hey bye 30s I'm back again for another lesson to start I want to do a little quick experiment right there so what I need you to do is take both hands and simply just put them together like this okay now take my heart let's do that again but really pay attention to what thumb goes on top so we'll do it one more time here what thumb goes on top for me it's always my left thumb the left thumb feels the most comfortable okay bring apart let's do it one more time this time though try putting the opposite thumb on top so we're gonna do it one more time here the right thumb it doesn't feel quite right left thumb just feels about best for me but for you it might be different so that is actually a genetic trait that you inherit through your chromosomes in your DNA now putting your left thumb on top is seen as the dominant version of that trait and the right thumb on top is the recessive version of that trait so here's a little trick there that you can see if you're either a dominant or recessive for that trait so leading it to the lesson today we're going to talk about dihybrid crosses we've done some crosses looking at one singular trait we're now going to look at a cross that includes two traits all the rules still apply but now we're going to do a bigger punnett square because of it so getting back to Mendel and his laws his laws of heredity were pretty simple right they said that or he said that traits are inherited from your parents okay they're controlled by genes and they come in pairs right one for mom and one for bed and then how they interact with one another can lead to the idea of dominance or incomplete dominance or codominant so in some cases one gene will mask the other this leads into the law of dominance where one allele or one trait will mask the other in the head Tyga's genotype so the heterozygous genotype if we have one big art when little R in this case per round or wrinkled seeds if you were heterozygous you would have brown seeds assuming that it was complete dominance so for example we mix one homozygous dominant with one homozygous recessive we have all of the individuals date hybrids and these hybrids all the brown seeds why because the round seed allele masks or dominates the wrinkled ceiling wrinkled seed allele okay his second law was the law of segregation so he came up with the law of segregation okay basically what it says is during the formation of eggs and sperm the alleles will separate okay so think back to when we talked about meiosis right the chromosomes or the alleles separate during meiosis so we have 23 going over here to this sperm 23 over here going to this verb or 23 over here to this secondary oocytes and 23 over here to this first polar body so they separate or they segregate so this individual that is a hybrid could either donate a big R allele or a little R allele we don't know which one they gave and if you are same thing over here they can donate a big R and they could go into a little art you don't know which one they gave and then when they recombine this is how we get our variety of traits so maybe two big R's combined maybe but big R in a little art maybe a big R in a little R or maybe a little R little art huh and that gets me a ratio and if you remember from the previous lesson what would that ratio be a 1 to 2 to 1 ratio so law of segregation simply during meiosis the chromosomes will say great or separate which means you're getting one allele from each parent so how do we apply that what we apply it in the punnett square right so we have let's say for example tall or short female parent being a heterozygous individual could donate tall allele or the short allele once again we're not sure which one donate until actual fertilization occurs male parent they can donate the tall allele or the short allele and now we have 75% chance that the individual is tall 25% chance that the individual is short of course our genotype ratio being one to two to one our phenotype ratio being three to one okay and in the final law I want to talk about here's the law of independent assortment this is very important basically the alleles for different traits assort themselves independently of one another and this is what leads into our dihybrid cross discussion today what does this mean it means your eye color has no connection to your hair color right they assort themselves independently so you could be dominant for eye color but recessive for hair color you could be dominant for your earlobes or you could be recessive for the length of your fingernails all of these traits are independent of one another now that's not saying all traits are independent some traits actually are found on the same chromosome and they link together so for example you might see someone with say red hair freckles and fair skin those traits tend to be linked together on the same chromosome but usually when we're looking at crowd we're looking at traits especially on different chromosomes they are independent one another so whatever happens on one trait does not impact the other training that's the law of independent assortment right so when we look at how chromosomes are sort themselves independently we can have ourselves going through meiosis one and then of course meiosis two and when we look at this right this cell right here is a big G in a big one this one is a big gene a big Y but this one is a little G little Y ability little light and then so on and so forth so what you get for the G trait does not impact what you get for the light rate so they assort themselves independent of one another these two traits are completely different they would have no linkage whatsoever so this of course leads into dihybrid crosses we were looking at two traits of sorting themselves independently and the locator on different locations or loci all the new chromosomes so same laws apply as a monohybrid cross for this diagur cross so let's get into it what are we gonna do here well let's say we're going to cross yellow seeds that are round with green seeds that are wrinkled all right yellow being the dominant trait and round being the dominant trait so let me take a look at this here we have a homozygous individual for both traits homozygous individual for both traits double dominant double recessive well it's a pretty easy cross because we know they're all going to be hybrid for color and they're all gonna be hybrid for shape so we end up having all of our individuals being yellow around that's a pretty simple cross right we know that when we mix two individuals one homozygous dominant one homozygous recessive they end up having all hybrid f1 individuals now let's do an f1 cross let's see what happens when you would have four girls okay when we do an f1 cross this is where things get a little complicated so I'm gonna break this down into steps for you step one you want to write out your allele symbols now I don't think you need to do step one but if you're having trouble organizing these problems I recommend you do step one so what I've done here is I've wrote out my symbols or a little legend for myself so I know that big R is around little R is wrinkled big Y is yellow little Y is green and remember these traits have nothing to do with each other so oftentimes I'll see people writing genotypes out like this this is wrong okay those traits don't be directive one another to write a proper genotype you must do this you have to keep the two alleles together so that's correct so step one write at a legend if you're having difficulties organizing your question step two you want to write out the genotypes out of your parents so that's what I've done here already so genotypes of my parents once again we're doing an f1 cross so is the genotype of parent one and the genotype of parent two hybrid for round hybrid for yellow hybrid for round hybrid for yellow step three write out your allele combinations for the Gammy's these are not genotypes these are gametes meaning they have essentially half the genotype so let's think about this okay this individual right here could donate a big arm they could also donate a big wide so that's one possible combination that they could have if you donate a big R and a big one they can also donate a big R and a little Y so it's a second possible combination we could have they could donate a little R and a Big Y that's a third possible combination and they could also donate a little R and a little wide so an individual that is heterozygous for both traits would have four possible gamete formation big R big lie big R little ID little R Big Y little R little Y and this individual would have the same gamete combinations so we need to make sure that we're writing out all of the possibilities right all of the possibilities all right let's set up a punnett square it's a little larger than normal why because we need to put all of our possibilities on the top and the side that means we have 1 2 3 4 possibilities here 1 2 3 4 possibilities here instead of having a 2 by 2 punnett square we now have a four by four punnett square giving us 16 boxes in total now we would fill it in just like we normally would so for example we match these two up they'd our big R big Y big wide so this individual would be round seeds yellow color over here big R loops big R big R and Big Y little Y this two would be round seeds yellow color so I'm not going to do all of it for you already done it here we fill in all the genotypes and then afterwards we can get our phenotype ratio genotype ratio is going to be a little difficult here so I'm just focusing on the phenotype ratio if we look at the phenotype ratio I'll do this in different colors here you'll notice that for rowdy yellow so rounding yellow we should have nine individuals let's count up nine individuals that are rather yellow okay this individual right here round yellow brown and yellow brown and yellow and brown and yellow there's four okay this one right here round and yellow yep this one nope too little wise to green use that wouldn't count there's another one this one would also be green so that doesn't count this one's good this one's good no good no good this one's good that one's no good that one's no good and now it's all good so now we have one two three four five six seven eight nine individuals that are dominant for both traits rounded yellow okay what about brown and green let's take a look at rounded green here there should be three individuals that are round in green so we need at least one big R and two little wise there's one there's two and there's three there are three round and green individuals okay let's move on to our wrinkled and yellow so wrinkled and yellow so this would be two little arms and one big wide right there right there and right there and finally we only have out of 16 there's the waste side there so I'm 16 we only have one individual right here that is wrinkled and green so double recessive so one is 16 chat so very unlikely but still possible so we look at it all together here nine individuals having the double dominant phenotype so brown and yellow three individuals being green and brown three individuals being yellow and wrinkled and one individual down therapy degree in wrinkled so once again all the same rules apply we just have to make sure that we're writing on all of our genotype or sorry all of our genotype combinations leading to our allele combinations we put them into our punnett square we match up our genotypes and then from there we can get a phenotype ratio now remember I said that the one to two to one ratio was one of the ones you want to remember this is another ratio that you want to remember as well very common ratio what we're doing dihybrid crosses a nine to three to three to one ratio once again this is a phenotype ratio not a genotype ratio so that's all I have for you today next lesson we'll be getting into traits on the X chromosome so we're gonna look at x-linked traits and we'll talk a little bit about how males and females differ because of it so thanks again for watching and we'll chat soon