welcome biologists and in today's session we're going to be looking at specification point b from patterns of inheritance now this is quite a big specification point so today we're just going to be focusing in on this monogenetic inheritance so mono means one and in this particular instance we're looking at one gene and the inheritance of one particular gene so what we're looking at here is some seeds and the in particular peas actually so we're looking to see here if i can what i will get from the cross of a homozygous dominant round pea plant and also a homozygous recessive wrinkled individual so the parental phenotypes here are going to be round and wrinkled now in an exam this is what they are looking for so i've got the genotype here at the top and what i would then need to do is identify the gametes that are present so the gametes obviously like in in this case of a plant it would be the eggs and also the pollen um so the gametes produced here from this individual here i can only get a large r gamete so there we are for that individual from this individual here the ring called individual i'm only going to get a small r that's the only gamete that i could get um now the genotype of my f1 generation would be determined from doing a punnett square but just to show you where i'm going to get all these from um you could also work out like this however you're not going to get max news exam for showing it this way and now as you can see all my individuals here all the offspring would have a big r and a smaller and because the big r which codes for round is dominant over the small r it means all the individuals here are going to be round um okay so if i did the cross in a punnett square which is what you get marks for your exam for showing this so i'll be doing the bigger and then the smaller and then again the same for this one bigger and smaller and the same for the bottom two so again as you can see here i'm just doing this cross and i'm finding that all individuals here i've got a big r and a smaller so therefore i have a 100 round offspring of my f1 generation now quite often so i've covered all those points there now quite often what they ask you to do is then cross two individuals from the f1 generation to create an f2 generation so my parental genotypes would be a big r and small r each they're both be round individuals the gametes produced by each are a bigger and a smaller each and i put them into my punnett square to do my cross so i've got a big arrow big r together i've got a round individual there a big iron a small r here again another round individual you'd put these are heterozygous now another heterozygous individual here but again still round and then i've got a homozygous recessive so this individually would be wrinkled so my phenotypes here for my offspring i've got a round round and a wrinkled one so this means my f2 generation ratio is going to give me a three to one phenotypic ratio whereby i have three round to one wrinkled it's really important you say what the numbers are representing as well not just given a three to one but you're actually saying which which is three and which phenotype is the one and if i was looking at a genotypic ratio however it'd be a one to two to one now a genotypic ratio is very rarely asked for in an exam it's normally always the phenotypic rational phenotype is how the offspring appears to be so in this case round or wrinkled a genotypic ratio is more to do with the actual letters involves the actual genes involved and the alleles involved there so that's monogenetic inheritance in the next video we'll look at dihybrid inheritance so monogenetic is one gene and dihybrid is two genes looking at the crossing of two genes in particular so guys in your exam please remember that box that we need to have we need to include all of those bits of information here in each of your exam questions so please make sure you've got those in each rigsum questions to access all the marks so even if you get the wrong ratio at the end if you've done all of these points here you may pick up marks for you working out so please please please make sure you include that and good luck with your exams