by the end of this video you're gonna have a full understanding of dihybrid crosses this is a process that will allow us to track the inheritance of two traits at once if that sounds like a lot don't worry I'm gonna walk you through the entire thing and you'll be a pro in no time but before we get to dihybrid crosses we have to brush up on a few key Concepts first this genotype this refers to the genetic makeup of an organism then there's phenotype this refers to the physical expression of those genes for example in pea plants we have different genotypes for the height of the pea plant with the big T allele coating for tall and a little tlu coating for short we also have to understand the concept of dominant versus recessive in our example Big T is a dominant trait and Little T is recessive that means that once we have the presence of the dominant allele that trait is a one that's going to be expressed physically and if the recessive allele is present its presence is going to be massed by the presence of the dominant allele and as a reminder the possible options for genotypes are Big T Big T homozygous dominant Big T Little T heterozygous and Little T Little T homozygous recessive we also need to understand what a Punnett square is it's a simple and really useful tool that allows us to predict the possible genetic outcomes of a cross between two parents each Square in a Punnett Square represents a possible genotype for The Offspring now if any of that sounds confusing please check out my previous video I cover it in full detail it'll be clear after that I'll link to that in the description below in this video we're going to be using a Punnett Square again but this time to explore dihybrid crosses so what is a dihybrid cross it's a breeding experiment that tracks the inheritance of two traits simultaneously in my previous video video we looked at a monohybrid cross that only studied one trait the height of the pea plant for example in this video we're sticking with our trusty pea plant but we're going to look at two traits height and flower color once again the height can be tall or short now the flower color that can be purple or white and for that trade purple is going to be the dominant one and white is recessive so we use big p for the dominant purple allele and Little P for the recessive white allele now just a note the the letter that we use is usually the first letter of the dominant trait okay so let's start with the first cross we're going to start with a parent generation with one parent plant being tall with purple flowers and remember those are the two dominant traits so the genotype will be Big T Big T big p big p for the other parent we're gonna go with all recessive so it'll be short with white flowers so little T Little T Little P Little P I know I might sound weird and I'm saying big and little instead of capital and lowercase but it's easier to say and I grew up seeing it so that's what we're doing big up to Saint Martin yeah so they're both homozygous for both traits meaning they have two of the same types of alleles for each trade with me so far awesome now when we cross the parent plants the result is pretty simple we don't really need to do a Punnett square for that we can if we want but it's more work for no additional benefit the result is pretty simple since we have one parent with all dominant alleles and one parent with all recessive alleles all of The Offspring in the resulting F1 generation they're gonna be heterozygous so that's Big T Little T big p Little P these plants get one of each allele from each parent so it makes sense and since tall and purple flowers are the dominant traits these plants will all be tall with purple flowers now let's get to our Punnett Square the Punnett square for our our dihybrid cross is going to be similar to that of our monohybrid cross but instead of being two by two it'll be four by four giving us 16 different combinations now for each parent we'll list out the possible gamete combinations we can have big T big p Big T Little P Little T big p and Little T Little P we're going to take these options and put them at the top for one parent and also at the side for the other parent now for each box within the Punnett Square we're gonna bring down the alleles from the parent at the top and bring over the alleles from the parent at the side this gives us all the possible genotypes of The Offspring that could result from this combination of gametes and when we tally up the phenotypic results you're gonna see that out of the 16 plans on average nine will be tall with purple flowers three will be tall with white flowers three will be short with purple flowers and one will be short with with white flowers that gives us a ratio of nine to three to three to one and that's what we see with dihybrid crosses so to bring this all home a dihybrid cross if we're starting with homozygous parents one that's dominant for both traits and one that's recessive for both traits it's gonna yield a heterozygous F1 generation when we cross that F1 generation we're gonna get a ratio of nine to three to three to one and you can always use a Punnett Square to determine what the genotypes and the phenotypes will be now in the next video we're going to look at sex-linked traits and genetic disorders to understand how certain traits and disorders are passed down through generations so make sure to check that one out next my name is Leslie Samuel from interactive biology where we're making biology fun that's it for this video and I'll see you in the next one