lean genes versus unlinked genes when we're talking about linked genes versus unlinked genes we're really comparing the relationship between two genes at two different loci when two genes are linked it means that these two genes are on the same chromosomes unlinked genes or non-linked genes means that these two genes are separated and are located on two different chromosomes a figurative example over here gene a and gene b are linked in this picture because they are located on the same chromosome but at different location of the chromosomes to the right is an example of a pair of unlean genes because gene a and gene b are on two different chromosomes and these two chromosomes are non-homologous to each other so now let's look at the first scenario in which the two genes they are unlinked a parent cells over here contains gene a and gene b on two separate chromosomes each chromosome has a homolog that contains a recessive version of gene a and gene b after meiosis we're going to have an equal chance of four different gamuts big a big b big a little b little a big b and little a little b this is because of independent assortment of the homologous chromosomes regardless of crossing over the arrangement of the chromosomes during metaphase 1 of meiosis is random so i could have big a with little b or little a with big b and the chance are random so if you get big a you don't necessarily have to get big b you can get little b together with big a as well so after meiosis a cells might get chromosomes with big a we and pair up with either chromosomes that has big b or little b so now let's do a non-linked dihybrid cross the male is going to produce four of these gametes by random chess with equal probability and the female just like the male is also going to produce four gametes with different genotypes by random chance and with equal probability and i lists are all the results of the fertilizations in the punnett square here we're assuming that the ab genes follow a single mendelian dominant recessive inheritance pattern and you're going to get a 9 to 3 to 3 to 1 ratio of for non-linked genes in which nine represents double dominant three represent one dominant one recessive another third represents one recessive one dominant and one represents double recessive this is the ratio when both parents are heterozygous for both genes and the genes are not linked now let's look at situations when both a and b are linked which means they are located on the same chromosome but we're going to assume that there is not going to be any crossing over between the chromatids of the homologous chromosomes during meiosis if the gametes receive a big a it will definitely receive a big b because the big a and big b genes are located on the same chromosomes and there's not going to have any crossing over the four possible gametes after the meiosis you're going to get big a with big b and little a with little b because there's no crossing over the yellow is going to represent the gamete that contains big a and big b genotype together and the green represents little and little b together let's set up the dihybrid cross again this time for gene and gene b with no crossing over but they are linked you can see that because of these possible gametes you're going to get a perfect 3 to 1 ratio 3 out of 4 is going to be double dominance and 1 out of 4 is going to be double recessive phenotype this is because the big a is always associated with big b in the gamut and little a is going to be associated with little b in the gamut because they are in the same package they are connected that's why it's linked and there's no crossing overs so the gap the genes doesn't get exchanged right your gamut choice is only restricted to big a big b or little a little b and of course the fertilization has to be random then the ratio will be 3 to 1. now let's look at the scenario in which both gene a and gym b are linked but crossing over can happen crossing over is a random event in which the homologous chromosome exchange the gene segment but this event happens quite rarely so most of the time you're still going to get big a with big b and little a with little b this is most of the time but during but when crossing over do happen the big a allele will be on the same chromatids as the little b and the big b will be on the same chromatid as the little a right because the gene segment for the chromosomes in between the homologous chromosomes are going to flip around like this so now the big a could be with big little b but if there's no crossing over that big a will be still associated with big b and the little a will be associated with little b and those that does have crossing over they're going to have big a with little b or little a with big b this occurs rarely so you have low quantity so now let's look at the dihybrid cross between two heterozygous for both gene a and gene b most of the male are going to have the gamut that is big a big b or little a little b very little quantity will be of the recombinant game gametes same as female during meiosis they're going to have a majority of the gametes being big a big b or little a little b with rare chance they're going to get big a little b and little a big b games so we're going to settle the punnett square and do fertilization a majority of the offspring are going to be double dominant and double recessive with the green being the offspring that contains the phenotype with double dominance for gene engine b and yellow being double recessive but if the male and female do have recombinant gametes right and if you combine the recombinant gametes through the process of fertilization you will still occasionally get a double dominant phenotype with being marked as green but notice that another pattern will occur in which you have one of the gene expressing dominant phenotype and another gene expressing recessive phenotype in here the pink is marked as gene a being dominant and gene b being recessive phenotype and for the purple it marks gene a being the recessive phenotype and gene b being the dominant phenotype so the number of this offspring is a result from the recombinant gamuts and the frequency in which observe all these recombinant gamuts will be very low as compared to the non-recombinant offspring so if we look at the double dominant phenotype to the double recessive phenotype the ratio will be close to three to one it's not exactly three to one but it's close to three to one but if you compare all four phenotypes right you compare the green to the yellow to the purple and to the pink you'll notice that the pink and the purple because they come from the recombinant gamuts that occurs rarely you rarely will get the single dominant recessive recombinant offspring so if you observe this kind of ratio coming out from a dihybrid cross between two genes that are linked with crossing over then the ratio should be something like 3 2 1 to very low number and to very low number of single dominant recessive phenotype with that i hope i clarified the idea between linked jeans and non-linked jeans have a good day