In selective breeding experiments, it is frequently observed that the strains respond to artificial selection for many generations, with the selected phenotype changing in the desired direction. Often, however, the response to artificial selection reaches a plateau after many generations, and the phenotype no longer changes as it did in past generations. What is the genetic explanation for the plateau phenomenon?

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Hi, everyone. Let's look at our next problem. The plateau phenomenon in selective breeding can occur because of which of the following reasons. And our answer choices are a trait being selected has reached its maximum expression level. B trade is being controlled by only a few genes. C trait under selection is genetically correlated to negative traits or d all of the above reasons. So, what is the plateau phenomenon? Well, we're talking about selective breeding. So the idea of it's not just natural selection, uh mating choice, things like that, but human directed choice of mates to improve a certain trait or create a certain phenotypic outcome. So the plateau phenomenon is the point at which a desired trait cannot be improved further. So the trait you're aiming for perhaps uh size, perhaps you're going for a larger chicken for instance. And you cannot get your chickens any bigger no matter how much you feed them, they've reached their maximum potential or a specific confirmation body type color that you're looking for and you reach a point where no matter what kind of breeding you do, you can't improve that trait any further. That would be the plateau phenomenon. So what would be the reason for this occurring? So let's look at our answer choices. Recalling that we do have that choice d all of the above. So we can have more than one correct answer. So a says the trait being selected has reached its maximum expression level and this would be a reason. So perhaps the chicken just cannot get any bigger. If it gets any bigger, it can't walk around or you're looking for a miniature animal. And if it gets any smaller, it can't reproduce successfully. So you just can't get any further in the expression level of what you're looking for. Choice B says the trade is being controlled by only a few genes. Well, this would be another reason that you would see a plateau when you have a trait that's only controlled by one or two genes. There's only a certain number of phenotypes possible as opposed to a polygenic inheritance pattern where you have many genes working together and you have a whole uh gradient of possible phenotypes. But if you just have one or two, you have a certain limited number of phenotypes and therefore can only go so far in trying to improve a particular trait. Then joy see is the trait under selection is genetically correlated to negative traits. And this is also true. If as you try and increase a certain trait, you're creating uh also along with it, things that create health issues for the animal, you will then run up against the limitation. You see this in some dog breeds. Uh so pugs with their dogs, with their noses being so kind of squished, have a lot of health issues that come along with it. That would prevent you from creating a more extreme version of that phenotype because you would end up causing so many health problems, you decrease the animal's fitness. So all three of these are reasons why you would see a plateau phenomenon in selective breeding. So our answer choice will be d all of the above reasons. See you in the next video.

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Key Concepts

Artificial Selection

Genetic Variation

Genetic Drift and Fixation