves who Mendel was. Mendel is recognized by science as the father of genetics. Mendel is recognized by science as the father of genetics. He died in 1822 and died in 1894. He died in 1822 and died in 1894. He was a monk and naturalist and conducted genetic experiments in a garden at the Burnet Monastery in what is now the Czech Republic. He was a monk and naturalist and conducted genetic experiments in a garden at the Burnet Monastery in what is now the Czech Republic. So Mendel was a monk, So Mendel was a monk, he lived in 1800, he lived in 1800. so we need to start right away to get an idea of what Mendel already knew about biology. So we need to start right away to get an idea of what Mendel already knew about biology. Mendel did not yet know about the existence of DNA. Mendel did not yet know about the existence of DNA. At that time, At that time, the existence of this molecule that holds the information that will then be translated into the expression of particular proteins that will then give the characteristics to human beings and to any living being was not yet known. The existence of this molecule that holds the information that will then be translated into the expression of particular proteins that will then give the characteristics to human beings and to any living being was not yet known. Therefore, Therefore, Mendel's studies used terminology that is not current and even if in a certain sense they predicted the existence of DNA, Mendel's studies used terminology that is not current and even if in a certain sense they predicted the existence of DNA, they did not take it into consideration because this molecule was not yet known. they did not take it into consideration because this molecule was not yet known. And starting from what Mendel did not yet know, And starting from what Mendel did not yet know, we are going to give some definitions that are instead very important for us. we are going to give some definitions that are instead very important for us. Obviously, Obviously, as already mentioned, as already mentioned, we must keep in mind that Mendel did not know about it, we must keep in mind that Mendel did not know about it, but today, but today, especially after 1900, especially after 1900, when DNA, when DNA, this molecule that holds genetic information, this molecule that holds genetic information, was discovered, was discovered, we know that biological mechanisms work in this way. we know that biological mechanisms work in this way. We have information written in DNA. We have information written in DNA. As you can see from the image, As you can see from the image, A chromosome is represented, a chromosome is represented, which is the form in which the DNA condenses when the cell wants to replicate. which is the form in which the DNA condenses when the cell wants to replicate. We must also remember, We must also remember, as already mentioned in lesson number 5, as already mentioned in lesson number 5, that during the life of the cell, that during the life of the cell. DNA is found in a scattered form in the nucleus of a eukaryotic cell, DNA is found in a scattered form in the nucleus of a eukaryotic cell, in a form called chromatin. in a form called chromatin. However, However, for convenience, for convenience, it is always represented in the form of a chromosome, it is always represented in the form of a chromosome, because then, because then, when the cell reproduces, when the cell reproduces, it condenses the DNA in the form of chromosomes. it condenses the DNA in the form of chromosomes. So we have a portion of DNA that holds the information, So we have a portion of DNA that holds the information, that is, that is, on that portion of DNA it is written how a certain characteristic must be made. on that portion of DNA it is written how a certain characteristic must be made. And so we see how the stretch of DNA present on the chromosome in which there is the information that will then produce a particular protein is called a gene. And so we see how the stretch of DNA present on the chromosome in which there is the information that will then produce a particular protein is called a gene. So the gene is that stretch of DNA that will then code for a particular protein. So the gene is that stretch of DNA that will then code for a particular protein. This protein will then obviously determine a particular characteristic. This protein will then obviously determine a particular characteristic. Let's take an example. Let's take an example. Here, Here, I will always use examples of skin color, I will always use examples of skin color, eye color, eye color, hair color, hair color, But let's keep in mind that as far as human beings are concerned these are slightly simplistic examples. but let's keep in mind that as far as human beings are concerned these are slightly simplistic examples. This is because in human beings these characteristics are described by multiple genes. This is because in human beings these characteristics are described by multiple genes. But let's not worry about that now, But let's not worry about that now, let's simplify the whole concept a lot and say that on this gene, let's simplify the whole concept a lot and and say that on this gene, for example, for example, On this stretch of DNA, on this stretch of DNA, it could be written what color our eyes are. it could be written what color our eyes are. For example, For example, it says, it says, eyes must be brown. eyes must be brown. And so this stretch of DNA is called a gene and that is, And so this stretch of DNA is called a gene and that is on this gene it is written that our eyes must be brown. On this gene it is written that our eyes must be brown. This gene will then be translated, This gene will then be translated, thanks to the ribosomes, thanks to the ribosomes, into a protein, into a protein, in this case melanin, in this case melanin, which will make our character, which will make our character, that is, that is, brown eyes, brown eyes, emerge. emerge. However, However, we must remember that in our genetic heritage we have a copy of each chromosome. we must remember that in our genetic heritage we have a copy of each chromosome. This is because we possess homologous chromosomes, This is because we possess homologous chromosomes, that is, that is, we have a chromosome that comes from our father, we have a chromosome that comes from our father, a chromosome that comes from our mother, a chromosome that comes from our mother, and so, and so, always keeping in mind the previous example, always keeping in mind the previous example, we will have a gene that codes for eye color on the chromosome that we got from our father, we will have a gene that codes for eye color on the chromosome that we got from our father, and the same gene that codes for eye color, and the same gene that codes for eye color, more than anything else a gene that codes for the same trait, more than anything else a gene that codes for the same trait, but that comes from our mother. but that comes from our mother. Maybe on our mother's chromosome we have written that the eye color must be blue. Maybe on our mother's chromosome we have written that the eye color must be blue. So here we have a gene that comes from our father that tells us what the eye color should be, So here we have a gene that comes from our father that tells us what the eye color should be, a gene that comes from our mother that tells us the same thing. a gene that comes from our mother that tells us the same thing. So we have a gene that codes for a trait that I have simply called trait A and we have another gene on the homologous chromosome that comes from the other parent, So we have a gene that codes for a trait that I have simply called trait A and we have another gene on the homologous chromosome that comes from the other parent, but that codes for the same trait, but that codes for the same trait, trait A. trait A. The two genes present on homologous chromosomes that code for the same trait are called alleles. The two genes present on homologous chromosomes that code for the same trait are called alleles. So what are alleles? So what are alleles? They are precisely the two genes that code for the same trait. They are precisely the two genes that code for the same trait. For example, For example, the two genes that come from our mother and father, the two genes that come from our mother and father, one that codes for eye color. one that codes for eye color. Very simply, Very simply, these are the alleles. these are the alleles. So we have the first allele present on the paternal chromosome and the second allele present on the maternal chromosome. So we have the first allele present on the paternal chromosome and the second allele present on the maternal chromosome. Having said that, Having said that, let's define some terms. let's define some terms. In terminology, In terminology, it is very important to know what a genotype is. it is very important to know what a genotype is. Let's remember that Mendel still didn't know what DNA and chromosomes were, Let's remember that Mendel still didn't know what DNA and chromosomes were, so he had no idea what a genotype was. so he had no idea what a genotype was. But we are already studying it because it will be very useful for understanding genetics. But we are already studying it because it will be very useful for understanding genetics. Obviously, Obviously, It is useless to go over all of Mendel's discoveries as if we were in 1800 because it is 2020 and therefore we know perfectly well how genetics works, It is useless to go over all of Mendel's discoveries as if we were in 1800 because it is 2020 and therefore we know perfectly well how genetics works, so it is better to study it with the terminology that we know today. so it is better to study it with the terminology that we know today. Obviously, Obviously, however, however, Mendel's laws, Mendel's laws, Mendel's three laws, Mendel's three laws, are still present today, are still present today, so much so that they are also the basis for modern genetics. so much so that they are also the basis for modern genetics. We will discover that there are exceptions to these laws that have obviously been discovered over time. We will discover that there are exceptions to these laws that have obviously been discovered over time. So what is a genotype? So what is a genotype? The genotype, The genotype, as the definition says, as the definition says, is the genetic makeup of an organism corresponding to the set of alleles present for each gene that governs the expression of the somatic trait, is the genetic makeup of an organism corresponding to the set of alleles present for each gene that governs the expression of the somatic trait, that is, that is, the phenotype that we are going to see now. the phenotype that we are going to see now. So, So, to put it simply, to put it simply, what is the genotype? what is the genotype? I'll tell you in simple terms. I'll tell you in simple terms. The genotype is what we have written in our DNA, The genotype is what we have written in our DNA, that is, that is, the genotype and what is written on the chromosomes, the genotype and what is written on the chromosomes, what is written on the alleles. what is written on the alleles. Here it is written on L1, Here it is written on L1, capital letter A. capital letter A. Here it is written on L2, Here it is written on L2, lowercase letter A. lowercase letter A. This is exactly the genotype, This is exactly the genotype, capital letter A, capital letter A, lowercase A. lowercase A. If we want to give a slightly more practical example, If we want to give a slightly more practical example, let's take for example the two chromosomes of paternal and maternal origin, let's take for example the two chromosomes of paternal and maternal origin, as we did before, as we did before, and let's take for example the trait of eye color. and let's take for example the trait of eye color. Well, Well, L1 could be the color of brown eyes. L1 could be the color of brown eyes. L2 could be the color of blue eyes. L2 could be the color of blue eyes. So what is written on our DNA? So what is written on our DNA? Our DNA says brown eye color, Our DNA says brown eye color, blue eye color. blue eye color. Which will prevail? Which will prevail? Which will be expressed? Which will be expressed? Well, Well, we will only find out by defining the phenotype. we will only find out by defining the phenotype. Because what is the phenotype? Because what is the phenotype? The phenotype is the observable trait, The phenotype is the observable trait, meaning that we can have many things written in our DNA, meaning that we can have many things written in our DNA, but only some of them could be expressed. but only some of them could be expressed. A classic example is eye color or hair color. A classic example is eye color or hair color. So the genotype is what is written in the DNA. So the genotype is what is written in the DNA. while the phenotype is what is visible. while the phenotype is what is visible. So we can immediately understand from this definition that there could be something written in the DNA that does not come out, So we can immediately understand from this definition that there could be something written in the DNA that does not come out, that cannot be seen. that cannot be seen. And let's see why, And let's see why, let's move on to examining Mendel's three laws. let's move on to examining Mendel's three laws. Let's start with the first one. Let's start with the first one. So, So, as you can see in the first image, as you can see in the first image, I have put a black mouse, I have put a black mouse, I have also put the terms genotype and phenotype, I have also put the terms genotype and phenotype, and so we can immediately understand that I will explain Mendel's law to you in a very elaborate way. and so we can immediately understand that I will explain Mendel's law to you in a very elaborate way. Why is that? Why is that? Because earlier we said that Mendel was a botanist. Because earlier we said that Mendel was a botanist. Mendel certainly didn't work with mice. Mendel certainly didn't work with mice. Mendel worked with plants, Mendel worked with plants, he mainly worked with pea plants, he mainly worked with pea plants, but I never like to use plants as examples because they are very distant from human beings. but I never like to use plants as examples because they are very distant from human beings. I prefer to use organisms that are much more similar to us, I prefer to use organisms that are much more similar to us, such as mice, such as mice, which are mammals. which are mammals, are very similar to human beings and are also much cuter than pea plants. are very similar to human beings and are also much cuter than pea plants. Even if the name always elicits a bit of laughter, Even if the name always elicits a bit of laughter, but mice are certainly cuter. but mice are certainly cuter. So, So, we see that Mendel had certain individuals crossbred several times until he obtained a pure line. we see that Mendel had certain individuals crossbred several times until he obtained a pure line. In this case we will talk about black mice and white mice. In this case we will talk about black mice and white mice. Why is that? Why is that? Because we will use the example of the color of the mice's fur. Because we will use the example of the color of the mice's fur. Mendel did the same thing. Mendel did the same thing. But on the color of flowers, But on the color of flowers, peas are the color of pea pods. peas are the color of pea pods. He did the same thing, He did the same thing, so he always used color to make his observations, so he always used color to make his observations, but he used plants, but he used plants, while we will look at the color of animals a little more similar to us, while we will look at the color of animals a little more similar to us, such as mice. such as mice. Mendel had his individuals crossbred several times to obtain a pure line and therefore homozygous mice that we would call black-black, Mendel had his individuals crossbred several times to obtain a puruline and therefore homozygous mice that we would call black-black, meaning they had black written on one chromosome and black written on the other chromosome. meaning they had black written on one chromosome and black written on the other chromosome. Obviously, Obviously, a mouse where one allele says black and the other says black, a mouse where one allele says black and the other says black, the only color it will be able to express will be black. the only color it will be able to express will be black. In fact, In fact, these mice had a black phenotype. these mice had a black phenotype. He called them purebred. He called them purebred. Well, Well, today we would say the genotype is black, today we would say the genotype is black, he called them purebred. he called them purebred. So he took the homozygous black black mice with the black trait expressed and had them crossbred with mice that he also called purebred, So he took the homozygous black black mice with the black trait expressed and had them crossbred with mice that he also called purebred. but homozygous white white. But homozygous white white. meaning they had white written on one allele and the other two. meaning they had white written on one allele and the other too. Obviously, Obviously, the character that came out of it, the character that came out of it, if they only wrote white on one chromosome and white on the other, if they only wrote white on one chromosome and white on the other, the color white would necessarily come out. the color white would necessarily come out. He crossed these purebred black mice with purebred white mice and let's see what he got. He crossed these purebred black mice with purebred white mice and let's see what he got. To understand which offspring will come out of this cross, To understand which offspring will come out of this cross, you have to use the square or the pinette panel. you have to use the square or the pinet panel. How do you do it like this? How do you do it like this? Very simple. Very simple. You draw three vertical lines, You draw three vertical lines, three horizontal lines. three horizontal lines. We are going to position the father either above or to the left or the mother above or to the left. We are going to position the father either above or to the left or the mother above or to the left. It doesn't matter. It doesn't matter. The important thing is that we use the alleles separate from each other. The important thing is that we use the alleles separate from each other. We'll understand why later. We'll understand why later. So, So, for example, for example, let's put the black mouse black on top, let's put the black mouse black on top, the mouse with the white characters white below. the mouse with the white characters white below. Now we're going to do exactly like Battleship, Now we're going to do exactly like Battleship, that is, that is, we're going to cross the letters and then we'll see that the genotypes for the offspring will be in this case NB, we're going to cross the letters and then we'll see that the genotypes for the offspring will be in this case NB, in this case NB, in this case NB, in this case also NB and we discover that in the last case we will also have a genotype NB black and white. in this case also NB and we discover that in the last case we will also have a genotype NB black and white. What follows from this? What follows from this? This is where Mendel's observation comes from. This is where Mendel's observation comes from. All the new mice that were born from a cross between a homozygous black-black individual and a homozygous white-white individual, All the new mice that were born from a cross between a homozygous black-black individual and a homozygous white-white individual, that is, that is, between a purebred black mouse and a purebred white mouse, between a purebred black mouse and a purebred white mouse, were all black. were all black. The very interesting thing is that if we take a slightly more detailed look at these mice, The very interesting thing is that if we take a slightly more detailed look at these mice, we will discover that these mice did indeed have the black phenotype, we will discover that these mice did indeed have the black phenotype, but they are heterozygous, but they are heterozygous, nitrogen monoboride, nitrogen monoboride. black-white, Black-white, so they have black-white written in their DNA. so they have black-white written in their DNA. And so this is where Mendel's first law, And so this is where Mendel's first law, called the law of dominance, called the law of dominance, comes from. comes from. What does the law of dominance say? What does the law of dominance say? It says exactly what we have just observed, It says exactly what we have just observed, namely, namely, individuals born from a cross between two homozygous individuals that differ in one allelic pair will have the phenotype, individuals born from a cross between two homozygous individuals that differ in one allelic pair will have the phenotype, that is, that is, the expressed trait, the expressed trait, given by the dominant allele. given by the dominant allele. That is, That is, Mendel discovered that there was an allele that dominated the other, Mendel discovered that there was an allele that dominated the other, and therefore a color that dominated the other color. and therefore a color that dominated the other color. That is to say, That is to say, if the DNA said, if the DNA said, black and white, black and white, only black would be expressed. only black would be expressed. And this is exactly Mendel's first law. And this is exactly Mendel's first law. That is to say, That is to say, Mendel, Mendel, with the first law, with the first law, precisely explained the issue of dominance, precisely explained the issue of dominance, that is to say that in some traits there are alleles that dominate over others. that is to say that in some traits there are alleles that dominate over others. But then Mendel was still not satisfied and continued with his experiments. But then Mendel was still not satisfied and continued with his experiments. He crossed the mice that had come from the previous cross again. He crossed the mice that had come from the previous cross again. He took two mice with a black phenotype, He took two mice with the black phenotype, but these mice had black and white written in their DNA, but these mice had black and white written in their DNA, that is to say they were heterozygous, that is to say they were heterozygous, they had both the color black and the color white written in their DNA. they had both the color black and the color white written in their DNA. He took two mice, He took two mice, therefore black heterozygous, therefore black heterozygous, and crossed them. and crossed them. To see what Mendel observed, To see what Mendel observed, we can use the Panet square as before and draw our lines. we can use the Panette square as before and draw our lines. We place a black heterozygous mouse at the top and obviously on the left the other mouse which will however be identical in terms of genotype, We place a black heterozygous mouse at the top and obviously on the left the other mouse which will however be identical in terms of genotype, black heterozygous. black heterozygous. At this point, At this point, let's plot the crosses, let's plot the crosses, therefore the probabilities of the genotypes of the offspring. therefore the probabilities of the genotypes of the offspring. We see that black with black will give a genotype NN. We see that black with black will give a genotype NN. Here there will be a genotype NB. Here there will be a genotype NB. Genotype NB, Genotype NB, genotype BB. genotype BB. So what do we discover? So what do we discover? We discover that Mendel observed, We discover that Mendel observed, in turn, in turn, that 25%, that 25%, that is, that is, a quarter, a quarter, of the mice that were born from this cross between two heterozygous black mice, of the mice that were born from this cross between two heterozygous black mice, were born homozygous black and, were born homozygous black and, therefore, therefore, with black black written in their DNA, with black black written in their DNA, and the mouse was purebred, and the mouse was purebred, exactly like its grandfather. exactly like its grandfather. Essentially, Essentially, 50%, 50%, that is, that is, half, half, were born heterozygous black, were born heterozygous black, that is, that is, just like their parents. just like their parents. They had black white written in their DNA, They had black white written in their DNA, Okay. their trait, Their trait, their phenotype was black. their phenotype was black. 25%, 25%, that is, that is, again one quarter, again one quarter, were born homozygous white and that is, were born homozygous white and that is, they had white white written in their DNA and the trait expressed, they had white white written in their DNA and the trait expressed, that is, that is, the phenotype, the phenotype, was white. was white. And then Mendel noticed something very interesting. And then Mendel noticed something very interesting. The white color was popping up again. The white color was popping up again. In other words, In other words, If in the previous generation we hadn't seen any mice with the white trait, if in the previous generation we hadn't seen any mice with the white trait, Mendel noticed that in the second generation the white trait would return in the offspring again. Mendel noticed that in the second generation the white trait would return in the offspring again. So he went on to denounce the second law. So he went on to denounce the second law. Mendel's second law is called the law of segregation and it tells us that during the generation of offspring, Mendel's second law is called the law of segregation and it tells us that during the generation of offspring, the alleles associated with the same gene separate from each other, the alleles associated with the same gene separate from each other, so that only one of the alleles reaches each of the two gametes. so that only one of the alleles reaches each of the two gametes. What is the consequence of this event? What is the consequence of this event? The consequence is that there will therefore be an allele that will remain hidden or may remain hidden until subsequent generations. The consequence is that there will therefore be an allele that will remain hidden or may remain hidden until subsequent generations. In the case we just saw, In the case we just saw, we saw how one quarter of the offspring of black heterozygous mice could be born homozygous white. we saw how one quarter of the offspring of black heterozygous mice could be born homozygous white. In this case, In this case, the trait that remains hidden and can emerge in subsequent generations is called recessive. the trait that remains hidden and can emerge in subsequent generations is called recessive. That's why I like to call this law the law of recessiveness. That's why I like to call this law the law of recessiveness. It doesn't exist as a term, It doesn't exist as a term, but if the first is the law of dominance, but if the first is the law of dominance, it would be much more coherent to call it the law of recessiveness. it would be much more coherent to call it the law of recessiveness. Obviously we will call it by its correct name, Obviously, we will call it by its correct name, that is, that is, the law of segregation, the law of segregation, but to remind you, but to remind you, you can mark it as the law of recessiveness, you can mark it as the law of recessiveness, which is what the term recessive is described in. which is what the term recessive is described in. And now let's look at Mendel's third law. And now let's look at Mendel's third law. Mendel's third law is called the law of independent assortment. Mendel's third law is called the law of independent assortment. Mendel realized that if he crossed several characters in an experiment, Mendel realized that if he crossed several characters in an experiment, these characters would come together in a completely independent way. these characters would come together in a completely independent way. And that is to say, And that is to say, let's take an example because obviously here there is a beautiful image that takes into consideration Mendel's experiment with pea plants. let's take an example because obviously here there is a beautiful image that takes into consideration Mendel's experiment with pea plants. But let's take a much more appropriate example, But let's take a much more appropriate example, perhaps involving a human being. perhaps involving a human being. So what does Mendel's third law tell us? So what does Mendel's third law tell us? It tells us that characters are reassembled, It tells us that characters are reassembled, they are recomposed in a completely independent way from each other. they are recomposed in a completely independent way from each other. That is, That is, if we have a father with brown hair and dark eyes and a mother with blonde hair and blue eyes, if we have a father with brown hair and dark eyes and a mother with blonde hair and blue eyes, then the children of the latter could have brown hair and blue eyes, then the children of the latter could have brown hair and blue eyes, brown hair and dark eyes, brown hair and dark eyes, exactly like the father, exactly like the father, blonde hair and blue eyes, blonde hair and blue eyes, exactly like the mother, exactly like the mother, or blonde hair and dark eyes. or blonde hair and dark eyes. So the two traits could come together completely independently, So the two traits could come together completely independently, that is, that is, it would not be mandatory for the child to be born exactly like the father or exactly like the mother. it would not be mandatory for the child to be born exactly like the father or exactly like the mother, But since there are two traits, but since there are two traits, these traits can mix together. these traits can mix together. We take this for granted, We take this for granted, but at the time it was not at all. but at the time it was not at all. So what does the law of independent assortment say? So what does the law of independent assortment say? Mendel's law number 3 states that during the formation of gametes. Mendel's law number 3 states that during the formation of gametes, Different genes are distributed independently of each other, Different genes are distributed independently of each other, that is, that is, in the cross of two subjects with different traits. in the cross of two subjects with different traits. So in this case, So in this case, two traits are taken into consideration, two traits are taken into consideration, that is, that is, two pairs of phenotypes. two pairs of phenotypes. Each of them is transmitted and inherited independently of each other. Each of them is transmitted and inherited independently of each other. And with this we conclude the video on Mendel's three laws. And with this we conclude the video on Mendel's three laws. In the next video we will will examine the exceptions to these laws. In the next video we will examine the exceptions to these laws. Alright, Alright, if you enjoyed the video, if you enjoyed the video, leave a like and subscribe to support the channel and we'll meet again for the next lesson. leave a like and subscribe to support the channel and we'll meet again for the next lesson. Nay. Nay.

we're going to discuss Mendel's three laws. This will be the first of three videos about genetics. This will be the first of three videos about genetics. As for the previous videos, As for the previous videos, I recommend that you watch lesson number five.

I recommend that you watch lesson number 5, where we talk about the DNA nucleus. where we talk about the DNA nucleus. It will be very important for understanding Mendelian genetics, It will be very important for understanding Mendelian genetics, as will the video in lesson 10, as will the video in lesson 10, which deals with meiosis. which deals with meiosis.

This is also essential for understanding how Mendel's laws of genetics work. This is also essential for understanding how Mendel's laws of genetics work. So, So, first of all, first of all, we must obviously start by asking ourselves who Mendel was.

we must obviously start by asking ourselves who Mendel was. Mendel is recognized by science as the father of genetics. Mendel is recognized by science as the father of genetics.

He died in 1822 and died in 1894. He died in 1822 and died in 1894. He was a monk and naturalist and conducted genetic experiments in a garden at the Burnet Monastery in what is now the Czech Republic. He was a monk and naturalist and conducted genetic experiments in a garden at the Burnet Monastery in what is now the Czech Republic. So Mendel was a monk, So Mendel was a monk, he lived in 1800, he lived in 1800. so we need to start right away to get an idea of what Mendel already knew about biology. So we need to start right away to get an idea of what Mendel already knew about biology.

Mendel did not yet know about the existence of DNA. Mendel did not yet know about the existence of DNA. At that time, At that time, the existence of this molecule that holds the information that will then be translated into the expression of particular proteins that will then give the characteristics to human beings and to any living being was not yet known. The existence of this molecule that holds the information that will then be translated into the expression of particular proteins that will then give the characteristics to human beings and to any living being was not yet known. Therefore, Therefore, Mendel's studies used terminology that is not current and even if in a certain sense they predicted the existence of DNA, Mendel's studies used terminology that is not current and even if in a certain sense they predicted the existence of DNA, they did not take it into consideration because this molecule was not yet known.

they did not take it into consideration because this molecule was not yet known. And starting from what Mendel did not yet know, And starting from what Mendel did not yet know, we are going to give some definitions that are instead very important for us. we are going to give some definitions that are instead very important for us.

Obviously, Obviously, as already mentioned, as already mentioned, we must keep in mind that Mendel did not know about it, we must keep in mind that Mendel did not know about it, but today, but today, especially after 1900, especially after 1900, when DNA, when DNA, this molecule that holds genetic information, this molecule that holds genetic information, was discovered, was discovered, we know that biological mechanisms work in this way. we know that biological mechanisms work in this way. We have information written in DNA.

We have information written in DNA. As you can see from the image, As you can see from the image, A chromosome is represented, a chromosome is represented, which is the form in which the DNA condenses when the cell wants to replicate. which is the form in which the DNA condenses when the cell wants to replicate. We must also remember, We must also remember, as already mentioned in lesson number 5, as already mentioned in lesson number 5, that during the life of the cell, that during the life of the cell. DNA is found in a scattered form in the nucleus of a eukaryotic cell, DNA is found in a scattered form in the nucleus of a eukaryotic cell, in a form called chromatin.

in a form called chromatin. However, However, for convenience, for convenience, it is always represented in the form of a chromosome, it is always represented in the form of a chromosome, because then, because then, when the cell reproduces, when the cell reproduces, it condenses the DNA in the form of chromosomes. it condenses the DNA in the form of chromosomes. So we have a portion of DNA that holds the information, So we have a portion of DNA that holds the information, that is, that is, on that portion of DNA it is written how a certain characteristic must be made. on that portion of DNA it is written how a certain characteristic must be made.

And so we see how the stretch of DNA present on the chromosome in which there is the information that will then produce a particular protein is called a gene. And so we see how the stretch of DNA present on the chromosome in which there is the information that will then produce a particular protein is called a gene. So the gene is that stretch of DNA that will then code for a particular protein.

So the gene is that stretch of DNA that will then code for a particular protein. This protein will then obviously determine a particular characteristic. This protein will then obviously determine a particular characteristic. Let's take an example. Let's take an example.

Here, Here, I will always use examples of skin color, I will always use examples of skin color, eye color, eye color, hair color, hair color, But let's keep in mind that as far as human beings are concerned these are slightly simplistic examples. but let's keep in mind that as far as human beings are concerned these are slightly simplistic examples. This is because in human beings these characteristics are described by multiple genes. This is because in human beings these characteristics are described by multiple genes. But let's not worry about that now, But let's not worry about that now, let's simplify the whole concept a lot and say that on this gene, let's simplify the whole concept a lot and and say that on this gene, for example, for example, On this stretch of DNA, on this stretch of DNA, it could be written what color our eyes are.

it could be written what color our eyes are. For example, For example, it says, it says, eyes must be brown. eyes must be brown.

And so this stretch of DNA is called a gene and that is, And so this stretch of DNA is called a gene and that is on this gene it is written that our eyes must be brown. On this gene it is written that our eyes must be brown. This gene will then be translated, This gene will then be translated, thanks to the ribosomes, thanks to the ribosomes, into a protein, into a protein, in this case melanin, in this case melanin, which will make our character, which will make our character, that is, that is, brown eyes, brown eyes, emerge.

emerge. However, However, we must remember that in our genetic heritage we have a copy of each chromosome. we must remember that in our genetic heritage we have a copy of each chromosome. This is because we possess homologous chromosomes, This is because we possess homologous chromosomes, that is, that is, we have a chromosome that comes from our father, we have a chromosome that comes from our father, a chromosome that comes from our mother, a chromosome that comes from our mother, and so, and so, always keeping in mind the previous example, always keeping in mind the previous example, we will have a gene that codes for eye color on the chromosome that we got from our father, we will have a gene that codes for eye color on the chromosome that we got from our father, and the same gene that codes for eye color, and the same gene that codes for eye color, more than anything else a gene that codes for the same trait, more than anything else a gene that codes for the same trait, but that comes from our mother.

but that comes from our mother. Maybe on our mother's chromosome we have written that the eye color must be blue. Maybe on our mother's chromosome we have written that the eye color must be blue.

So here we have a gene that comes from our father that tells us what the eye color should be, So here we have a gene that comes from our father that tells us what the eye color should be, a gene that comes from our mother that tells us the same thing. a gene that comes from our mother that tells us the same thing. So we have a gene that codes for a trait that I have simply called trait A and we have another gene on the homologous chromosome that comes from the other parent, So we have a gene that codes for a trait that I have simply called trait A and we have another gene on the homologous chromosome that comes from the other parent, but that codes for the same trait, but that codes for the same trait, trait A. trait A.

The two genes present on homologous chromosomes that code for the same trait are called alleles. The two genes present on homologous chromosomes that code for the same trait are called alleles. So what are alleles? So what are alleles? They are precisely the two genes that code for the same trait.

They are precisely the two genes that code for the same trait. For example, For example, the two genes that come from our mother and father, the two genes that come from our mother and father, one that codes for eye color. one that codes for eye color. Very simply, Very simply, these are the alleles.

these are the alleles. So we have the first allele present on the paternal chromosome and the second allele present on the maternal chromosome. So we have the first allele present on the paternal chromosome and the second allele present on the maternal chromosome. Having said that, Having said that, let's define some terms. let's define some terms.

In terminology, In terminology, it is very important to know what a genotype is. it is very important to know what a genotype is. Let's remember that Mendel still didn't know what DNA and chromosomes were, Let's remember that Mendel still didn't know what DNA and chromosomes were, so he had no idea what a genotype was. so he had no idea what a genotype was.

But we are already studying it because it will be very useful for understanding genetics. But we are already studying it because it will be very useful for understanding genetics. Obviously, Obviously, It is useless to go over all of Mendel's discoveries as if we were in 1800 because it is 2020 and therefore we know perfectly well how genetics works, It is useless to go over all of Mendel's discoveries as if we were in 1800 because it is 2020 and therefore we know perfectly well how genetics works, so it is better to study it with the terminology that we know today.

so it is better to study it with the terminology that we know today. Obviously, Obviously, however, however, Mendel's laws, Mendel's laws, Mendel's three laws, Mendel's three laws, are still present today, are still present today, so much so that they are also the basis for modern genetics. so much so that they are also the basis for modern genetics. We will discover that there are exceptions to these laws that have obviously been discovered over time.

We will discover that there are exceptions to these laws that have obviously been discovered over time. So what is a genotype? So what is a genotype? The genotype, The genotype, as the definition says, as the definition says, is the genetic makeup of an organism corresponding to the set of alleles present for each gene that governs the expression of the somatic trait, is the genetic makeup of an organism corresponding to the set of alleles present for each gene that governs the expression of the somatic trait, that is, that is, the phenotype that we are going to see now.

the phenotype that we are going to see now. So, So, to put it simply, to put it simply, what is the genotype? what is the genotype? I'll tell you in simple terms. I'll tell you in simple terms.

The genotype is what we have written in our DNA, The genotype is what we have written in our DNA, that is, that is, the genotype and what is written on the chromosomes, the genotype and what is written on the chromosomes, what is written on the alleles. what is written on the alleles. Here it is written on L1, Here it is written on L1, capital letter A. capital letter A. Here it is written on L2, Here it is written on L2, lowercase letter A.

lowercase letter A. This is exactly the genotype, This is exactly the genotype, capital letter A, capital letter A, lowercase A. lowercase A. If we want to give a slightly more practical example, If we want to give a slightly more practical example, let's take for example the two chromosomes of paternal and maternal origin, let's take for example the two chromosomes of paternal and maternal origin, as we did before, as we did before, and let's take for example the trait of eye color. and let's take for example the trait of eye color.

Well, Well, L1 could be the color of brown eyes. L1 could be the color of brown eyes. L2 could be the color of blue eyes.

L2 could be the color of blue eyes. So what is written on our DNA? So what is written on our DNA?

Our DNA says brown eye color, Our DNA says brown eye color, blue eye color. blue eye color. Which will prevail? Which will prevail?

Which will be expressed? Which will be expressed? Well, Well, we will only find out by defining the phenotype.

we will only find out by defining the phenotype. Because what is the phenotype? Because what is the phenotype? The phenotype is the observable trait, The phenotype is the observable trait, meaning that we can have many things written in our DNA, meaning that we can have many things written in our DNA, but only some of them could be expressed. but only some of them could be expressed.

A classic example is eye color or hair color. A classic example is eye color or hair color. So the genotype is what is written in the DNA. So the genotype is what is written in the DNA. while the phenotype is what is visible.

while the phenotype is what is visible. So we can immediately understand from this definition that there could be something written in the DNA that does not come out, So we can immediately understand from this definition that there could be something written in the DNA that does not come out, that cannot be seen. that cannot be seen.

And let's see why, And let's see why, let's move on to examining Mendel's three laws. let's move on to examining Mendel's three laws. Let's start with the first one.

Let's start with the first one. So, So, as you can see in the first image, as you can see in the first image, I have put a black mouse, I have put a black mouse, I have also put the terms genotype and phenotype, I have also put the terms genotype and phenotype, and so we can immediately understand that I will explain Mendel's law to you in a very elaborate way. and so we can immediately understand that I will explain Mendel's law to you in a very elaborate way. Why is that? Why is that?

Because earlier we said that Mendel was a botanist. Because earlier we said that Mendel was a botanist. Mendel certainly didn't work with mice. Mendel certainly didn't work with mice.

Mendel worked with plants, Mendel worked with plants, he mainly worked with pea plants, he mainly worked with pea plants, but I never like to use plants as examples because they are very distant from human beings. but I never like to use plants as examples because they are very distant from human beings. I prefer to use organisms that are much more similar to us, I prefer to use organisms that are much more similar to us, such as mice, such as mice, which are mammals. which are mammals, are very similar to human beings and are also much cuter than pea plants.

are very similar to human beings and are also much cuter than pea plants. Even if the name always elicits a bit of laughter, Even if the name always elicits a bit of laughter, but mice are certainly cuter. but mice are certainly cuter. So, So, we see that Mendel had certain individuals crossbred several times until he obtained a pure line. we see that Mendel had certain individuals crossbred several times until he obtained a pure line.

In this case we will talk about black mice and white mice. In this case we will talk about black mice and white mice. Why is that? Why is that?

Because we will use the example of the color of the mice's fur. Because we will use the example of the color of the mice's fur. Mendel did the same thing.

Mendel did the same thing. But on the color of flowers, But on the color of flowers, peas are the color of pea pods. peas are the color of pea pods. He did the same thing, He did the same thing, so he always used color to make his observations, so he always used color to make his observations, but he used plants, but he used plants, while we will look at the color of animals a little more similar to us, while we will look at the color of animals a little more similar to us, such as mice.

such as mice. Mendel had his individuals crossbred several times to obtain a pure line and therefore homozygous mice that we would call black-black, Mendel had his individuals crossbred several times to obtain a puruline and therefore homozygous mice that we would call black-black, meaning they had black written on one chromosome and black written on the other chromosome. meaning they had black written on one chromosome and black written on the other chromosome.

Obviously, Obviously, a mouse where one allele says black and the other says black, a mouse where one allele says black and the other says black, the only color it will be able to express will be black. the only color it will be able to express will be black. In fact, In fact, these mice had a black phenotype.

these mice had a black phenotype. He called them purebred. He called them purebred.

Well, Well, today we would say the genotype is black, today we would say the genotype is black, he called them purebred. he called them purebred. So he took the homozygous black black mice with the black trait expressed and had them crossbred with mice that he also called purebred, So he took the homozygous black black mice with the black trait expressed and had them crossbred with mice that he also called purebred. but homozygous white white.

But homozygous white white. meaning they had white written on one allele and the other two. meaning they had white written on one allele and the other too. Obviously, Obviously, the character that came out of it, the character that came out of it, if they only wrote white on one chromosome and white on the other, if they only wrote white on one chromosome and white on the other, the color white would necessarily come out. the color white would necessarily come out.

He crossed these purebred black mice with purebred white mice and let's see what he got. He crossed these purebred black mice with purebred white mice and let's see what he got. To understand which offspring will come out of this cross, To understand which offspring will come out of this cross, you have to use the square or the pinette panel. you have to use the square or the pinet panel. How do you do it like this?

How do you do it like this? Very simple. Very simple.

You draw three vertical lines, You draw three vertical lines, three horizontal lines. three horizontal lines. We are going to position the father either above or to the left or the mother above or to the left.

We are going to position the father either above or to the left or the mother above or to the left. It doesn't matter. It doesn't matter.

The important thing is that we use the alleles separate from each other. The important thing is that we use the alleles separate from each other. We'll understand why later.

We'll understand why later. So, So, for example, for example, let's put the black mouse black on top, let's put the black mouse black on top, the mouse with the white characters white below. the mouse with the white characters white below. Now we're going to do exactly like Battleship, Now we're going to do exactly like Battleship, that is, that is, we're going to cross the letters and then we'll see that the genotypes for the offspring will be in this case NB, we're going to cross the letters and then we'll see that the genotypes for the offspring will be in this case NB, in this case NB, in this case NB, in this case also NB and we discover that in the last case we will also have a genotype NB black and white. in this case also NB and we discover that in the last case we will also have a genotype NB black and white.

What follows from this? What follows from this? This is where Mendel's observation comes from. This is where Mendel's observation comes from. All the new mice that were born from a cross between a homozygous black-black individual and a homozygous white-white individual, All the new mice that were born from a cross between a homozygous black-black individual and a homozygous white-white individual, that is, that is, between a purebred black mouse and a purebred white mouse, between a purebred black mouse and a purebred white mouse, were all black.

were all black. The very interesting thing is that if we take a slightly more detailed look at these mice, The very interesting thing is that if we take a slightly more detailed look at these mice, we will discover that these mice did indeed have the black phenotype, we will discover that these mice did indeed have the black phenotype, but they are heterozygous, but they are heterozygous, nitrogen monoboride, nitrogen monoboride. black-white, Black-white, so they have black-white written in their DNA. so they have black-white written in their DNA.

And so this is where Mendel's first law, And so this is where Mendel's first law, called the law of dominance, called the law of dominance, comes from. comes from. What does the law of dominance say?

What does the law of dominance say? It says exactly what we have just observed, It says exactly what we have just observed, namely, namely, individuals born from a cross between two homozygous individuals that differ in one allelic pair will have the phenotype, individuals born from a cross between two homozygous individuals that differ in one allelic pair will have the phenotype, that is, that is, the expressed trait, the expressed trait, given by the dominant allele. given by the dominant allele.

That is, That is, Mendel discovered that there was an allele that dominated the other, Mendel discovered that there was an allele that dominated the other, and therefore a color that dominated the other color. and therefore a color that dominated the other color. That is to say, That is to say, if the DNA said, if the DNA said, black and white, black and white, only black would be expressed. only black would be expressed. And this is exactly Mendel's first law.

And this is exactly Mendel's first law. That is to say, That is to say, Mendel, Mendel, with the first law, with the first law, precisely explained the issue of dominance, precisely explained the issue of dominance, that is to say that in some traits there are alleles that dominate over others. that is to say that in some traits there are alleles that dominate over others.

But then Mendel was still not satisfied and continued with his experiments. But then Mendel was still not satisfied and continued with his experiments. He crossed the mice that had come from the previous cross again.

He crossed the mice that had come from the previous cross again. He took two mice with a black phenotype, He took two mice with the black phenotype, but these mice had black and white written in their DNA, but these mice had black and white written in their DNA, that is to say they were heterozygous, that is to say they were heterozygous, they had both the color black and the color white written in their DNA. they had both the color black and the color white written in their DNA.

He took two mice, He took two mice, therefore black heterozygous, therefore black heterozygous, and crossed them. and crossed them. To see what Mendel observed, To see what Mendel observed, we can use the Panet square as before and draw our lines.

we can use the Panette square as before and draw our lines. We place a black heterozygous mouse at the top and obviously on the left the other mouse which will however be identical in terms of genotype, We place a black heterozygous mouse at the top and obviously on the left the other mouse which will however be identical in terms of genotype, black heterozygous. black heterozygous.

At this point, At this point, let's plot the crosses, let's plot the crosses, therefore the probabilities of the genotypes of the offspring. therefore the probabilities of the genotypes of the offspring. We see that black with black will give a genotype NN. We see that black with black will give a genotype NN. Here there will be a genotype NB.

Here there will be a genotype NB. Genotype NB, Genotype NB, genotype BB. genotype BB.

So what do we discover? So what do we discover? We discover that Mendel observed, We discover that Mendel observed, in turn, in turn, that 25%, that 25%, that is, that is, a quarter, a quarter, of the mice that were born from this cross between two heterozygous black mice, of the mice that were born from this cross between two heterozygous black mice, were born homozygous black and, were born homozygous black and, therefore, therefore, with black black written in their DNA, with black black written in their DNA, and the mouse was purebred, and the mouse was purebred, exactly like its grandfather. exactly like its grandfather. Essentially, Essentially, 50%, 50%, that is, that is, half, half, were born heterozygous black, were born heterozygous black, that is, that is, just like their parents.

just like their parents. They had black white written in their DNA, They had black white written in their DNA, Okay. their trait, Their trait, their phenotype was black.

their phenotype was black. 25%, 25%, that is, that is, again one quarter, again one quarter, were born homozygous white and that is, were born homozygous white and that is, they had white white written in their DNA and the trait expressed, they had white white written in their DNA and the trait expressed, that is, that is, the phenotype, the phenotype, was white. was white.

And then Mendel noticed something very interesting. And then Mendel noticed something very interesting. The white color was popping up again. The white color was popping up again. In other words, In other words, If in the previous generation we hadn't seen any mice with the white trait, if in the previous generation we hadn't seen any mice with the white trait, Mendel noticed that in the second generation the white trait would return in the offspring again.

Mendel noticed that in the second generation the white trait would return in the offspring again. So he went on to denounce the second law. So he went on to denounce the second law.

Mendel's second law is called the law of segregation and it tells us that during the generation of offspring, Mendel's second law is called the law of segregation and it tells us that during the generation of offspring, the alleles associated with the same gene separate from each other, the alleles associated with the same gene separate from each other, so that only one of the alleles reaches each of the two gametes. so that only one of the alleles reaches each of the two gametes. What is the consequence of this event? What is the consequence of this event?

The consequence is that there will therefore be an allele that will remain hidden or may remain hidden until subsequent generations. The consequence is that there will therefore be an allele that will remain hidden or may remain hidden until subsequent generations. In the case we just saw, In the case we just saw, we saw how one quarter of the offspring of black heterozygous mice could be born homozygous white. we saw how one quarter of the offspring of black heterozygous mice could be born homozygous white.

In this case, In this case, the trait that remains hidden and can emerge in subsequent generations is called recessive. the trait that remains hidden and can emerge in subsequent generations is called recessive. That's why I like to call this law the law of recessiveness. That's why I like to call this law the law of recessiveness. It doesn't exist as a term, It doesn't exist as a term, but if the first is the law of dominance, but if the first is the law of dominance, it would be much more coherent to call it the law of recessiveness.

it would be much more coherent to call it the law of recessiveness. Obviously we will call it by its correct name, Obviously, we will call it by its correct name, that is, that is, the law of segregation, the law of segregation, but to remind you, but to remind you, you can mark it as the law of recessiveness, you can mark it as the law of recessiveness, which is what the term recessive is described in. which is what the term recessive is described in. And now let's look at Mendel's third law.

And now let's look at Mendel's third law. Mendel's third law is called the law of independent assortment. Mendel's third law is called the law of independent assortment. Mendel realized that if he crossed several characters in an experiment, Mendel realized that if he crossed several characters in an experiment, these characters would come together in a completely independent way.

these characters would come together in a completely independent way. And that is to say, And that is to say, let's take an example because obviously here there is a beautiful image that takes into consideration Mendel's experiment with pea plants. let's take an example because obviously here there is a beautiful image that takes into consideration Mendel's experiment with pea plants.

But let's take a much more appropriate example, But let's take a much more appropriate example, perhaps involving a human being. perhaps involving a human being. So what does Mendel's third law tell us? So what does Mendel's third law tell us? It tells us that characters are reassembled, It tells us that characters are reassembled, they are recomposed in a completely independent way from each other.

they are recomposed in a completely independent way from each other. That is, That is, if we have a father with brown hair and dark eyes and a mother with blonde hair and blue eyes, if we have a father with brown hair and dark eyes and a mother with blonde hair and blue eyes, then the children of the latter could have brown hair and blue eyes, then the children of the latter could have brown hair and blue eyes, brown hair and dark eyes, brown hair and dark eyes, exactly like the father, exactly like the father, blonde hair and blue eyes, blonde hair and blue eyes, exactly like the mother, exactly like the mother, or blonde hair and dark eyes. or blonde hair and dark eyes. So the two traits could come together completely independently, So the two traits could come together completely independently, that is, that is, it would not be mandatory for the child to be born exactly like the father or exactly like the mother.

it would not be mandatory for the child to be born exactly like the father or exactly like the mother, But since there are two traits, but since there are two traits, these traits can mix together. these traits can mix together. We take this for granted, We take this for granted, but at the time it was not at all. but at the time it was not at all. So what does the law of independent assortment say?

So what does the law of independent assortment say? Mendel's law number 3 states that during the formation of gametes. Mendel's law number 3 states that during the formation of gametes, Different genes are distributed independently of each other, Different genes are distributed independently of each other, that is, that is, in the cross of two subjects with different traits. in the cross of two subjects with different traits.

So in this case, So in this case, two traits are taken into consideration, two traits are taken into consideration, that is, that is, two pairs of phenotypes. two pairs of phenotypes. Each of them is transmitted and inherited independently of each other. Each of them is transmitted and inherited independently of each other. And with this we conclude the video on Mendel's three laws.

And with this we conclude the video on Mendel's three laws. In the next video we will will examine the exceptions to these laws. In the next video we will examine the exceptions to these laws.

Alright, Alright, if you enjoyed the video, if you enjoyed the video, leave a like and subscribe to support the channel and we'll meet again for the next lesson. leave a like and subscribe to support the channel and we'll meet again for the next lesson. Nay.

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Transcript for:Genetics Foundations and Mendel's Laws

Transcript for:
Genetics Foundations and Mendel's Laws