 So now we're going to dive into Gregor Mendel and his laws that he created to explain some of the genetic variation that occurs from offspring to offspring. First of all, the offspring that result from sexual reproduction are highly varied. We talked earlier about prophase I and the crossover effect that would happen when you have two chromosomes that are in physical contact with one another. This variation doesn't explain how exactly the offspring are so genetically varied not only from their parents, but even from each other in the same generation. So how does this genetic variation occur? It has to be more than just crossover. Gregor Mendel thought the same thing and he started to investigate these principles. The first thing he thought was maybe the genetic variability has to do to the random combination of gametes. One male, one female, unite in fertilization with no rhyme, reason, or logic behind the uniting other than this one's male and that one's female. Another one might think that maybe it was just crossover. Maybe the crossover was enough to lend enough genetic variability to create these variations from offspring to offspring even within the same generation. Gregor Mendel went a little bit more in depth with these reasonings and to do so he created two different laws. The first one is the law of independent assortment and the second is the law of segregation. These laws are what we're going to investigate today and by the end of our discussion we'll really understand why genetic variability results and why it's so important in the propagation of the species. So when Gregor Mendel started off with his genetic experiments he started with what's known as a monohybrid cross. A monohybrid cross is going to involve a parent generation or a P generation that are same except for one characteristic. For example, crossing purple flowers with white flowers. The only thing that they differ in is flower color. So Gregor Mendel first thought that when you take a purple flower and you cross it with a white flower that you should end up with some kind of blend, maybe a light purple colored flower. When he ended up finding in the first generation from this cross also known as the F1 generation is that all of the flowers were purple. Not quite understanding exactly what had happened he knew that there had to be a characteristic for purple and for white. Somehow the purple characteristic is masking the white characteristic or maybe it got rid of it all together. Maybe the purple characteristic removed the white. So he decided to test this by crossing the F1 generation with each other, taking this offspring and mating them to create what's known as an F2 generation. In this F2 generation, three-fourths were purple flowers and one-fourth were white flowers. So this proved that the white flower allele did not disappear. It was only being masked by the more dominant allele for flower color which is the purple. His conclusions can be broken down and summarized into four basic points and these basic points are things that we can carry with us throughout genetics to really understand why things happen and how things are divided up. The first one is that there are more than one version of a gene that is going to account for heriable traits. Different types of inheritable traits include flower color, we'll look into later, skin color, eye color, hitchhiker's thumb, attached earlobs. All of those are inheritable traits and they're coded for by a gene. Each person gets one version of the gene from the mother, one version of the gene from the father. These genes may be the same or they may be different just like purple versus white flowers. For each inheritable trait, remember we get one from the mother and one from the father, if these traits are the same then they're considered to be homozygous. If these traits are varying then they're considered to be heterozygous. Now with heterozygous alleles, one is dominant and you're going to see this abbreviated often by a capital letter. So a capital letter means that the trait is dominant and it will mask the recessive trait. The recessive trait is the lowercase letter. So if we were to look at purple flowers a capital P would represent purple because it is a dominant trait whereas a lowercase P would represent white because it's a recessive trait. Heterozygous individuals have one capital letter and one lowercase letter meaning that they are a carrier for the recessive trait. Homozygous dominant individuals mean that you have two dominant alleles, capital P capital P. Homozygous recessive individuals means that you have two recessive alleles, little P little P. Lastly with Mendel's laws each gamete only carries one allele for that characteristic. So each gamete whether it's from the male or for the female only carries one allele being it dominant or recessive it's only going to carry one and when the male unites with the female during fertilization those gametes will come together and the diploid number will be restored. So now that we've kind of discussed all of Mendel's laws and how he came about understanding mono-hybrid crosses we're going to look through one of his cross and we're going to discuss his conclusions to see if we can come up with the same ones from this illustration. So first we start off with the P generation. Remember for mono-hybrid crosses you start off with a true cross. A true cross is where you take a homozygous dominant meaning that you have two dominant alleles and you're going to cross it with a homozygous recessive plant which means you have two recessive alleles. This plant has red flowers, this plant has white flowers. When you cross them together you get an F1 generation which is heterozygous meaning they contain one dominant and one recessive allele. Crossing those together you end up with an F2 generation. This F2 generation contains a mix of homozygous dominant heterozygous and homozygous recessive plants. So let's go over his conclusions. The first conclusion that he had was that there are alternating versions of genes that account for variations in inheritance. We can see here that the one gene for flower color comes in two forms red and white. The second conclusion was that for each inheritable characteristic the offspring is going to get one allele from each parent. We can see with the F1 generation that is heterozygous in nature we've got the dominant allele from one parent I just love when that does that and one from the other parents that creates that heterozygous nature. Next with heterozygous allele one is going to be dominant and one is recessive which we can see heterozygous one's dominant and one is recessive. The recessive allele is going to be masked by the dominant allele as is evident when you look at the homozygous recessive plant. There is no masking effect taking place because there is no dominant allele. And lastly for each inheritable characteristic a gamete is only going to carry one allele for each trait. So the gamete from this plant you had one carrying a capital A and the other carrying a capital A. One lowercase, one lowercase. So four total gametes. One will unite with here to create either offspring. There's going to be a lot of variations and so a lot of times we use what's known as a punnett square to look at those variations. You can see that we took one parent of the F1 generation and laid out the possible gametes. Remember each gamete only carries one allele for each plant and then they cross together to create possible offspring combinations. You can see we have capital A capital A which is homozygous dominant two heterozygous and one homozygous recessive. This punnett square is going to be essential for solving a lot of genetic problems later on. So based on his conclusions we are able to see that the genetic composition of an organism isn't always reflected in the organism's appearance. For example there are various genotypes that one could have. Homozygous dominant, heterozygous or homozygous recessive. Remember that if there is a dominant allele a lot of times it masks the appearance of the recessive allele. And this appearance is going to be masked in the phenotype. The phenotype is the physical appearance of one's genotype. So it's something that you can actually see. It's tangible. So we have three different genotypes but only two different phenotypes. Remember these are going to wow look what I did. These are going to create a red flower. This is the only thing that creates a white flower. These ratios are not always going to be equal. So you could always have three genotypes, two phenotypes, sometimes they'll be equal. It just depends on that particular gene and that particular expression. So Mendel's law of segregation is going to describe the inheritance of a single characteristic. We know that alleles will separate that one gamete will contain one allele, another gamete will contain the other. It could be dominant or it could be recessive. But that chromosome is going to separate during metaphase that you create different varying haploid daughter cells instead of identical cells which are produced from mitosis. So this law of segregation explains that when you have separation of these chromosomes the sister chromatids are going to be on their own from that point on. And it's up to the luck of the draw to see which sister chromatid gets into which gamete and then which gamete is selected for the process of fertilization.