hi everyone welcome back to useful genetics this is lecture 1n we're talking about ploidy and recombination building on the basic understanding of life cycles from the previous lecture we're going to talk about how ploidy changes in sexual reproduction we alternate between haploid and diploid cells and then we'll talk about how the recombination that's part of the process of producing haploid gametes and bringing them back together to diploid how this creates new versions of genomes in two ways process is called reassortment and crossing over so first the essential terms a haploid cell these are terms that describe the genetic constitution of cells in terms of their chromosome sets so a haploid cell has one complete set of chromosomes and here again I've drawn chromosomes a set of five chromosomes as a complete set the chromosomes are distinguished here they're all the same color but they're distinguished by having different lengths a diploid cell has two homologous complete sets so here I've drawn two sets you can see that each set has the same number of chromosomes they have the same lengths and they're different shades of the same color to help us remember that they're just different versions of the same chromosomes a third term that's very useful is in sometimes lowercase in so the number of chromosomes in a set so this haploid set has in in equals five this diploid set has also N equals five the number of chromosomes in a complete set or we could write 2n equals 10 to describe this for humans N equals 23 and we are diploid so we have to normally have to M equals 46 chromosomes now one point it's important to make clear is that the concept of ploidy is not used to distinguish between the amounts of DNA in the cell before and after DNA replication so when we look at this cell we can't say that this cell must be haploid and we can decide that because I've drawn it with only three chromosomes it's an odd number so it must be couldn't be diploid and because the three chromosomes that I've drawn are all different lengths so this cell is haploid at this point it's replicated it's DNA but it's still a haploid cell because these aren't homologous chromosomes these two versions these are identical copies and then when the cell divides the daughter cells are still haploid now the most important issue that was that's a terminology boid question but there's very important genetic consequences of sexual reproduction because not only does sexual reproduction alternate us between haploid gametes and diploid all of our bodies but it the process of generating the haploid gametes and then fusing them together to give new diploid cells generates new genetic combinations from the previous generation and these new combinations arise in two ways first we get new combinations of chromosomes but then there's the second process called crossing over whereby even following a single chromosome we see that there's different versions of it than were present in either parent all of this will be discussed in great detail in module 7 for now what you'll learn in this lecture is really all you need to know so we'll consider inheritance as sets of chromosomes coming from your mother and your father again we're pretending there's only five chromosomes for simplicity your mum has two sets of chromosomes that she inherited from her parents a set from her mum and a set from dad I've drawn them as if they were kept separate but in fact they're mixed together in the cell when the egg meets the sperm the chromosomes mixed together and the cell doesn't keep track of which parent they came from again here's a set the two sets in your dad from his mother and his father and again these two sets will have been mixed together in his cells now when your parents produce gametes the eggs and the sperm they do so by the cell division called mitosis and meiosis takes the two combinations in the diploid cell and generates new cells that have only a single set of chromosomes but because the two sets of chromosomes were all mixed up the new set that you get is usually a mixture of chromosomes from each of your maternal grandparents same thing happens with the chromosomes you get from your father they're a mixture of chromosomes from your two paternal grandparents it's a complete set but it's a new complete set with different combinations than in either parent and then when you go on to reproduce your dam its contain new combinations all the two sets that you inherited because again these two sets are mixed together in your cells your body doesn't keep track of the source so your children inherit new combinations of the chromosomes that you got from all four of your grandparents and then they'll get another complete set different combinations of the chromosomes that your partner got from their grandparents now this is all the process are described as process one this is reassortment often called shuffling because it like shuffling the cards in a deck but there's a second process called crossing over to that creates even more genetic variation between the generations so now we're gonna consider just a single chromosome the two copies of it it could be say chromosome 8 the two copies in your mother and the two copies in your father when meiosis produces the gametes from your parents each gamete contains only one copy of chromosome 8 but it's not either copy it's a copy that's a new combination of segments from the two parents what's happened literally is that there have been breaks and joins let's see if I can put them in the right places there have been breaks and joined in the continuity of the chromosome so that the chromosome that you inherit has that information from your grandmother from your grandfather from your grandmother from your grandfather the same is true for the chromosome that you inherit from your father it's a new combination of the information that different versions he got from his two parents and again when you have children your children your gametes will contain new combinations produced by more crossing over so if we were to diagram the crossing over again we'd say well there's a cross nope there's a cross over there and there's a cross over so right now there's a cross over there never mind you can draw it for yourself um in the same way the chromosomes that your children inherit from your partner will also be new combinations of segments of the homologous chromosomes that your partner inherited from their grandparents so this even you can see even in following from your parents to your children there's an enormous amount of new genetic diversity created new combinations of the genetic variation that was present in your grandparents is present in very many new combinations in your children so as I said we'll be discussing this in much more detail later in the course but for now you simply need to understand that sexual reproduction changes the number of sets of chromosomes the ploidy of our cells so that our gametes have a single set of chromosomes and our body tissues have two sets of chromosomes and you need to realize that we generate a lot of genetic variation just by shuffling the variation that exists into new combinations both by Rhea sorting the chromosomes into new combinations and by breaking up the combinations within each chromosome by crossing over between the parental chromosomes so that's really all that you need to understand the mechanisms can wait coming up next we're now ready to think about genetic variation in populations which is going to bring us back to the issue of how different are we and did we really have sex with Neanderthals so I hope to see you there