I haven't been here in a long time... Okay, I’m having a ‘debate’ with my biology teacher. It’s over Mendel’s theory of the alleles and dominance and all that good stuff. When she began to talk about the whole Incomplete Dominance thing, I had a question. Does it only have to do with the Dominant allele (i.e. RB)? Why can’t two recessive alleles combine? I’m not talking about a homologous recessive (i.e. rr), but two different colors. There are four color mice: Black and White, Grey and Brown. The black and white are controlled by the letters BB and bb. Grey and brown are controlled by GG and gg. When a homologous Black and a homologous Grey come together, you get BG. This is a dark grey mouse. That is Incomplete Dominance. What I asked was if a white, bb, and brown, gg, came together, would the genotype be bg? That too is Incomplete Dominance, no? Well, I asked that, and she replied “No, because it doesn’t work like that.” I asked why. She said “Just because.” You know, the reply from when you were five. I thought she didn’t get what I was asking. I tried again, in a different way. 1) Why can’t it happen and 2) what will happen if it does. Her argument was because it says incomplete DOMINANCE it can’t involve different recessive alleles in it. I couldn’t get a straight answer out of her. They CAN breed, but they won’t have either color apparently. Well, what color could it be? If it isn’t brown, and it isn’t white, then what is it? Can they not breed? Another thing is, when she tried to explain it, she said something about Mendel’s flowers (One red, one white) that came together to form a pink flower. I asked if two other colored flowers bred, and produced purple, if it was possible. She said yes. I asked if a pink and purple flower mixed, what would happen. She said that was impossible. I asked why. She never said. At this point I had not gotten a real answer, and was starting to get annoyed. The ENTIRE class was agreeing with me, but she didn’t care. Yeah, that makes sense. So, black (BB) and white (WW) make grey, but purple(pp) and neon(mm) don’t make a color? Thanks for clearing that up! So, I’ve given my logical argument(and if you listen to me you are wrong because I’m wrong ho hum) and she’s given her [il]logical argument(just because). Now, please tell me, which of us is right? Can two recessive alleles make for an incomplete dominance (except with recessive)? Can two hybrids make a new color? If two homologous recessive organisms do NOT mix, what happens then? Ask if you need more clarification.
Your teacher might be right, but I can't be sure. If I'm not mistaken (I could be) incomplete dominance is expressed by two types of only-dominant alleles. That is, when working with RedxWhite=Pink, you're using RxW=RW, but I think it might be CRxCW=CRCW, where C is always dominant. Don't go with my thoughts though, this is just a thought! I, too, frequently involve myself in friendly scientific arguments with my biology teacher. Just last week, I was arguing my case regarding sex-linked inheritance, haha. To anyone that wants to save themself the lengthy read, XxSpix is asking: Can an organism exhibit incomplete dominance (mixed phenotype expression) by the result of heterozygous recessive alleles?
Yes it does, but the real question is does one of those traits take dominance over the over in a preset hierarchy or do they exhibit co-dominance? Good question, I don't know. Makes sense to me that they would form the trait of rb instead of R or B or maybe it's a 50-50 chance which one takes over.
I'm still unsure, but what I was trying to say is that, with incomplete dominance (particularly with the white red pink flower example), there aren't recessive alleles. But now I'm just confusing myself. What you're saying makes sense, but neither my biology book or the internet alludes to the answer at all. Sadface. I'm sure it's SOMEWHERE. Yahoo answers maybe?
Make a punnett square for science's sake! --- AB __ Ab __ aB __ ab AB | Ab | aB | ab ------------------ aabb
Thank god I'm no longer doing biology, this used to confuse the hell out of me. My only recollection of this topic is that hybrids can't combine to make a new hybrid, I don't know why but I think your teachers right on that one.
I don't know much about Biology, but i'm about to finish A-level physics, and from my experience i will tell you that science is full of it. It might sound patronizing when a teacher tells you "that's just the way it works, i can't tell you why", but unfortunately they are true. The exact truth in science is far too ambigious and complex to ever be taught properly in a class. There are parts where it will all make sense and parts where you won't understand their conclusions, but you have to just agree that there is a level at which you don't yet understand where the reasonings lie. The thing that annoys me is when you are taught something at an early age and then later shown how that is not to be true. Why teach it?
Pretty much what Matty said; many cases in Science some stuff just works but we don't know why at present. However in this cases there might actually be a reason for it. Take a normal example of a blood group. In general (ignoring o+ etc) you have IA, IB and Io (I stands for the locus while A/B/o stands for blood type). Depending on the gametes of parents you'll get a varitey of offspring genotypes. Blood groups A and B are dominant while blood group o is reccessive. When you get the Genotype IA IB co-dominace occurs (You called it Incomplete dominace) and so the blood group is AB. When you have a recessive and a dominant then the dominant is expressed and the recessive is not. From what you've said I believe you understand that, I was just running through the basics. When it comes to two reccessive breeding you teacher is right in that it wouldn't happen. This is due to a process known as [Recessive] Epistasis. The Greek word for 'stoppage' it basicalled means that one gene control the expression of a phenotype outside of the 'norm'. In cases like this recessive genes can be expressed regardless of other genes. Basically it means that when a certain set of conditions is met things happen. As a example I'll use flowers. So if there is the presence of a homozygous aa then that will cause the B (for purple) nor the b (for pink) to be expressed unless an A is present. So, what I can gather from what your saying is that basically if the two recessives were crossed then epistasis would take place and instead of both of them working in a complementary fashion one of the two recessives would mask the other thus producing what ever phenotype is shown. So yeah, you teacher is right in that it can't actually happen in anyway we know. Not too sure if what I said made sense so feel free to ask anything I might have confused you on.
Well, co-dominance and incomplete dominance are two different things With co-dominance, when they mix, you get a spotted, muddled, or whatever you want to call it, two colored whatever. i.e. Red with white spots With incomplete dominance, the two colors would come together and form a mix in the middle. i.e. Pink