IGCSE BIOLOGY - DICHOTOMOUS KEY

welcome to the biology section of ego's edgy concept YouTube channel in this video we want to look at the dichotomous Keys dichotomous keys are keys that are used to identify organisms based on a series of questions about their features dichotomus means branching into two and it leads to it leads the user through to the name of the organism by giving two descriptions at a time and asking them to choose each choice leads the user onto another two descriptions in order to successfully navigate a key you need to pick a single organism to start with and follow the statement from the beginning until you find the name you then pick another organism and start the beginning of the key again repeating until all organisms are named the general example of a diomas Kei is a as shown here okay so this is a general example of a d masin now I know what has been said so far might not make uh a lot more sense and relatable but I promise you as we look through the three examples two of which are direct uh extract from the igc igcc final examinations you will definitely understand and you never fail to answer any question that involves the diet kotas Keys now this is just the general thing you can pause and go through the instructions again so I'll be re explaining these things as I go through the examples so take note we have said die here die means two so that's why they will always be giving two sets of rules okay two sets of rules when one is valid you follow it which which can also Branch into two until you find what you're looking for so for example in here at this stage there were two sets of of roots that were given not fur and then one with fur so if something had fur then it was going to be this animal there if it did not have fair then you need to go ahead and ask yourself does it have feathers if it has feathers then it could be this thing here if it has got no feathers there are other two options does it have a dry skin or does it have a moist skin okay all right so those that is how you'll be questioning yourself so at this point where you're saying dry skin moist skin so dry skin it could have been this one if it was moist skin you go ahead and ask yourself two questions and you you find the answers you identify all the creatures that have been given to you so let's look at these examples I'll start with the very basic one and then we go to the examination ones so for instance here we are saying the first example here they've given us these are the uh organisms that they want us to identify a uh B c d and e so we want to identify them and usually they will give you the dichotomous key and then you have somewhere to fill in as you see from the other two exam standard examples so this is just for us to get started and have an idea so it is a very basic one so for example here it says we are starting with this one so we are going to choose a we want to find out what a is so you look at a you can tell the features of a by looking at it so you look at a now here it says jointed limbs do you think a has got jointed limbs now if it has got jointed limbs then you are supposed to proceed to two but if it has no jointed limbs then you have found its name so meaning a doesn't have any jointed limbs here as we can see meaning you have found it it's an ethm okay so meaning under a you have found it's an ethm then you go to B let's look at B so B is this guy here so jointed limbs does this have jointed limbs I can see all these limbs here all these all these in the Sid these are jointed limbs so jointed limbs so if it has jointed limbs then we go to number two here so number two they are saying more than five pairs of jointed limbs so if it has more than five pairs of jointed limbs then the name is going to be stiped if it has got five or fewer pairs of jointed limbs then we proceed to where to three but here where we read we check we have it has jointed lims but how many they are quite a lot 1 2 3 4 5 6 7 so without counting all of them I can confidently say it has got more than five so since it has got more than five then I know it's a centipede then again I'll choose C so every time you're trying to identify an organism you begin from one you don't just begin from anywhere from one the first instruction so C jointed limbs okay I can see joint jointed limbs yes so if it has got jointed limbs then I should go to number two then if it has got more than five pairs I don't think it has five pairs because all I can see it has got one pair here then it has got two the front ones and then there's this one here and the one which is behind so it only has three so it will fall into the category which ISS five or fewer pairs of jointed limbs yes so I'll go to number three so three they saying first pair of Limbs form large uh claes which it doesn't have any CLA this guy okay so no no large clothes we go to four we go to four so we go to four here four it says three pairs of Limbs it's a locust of which when we counted the pairs here we found three so we have identified it to be a locust then we go to D so D again jointed limbs yes we go to two it says five or fewer pairs of jointed limbs yes it has got five or fewer then if we say yes we go to three then three it says first pair of Limbs form large cloths which is true we can see the clo here you can see the close here okay so we have found this guy to be the crab so that is how we are finding locast crab and then we look at now e so we are on E we start again from one jointed limbs yes we go to two more than five pairs okay so let's look at the pairs first of all so I can see this pair here this is one then this this one here and this one that is two then this one here and that one that is three we have a question mark on this one whether they are limbs or not but let's just say for the sake of piece there are now four so 1 2 3 four okay for Simplicity sake so there are four four pairs here so now we start jointed limbs yes we go to two it says more than five pairs no so five or fewer pairs which is three so we proceed uh uh from from this one we go to three then it says first pair of Limbs form large claws okay no we go so no large cloth we go to four so four here we say four pairs of Limbs it's a spider of which we've counted four so that is how you deal with the dichotomous key because of these instructions we have found all the animals that we are supposed to find whether they are animals whatever these creatures are but we've used a dichotomous to find them so that is earth worm cped Locust crab and spider now you can also make your own dichotomous key or you have to do is just make sure they are paired two two two sets of characteristics if it's this one then it's that one if it's not you proceed to this one so you keep giving two pairs but that is a discussion for another session let's look at a sample exam question this is a direct extract from one of the exam papers the November Series so it says figure 6.1 shows sections of flowers from six different plant species so a b c d up to F and then it says use the key to identify each species write the letters of each species a to F in the correct box in the key so this is the key that we've been given so how can we proceed now to find these an so you pick each species one by one you find it and that's when you Pro proceed to the next so here we are beginning with uh a strategy so the strategy that you need here if you don't know about the various parts of the flower then you are gone you need to know the various parts of the flower that's when you'll be able to answer these question so our advice pause look at the various parts and then you resume back to the qu the questions okay very important so let's look at a we are looking at a so we start we are we are on this one so it says two petal visible do we see two petals I can see one here I can see two so yes two petals are visible okay I can see two petal are visible then we go to two so one stigma the flower yes I can only see one stigma here from there all the way so I can see one stigma here so one stigma flower then I'm going to three okay then I'm going to three so three here so go to three so three is here so three here it says stigma higher than anther of which we can see the stigma here is higher than all the aners so here where it says stigma higher than answer meaning the name name is this one here which is prunas domestica but they've told us to write the letters so the letter a is the one which you're going to put in here okay so the letter a is what you're going to put in here so you've identified that one okay that is a then we go to B so a done then we go to B now so B again you start from the beginning it says two P visible go to two of which do we have two P I can see this one I can also see this other one here so those are two petals visible yes okay two petals visible so we go to two two it says one stigma per flower okay one stigma per flower so one stigma per flower yes uh it may be confusing at the bottom here but a stigma it begins from all the way it contains um so this one here and then if we follow uh this one here so you find that this is the part here so yes it has got one stigma per flower okay so let me see now uh here we went first of say one stigma per flower so it it is taking us to three so we go to three it says stigma higher than answer no as we can see this stigma there are other aners that are higher so no then we say stigma at the same level or lower than an then this one will give us what will give us B so we have also found this one so ladies and gentlemen you just need to pay particular attention to these ones for those of you that have done subjects like special paper one special paper two at ecz it's one and the same thing there's nothing to memorize here but just to follow through the instructions and having a background knowledge of the subject content okay so we have done that one so that was a and then that was B and then we go to C okay so we have C here now okay so we have our C uh C we have one two and three petal so here it says two petal visible uh go to two so we are not dealing with a two petal visible we are dealing with three petals visible so three petal visible then we go to four okay so we skip all the way we go to four then it says two OV now two OV does it have two OV let's see okay so if you look at this one here in here and the other side here it seems like something that looks like two petals so uh two p uh two OV visible we go to five so the one I've just put this one and this one it looks like two yes so we go to five so let's look at uh let's look at what five says so five says answer smaller than Capel now you need to know that the Capel is everything from the ovary the stamin the filament and all this all this C thing here so it says it's smaller than okay it's smaller than so this one is smaller than so answer is smaller than the cap as we can see this thing is actually bigger so meaning our c is going to be in this box here and that is how we find our C we proceed uh we go to D okay we proceed we go to D so D I can see this one is h p and that one is a p so two p visible yes they are and then we go to so that was D and then we go to two so two it says one stigma per flower um yes there is actually one stigma flow because there's just one here which is connected all the way there it is so just one stigma per flow so one stigma per flow we go to three so when we come to uh sorry sorry sorry sorry so before we do that uh before we do that let's just check again so it says two petals go to two so we first go to two it says uh it says one stigma per flower uh I think this one is not exactly one stigma there's more than one stigma per flower so this one should be D because I can see there is this part here and there's also another part behind here so there are they are like two stigmas here so that one we going to get ourselves a D in here meaning it's more than one stigma per flower so that is a D in here uh we can proceed to e so e this one has been done we can proceed to e so e says uh here as you can see I think we can see one two and three petals so we going to start uh where they um we're going to start where they are three so three petals visible then we go to four so when we go to four when we go to four they are saying two OB uh uh so let's see to four they are saying two OBS visible then we go to uh five two OBS visible we go to five so I can see two OBS uh there and there so two ofs visible then we go to five so they are saying um uh two OBS visible then this other one they are saying more than two OBS visible let's let's count again let me just rrap this one so you really need to pay attention as we are dealing with these so let's just count uh properly if I told there two of you so I think there is one two here three four i i i coupled these two here so there are two here and two here so if we go here to four it says two OBS visible no I think there are not two there are more than two OBS forgot about this one meaning this one will be our e in here okay so that will be our e we have found it so you put your e in there and then you go to F so for those that feel lazy we have done everything there's only one box that is remaining which is the last one so you can even just write that one or you go through due process you start with again it says two P Vis two P visible no I can see one uh two three so three petal visible is where we're starting from and then we go to four so four it say is two OV yes there are two OV there's this one on the left and this one on the right so if two OV are visible then we go to five then it says answer large larger than Capel so you can see the aners here these aners and this one they are larger than this whole part which is here that is the cap so that is how you get your F and ladies and gentlemen that's how you do your dichotomus key but just in case you missed it with the flowers let's see if we can look at uh this other example here also an extract from the exam uh it says the student made care of drawings of seven types of alga found in the sample of Lake water the drawings are shown as here so again let's identify them using this dichotomous key here okay let's identify them using this dichotomus key here so here it says uh we start with a so you always start with one and run through with it until you discover what it is exactly so you follow it so alga has four spikes like extensions now this one doesn't have any spikes you might be tempted to think these are spikes these are not spikes okay so these are not spikes they are just extensions so we are not going with this route we are going with this one alga does not have four like extension so we go to three so we go to three alga has a round shape no alga this a does not have a round shape alga does not have a round shape we go to four so we go to four it says alga Narrows towards the ends no it doesn't so we follow this one alga does not narrow towards the end so we go to Six alga is Branched is alga Branched yes we have these branches here so this one is where we are going to put our a okay so this is our a alga is Branched you put it in the appropriate here you go to the B so B here it says uh we go to B it says alga has four spikes like extensions no it doesn't so again we are following this rout alga does not have four uh spikes like extension we go to three so alga has a round shape alga has a round shape so can we put our B here can we put our B here so yes because it has a round shape alga has a round shape so yes um it actually has a round shape so our B is actually going to be here we proceed so that is where our B is we proceed let's look at C so alga has four Spike like extensions no it doesn't alga does not have so we go to three so three alga has a round shape no alga does not have a round shape alga so we go to four since alga does not have a round shape we go to four then it says alga Narrows towards the end no alga does not have narrow ends yes so we go to six so alga is Branched so when we look at this one alga is not branched so this one it says alga is not branched so if alga is not um so we looking at C here okay uh we looking at C let's just check properly uh this this one here whatever shape it is okay let's let's just uh go through this one uh that is the thing you really have to pay attention to exactly what you are doing okay uh so let's let's go again so it says AA has four spikes like we know it doesn't have so alga does not have so let's go to three again alga has a round shape no we know that it doesn't have a round shape this one here we looking at C then alga does not have a round shape so we go to four yeah we had skipped four so alga Narrows towards the ends actually it doesn't so alga does not uh does not narrow so now we are going to H so let's see alga Narrows towards the end okay let's see here does it narrow do do you think this is a narrowing uh do you think this is a narrowing um okay yes let's assume it's narrowing because it's coming and then there's this Bend here so it's it's narrowing so alga uh so we are saying uh we go to five so let's go to five uh Alo cell is semicircular in shape now it's not semicircular actually so alga cell is not semicircular in shape so this this box at the end this is where we can find our C so that is where we can find our C then we go to this D so D will also follow the same thing because we know does not have four Spike like extension so we go to three alga has a round shape okay it doesn't have uh so let's see alga has a round shape okay we assume alga does not have a round shap so here alga Narrows towards the end uh yes it does okay alga Narrows towards uh the end uh so let's see yes it does if we go to Five Al is semicircular it's clear this is semicircular so that is a d there we put our D in there then we go to our e okay so we go to our uh our e so e it's this this guy here they're saying alga has four uh has four spine like extensions no it doesn't have so we go to three so three they saying alga has a round shape no it doesn't have so alga does not have a round shape let's go to four alga Narrows towards the ends no alga does not narrow so we go to Six we follow this one so in six alga is Branched no alga is not branched so this is where we're going to put our e because alga is not branched okay so we continue we go to F so far we have found uh those ones there okay so we have found those ones so let's look at F let's look at F now here they are saying Al for uh alga has uh so alga has four spine like extensions we go to two yes it has 1 2 3 4 so 1 2 3 4 yes it does so we go to two so two it says alga has spine like extensions that are all the same length no they are not so because these These are different lengths so we go to this one Al spying like extensions that are different length yes so this one is likely to be F okay then let's let's go to G G says a has four Spike like so we have this Spike that Spike that Spike that Spike and these are all the same so alga has four Spike like extensions we go to two alga has Spike like extensions that are all the same length so this is the one because they are all of the same length and we are found for all of them now so ladies and gentlemen that is how you use a dichotomous key it's just about observing the features and following the rules and you have to do that for all the species that are provided before you thank you so much for visiting our Channel please keep liking it subscribe and keep visiting our channel for more information on STEM related content following the Cambridge International syllabus thank you so much bye-bye