here we're going to look at four examples of types of inheritance that don't match normal mendelian genetics so we kind of call these non-mendelian genetics all right so our first one is the example of incomplete dominance so uh we take our p generation we're gonna use chickens as our example uh and these i think are and illusion chickens anyway so if we have a true breeding or pure breeding black chicken and a pure breeding white chicken and we cross them we find that all of the f1 are heterozygous so here in the case of incomplete dominance the heterozygote offspring actually is like a blending of the two parents so this if this followed normal mendelian genetics with complete dominance let's pretend uh black feathers were completely dominant then all of the f1 would have black feathers but that is not the case instead they're all a mixture of the two and if we cross two f1 heterozygote chickens with each other what we find in that phenotypic ratio in the f2 generation is a one to two to one we get one black chicken two like grayish blue chickens and one white chicken so this type of inheritance uh of incomplete dominance is where neither allele is completely dominant over the other and the heterozygote is the result of like a blending of the two other or of the two alleles um it's like an intermediate form of the two parents or of the two um pure breeding or true red lines now this is similar but also different than codominance it's similar because the f1 generation the heterozygotes do appear different than what you would expect under mendelian again if it was mendelian genetics you would expect the f1 a chicken to like have black feathers but instead in co-dominance the f1 have both traits expressed so here we see a chicken with both black and white feathers and if you cross two um heterozygotes again you see that there are one to two to one ratio with the heterozygotes expressing both of the alleles now you can remember this because co the prefix co means together like co-worker collaborate co-captain like they work together or they show up together uh okay so let's go ahead and see an example so here we have two um like generation setups with flowers and what i want you to do is think to yourself which one is incomplete dominant and which one is co-dominant all right all right so over here on the left we have incomplete dominance we can see how that f1 is like a blending of the two parents or of the two pure breeding genotypes and on the right we have codominant where the heterozygote has both um phenotypes expressed okay so let's go ahead and see this example of codominance with another new idea called multiple alleles so blood types in humans come in a couple different options so we have blood type a and what i want to point out is when we talk about traits traits are based on proteins that you make so in people with the genes for blood type a the protein they make are represented by these blue spikes on this blood cell versus a person with blood type o has like i guess a mutation where they don't make any of these proteins they don't have any proteins on the surface of the red blood cells so it turns out that having type o blood is actually like homozygous recessive um both copies of their of their gene do not have the directions on how to make proteins whereas type a blood is dominant over o so a person with type a blood could be homozygous dominant for a or they could be heterozygous as long as they have one copy of that a allele they will make the a proteins found on the surface of the red blood cell now a person though who is titan b has the dna directions to make protein b on the surface of the red blood cells but protein b or gene b is also dominant over type o so as long as a person had one copy of that gene coding for the b protein they would have type b blood now um here comes in the codominance aspect if a person inherits one chromosome that says have type a and one chromosome that says type b each chromosome codes for their own proteins so the chromosome carrying the gene for a will code for a proteins and the chromosome carrying the gene for b will code for b proteins so both proteins show up together this is an example of codominance not incomplete dominance because they're both expressed in the phenotype so here when we look at this blood type trait it is different than what we saw with mendel in mendel luckily for him and him figuring out the patterns of inheritance and how traits were inherited his traits with pea plants only came in two possible alleles either purple or white flowers yellow or green peas but in reality traits can have multiple alleles i hear with blood type there's three alleles possible but as a diploid organism we only inherit two so multiple alleles means there could be more than two options out there um and it just depends on which two you get that will determine your phenotype now our last example of non-mendelian oh just king here's a punnett square to show blood type and how you would do a cross for it so let's take a blood type a and a blood type b a couple and uh cross them to see what are possible in their offspring so let's pretend they're heterozygous for type a so they have type a blood but they carry the allele for type o same thing with our type b blood let's say they have the dominant b allele but also the recessive o so when we look at their first possible possibility in offspring this child would ex exhibit co-dominance and have type a b blood and then um they could have a child with type a a child with type b or a child with type o all four possibilities are possible between these two parents so this dna here would code for both a and b proteins only a proteins only b proteins or no proteins okay great job and now our last term to use that falls under the non-mendelian genetics category is polygenic inheritance i want you to think about this word poly poly means many and genic is genes so here we are looking at a single trait that is due to many genes so here is a a human karyotype and let's say we have gene a gene b and gene c all located at different loci throughout the chromosomes throughout the genome so we have multiple genes or polygenic here so protein emember genes code for proteins so proteins from gene a proteins from gene b and proteins from gene c are all going to work together to determine the single trait of skin color in humans so proteins from multiple different genes all lead to the beautiful variety that we can see in human skin colors and it all depends on which combinations of these alleles a person inherits that will determine their skin color so here if you have two parents that are heterozygous for both traits i mean heterozygous for all of the genes involved in skin color this right here represents all of the possible combinations and gametes that this one parent could make and then over here are all the possible combinations that this parent could make so this large punnett square is showing us all the possibilities in phenotype of their offspring so they could have oops i'm still writing they can have an offspring right here with like a lot of melanin produced and their very next child could be this child both of them are possible from these two parents so polygenic inheritance is when many genes influence a single trait and that's it there are a few other non-mendelian things like epistasis and stuff that i'm just not including in this video uh but otherwise great job guys