 Now, we talk about the importance of meiosis. What is the importance of meiosis? The most important feature of meiosis is the genetic variation by recombination. As we know that the natural environments in which the organisms are living, they are always changing. The temperatures change, the sometimes earthquake comes and the whole environment is changed. There are different types of environmental factors which always they are modifying, the availability of foods, the availability of shelters and everything changes and the organisms have to modify according to these environments. These recombinations by the crossing over in the prophase one of the meiosis provides the basis of new recombinations and this provision of new recombination makes new characteristics. These characteristics if are suitable for the organism and makes it more fit for the environment, the organisms survival is improved. So, usually the organisms have a better survival if they have new good characteristic, but it could be otherwise if the characteristic which appears is not good, then the survival will be reduced and as we know that the Darwin's theory says that the fittest will survive in an environment. The second benefit is this that because of meiosis half number of chromosomes go to one gamete and then the gametes have to combine and during as we know from the anaphase because we covered anaphase. In anaphase this is not necessary that whole set coming from the grandparent will go to just gamete, just one gamete. There will be a mixture because in anaphase the chromosomes are arranged in homologous pairs and in each pair there is this is not necessary that the paternal chromosome from father will be on one side and the chromosome from mother is on the other side, but this may change. So, there is a fifty-fifty chance that the gamete will have the paternal chromosomes or the maternal chromosomes. So, in each gamete there is a mixing or combination of few paternal and few maternal chromosomes. This provides another degree of recombination. This is the most important feature of meiosis. This is the importance of meiosis, the genetic variation which makes usually the organisms more fit to survive in the changing environments. Just have a look in the diagram which shows a homologous pairs and then shows a recombination, a chiasmata formation at two places and this chiasmata formation at two places as you can see if we look at their genes in the initial chromosomes there is a combination of capital Y, small Y for example, in a plant this is for yellow and green. These are the genes for yellow and green seeds. After recombination we can see that the first one will become yellow, the second one may become a green, the other one may become due to a recombination and in between color. So, a genetic variation arises due to these recombinations in meiosis. Now what happen if an error occur in meiosis? Errors in meiosis could have serious consequences or disadvantages. Usually errors in meiosis comes from non-disjunction. To understand the non-disjunction we have to look at a diagram. As we know that meiosis have two phases meiosis one and the meiosis two which results in formation of gametes. As you can see in the first diagram on the left in the n-phase two when the chromosomes have to move from the center towards poles. If the homologous chromosomes are not separated in n-phase one then what will be the result that the whole homologous pair will go towards one cell and the other cell will not have any chromosome on its place. So, that will be empty for that chromosomal pair that whole homologous pair. Then these two cells will divide by meiosis one they will result in unequal formation of gametes. As you can see in the first one it shows an n plus one that is one extra chromosome is here. The second also have an n plus one but the other one where no part of homolog went the chromosome is number is n minus one that is one chromosome less than the haploid number and the fourth one is also n minus one that is one chromosome less than the normal haploid number n. This is called non-disjunction that is in n-phase the homologous set which should naturally or normally separate are not separated but they moved towards one pole only and the other pole did not receive any chromosome. This non-distinction could occur in the meiosis two as well sometimes what happened that meiosis one completes correctly and both of the cells which are the result of meiosis two they have the correct number of chromosomes and then they start with the meiosis two but at n-phase two when they have to separate as the diagram on the right shows when they have to go to the poles in one of the cells division is normal and the chromosome number is m but in the other cells which is divided into two one chromosome with both of its sister chromatids move towards one pole and on the other pole there is no chromosome because one sister chromatids as normally as you can see in the last right diagram should go towards one pole and the other sister chromatids should go towards the other pole but what happened that both sister chromatids keep attached to each other did not separate and they move towards one pole only and the other pole remains blank the result is again we call it a non-distinction no separation and the result is one gamete which is n plus one that is haploid number plus one extra chromosome and the other one is n minus one that is haploid number with one chromosome less now as you can observe in these two diagrams and if you compare both of these non-distinctions the first one non-distinction in meiosis one and the second one the non-distinction in meiosis two if we compare those we can see that the non-distinction in meiosis one is more dangerous because the result is all the four gametes have abnormal number of chromosomes but in the case of non-distinction in meiosis two we have two normal gametes and two abnormal gametes so it means that if non-distinction occur in the meiosis one then the result will be all abnormal gametes more dangerous but in case of non-distinction at meiosis two we will have two normal gametes and two abnormal gametes that is 50 percent correct and 50 percent incorrect so this is non-distinction and what will be the result if these unequally distributed chromosomes in the gametes will combine with a normal ag sperm and will be fertilized to make a zygote the individual which is formed will not be normal this results in different types of diseases at birth you can see here an individual the picture with a down syndrome which is a mental disorder the child have inabilities intelligence inabilities and have many other problems and the reason is that if you can see in the yellow circle the chromosome number 21 have rather than two normal parts of a homolog it have three homologs just non-distinction in one chromosome only out of out of 23 pairs in just one pair there is a non-distinction and the fusion of the gametes I mean one gamete from normal parent the normal gamete and one gamete from a parent where a non-distinction occurred which have an extra chromosome the result is a very serious disorder down syndrome all the chromosomes you can see the one two three four five six seven eight are normal the last is also normal just at one chromosome and there is we call that three chromosomes are present in the set rather than two homologs there are three homologs which are present it means that a slight disorder a slight problem a slight non-distinction just even in one chromosome may result in a very very serious disorder this is and there are a lot many other disorders which may occur due to non-distinction at different types of chromosomes so errors in the meiosis may result in serious inborn disorders and as you can see here it's a very serious mental disorder in person is in unable to run the normal life in normal circumstances and they need special attention now the last topic for today twins twins we know if we look in in our lives normally many pairs of the twins one sometimes two pair maybe three or more there are two types of twins one is called fraternal twins the other are called identical twins we sweetens are in our normal life many times we see that they are just like each other in in their height in their face in their color in their features everything is just alike and they are of same gender both are male or both are female both boys and both girls but sometimes we see the twins which are actually very different from each other and who don't like don't look like twins they have different heights they have different features they have different religions different character other characteristics actually according to the divisions there are two types of twins one is called fraternal twins fraternal twins are those twins which look to us different from each other just as the siblings are different from each other which which are born at different times what happened that they are actually produced by different eggs sometimes the more than one eggs are produced by the ovary and both eggs are separately fertilized the result is two genetically different individuals they are as different as those individuals are which born in different times but the identical twins they are produced by the division in the same egg what happened that an egg is produced with the haploid number but it somehow divides by mitosis into two and when the egg is fertilized after its fertilization it divides into two which means that zygote is divided into two we also call them mono zygotic twins because they are product of same zygote they have exactly same genetic features they have exactly same type of chromosomes so they look just just as each other they must be they must have same gender both our boys or both our girls they are identical totally thoroughly in their genetic recombinations and in their genetic characteristics but there is there is another thing which may makes them different as we know that characteristics of an individual particularly its behavior does not only depend upon the genes genes play their role but the interaction of environment with the genes or we can say the gene environment interactions make certain characteristics particularly the behavioral characteristics and the intelligence by various studies in different parts of the world shown that if identical and mono zygotic twins they are grown up in different situations and conditions they develop different types of intellectual skills they develop different types of behaviors according to the environment in which they are grown up they are brought up it means that genes alone are not responsible for all the characteristics of an individual but the environment plays its role and the environment and genes both of these interact with each other to produce a full individual a complete individual with all its characteristics life is a production of genes in interaction with the environment so this was all about meiosis its significance its errors the results of its errors the twins interaction of the genes and environment I hope you understand