Cat breeders are aware that kittens expressing the X-linked calico coat pattern and tortoiseshell pattern are almost invariably females. Why are they certain of this?

Video transcript

Welcome back. Let's look at our next problem. It says the retina of a female who is heterocyclic for the color blindness gene will be. Well let's recall of course that the gene for color blindness is X linked. It's found on the X chromosome. So a woman who has had a rose I guess for the color blindness gene in every cell of her body, one of her X chromosomes carries the gene for color blindness and one has a gene for normal vision. But we recall according to the lion hypothesis that to prevent there from being issues from the fact that females have two X chromosomes in every cell and men only have one that early in fetal development that one of each somatic X chromosome is selected at random and inactivated in the female embryo. So in every single cell at that early embryonic stage, one of those X. Isn't activated. And then of course every one of its descendants cells will have that same X inactivated. And that leads to the phenomenon of moses is um since every cell descended from each embryonic cell will have one X inactivated. So if you've got a gene that has a different alley on each of those X chromosomes, um it will be expressed in some cells and not in others. So, returning to our question, we would expect that the cells of the retina, some will be colorblind and some will have normal vision depending on whether the cell they were descended from had the gene the X chromosome with the gene for color blindness and activated or not. So let's look at our answer choices and see which one expresses that. And we see that choice C. Says mosaic with patches of defective and normal color perception. And that's our correct answer. When we look at the other answer choices, we see choice A. Says defective in color perception. But again, since she has some retinal cells that can perceive color, um that is not the answer choice that um occurs in a male who has only a single X chromosome with a gene for color blindness but not in a female who's hetero choice B says normal in color perception. Well the the female overall will have normal color perception in her overall site. But if we're specifically talking about retinal cells again, we'll expect to see that mosaic effect. Lemon choice. The defective and dim light perception but dim light perception is determined um by the rod cells. Whereas color vision is determined by the cone cells. So dim light perception isn't isn't affected by a gene for color blindness. So that's why d is not correct. So again, the retina of a female who's heterocyclic for the color blindness gene will be choice. See mosaic with patches of defective and normal color perception. See you in the next video