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Lec 4 - Epigenetics

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University of Toronto St. George
Human Biology
Michelle French

Thursday, January 15, 2009 - The Olsen sisters they are derived just like all identical twins from an individual egg that was fertilized and it started to divide and then split so early, early on it split and the same genetic material was in both cells, both embryos and both offspring. - So that is a very good demonstration that our genotype, in other words the genes that we inherit determines our phenotype because we can see that these individuals just from appearances look very similar. - Now that concept of genotype to phenotype is really something were going to see throughout the course & Prof Campbell will go into great detail to show us that genotype does in fact determine phenotype. But that isnt the entire story & we shall see why. - Here we have two mice, they are genetically identical and yet you see one mouse is yellow and rather on the hefty side and one is kind of brown coloured. So what could account for this huge difference in phenotype if it isnt the genes? - If you look at the litters from these mice that have been mated looking at genetically identical parents can give rise to a litter with variable phenotype like this: going from the big fat yellow mouse to the skinny brown mouse. These are all brothers or sisters. This is the same kind of idea, these are the extremes. - These mice carry a genotype which is designated agouti variable A & then a small a. This is the 1 time weve been shown any genetic notation, what this shows you is that there are 2 alleles of the same gene, this is called the agouti gene & the 2 alleles are 2 variants of the same gene so it is just a way of annotating that there are 2 variations of this agouti gene. The slash like that shows the 2 alleles, just the way they notate it. Usually genes are shown as italicized so youll see genes written in italic fonts. - It turns out that these particular phenotype changes are due to epigenetic changes, changes that dont involve the gene per say. It turns out that these mice all have this genotype and yet they have a variable phenotype. Now to talk what these epigenetic changes are and how they can influence the phenotype. - Epigenet ics is a co nc ept where there is a mo dific at io n of the gene fu nct io n that doesnt relate to the DNA sequence so you have the same DNA sequence in both instances but the gene functions in a different way. Anytime you have that circumstance that is called an epigenetic effect so modifications in gene function that are not due to changes in the sequence of the DNA. - Those mice had the same DNA sequence but their phenotypes were different so in other words there was a modification of the gene function. Incidentally in those yellow mice, not only are they obese but they are prone to diabetes as well. It is better to be the brown mouse than the yellow mouse. - In that video youll actually see the narrator picking up an agouti mouse and he hasnt learned how to handle mice properly bc he holds it by its tail & pokes its abdomen. When youre holding a mouse, never let it bite your fingers, hold your hand above its mouth. Another thing he does is that he puts it on the sleeve of his coat so it will probably have to go to the dry cleaners. If you ever handle mice, hold them at the base of the tail & dont put them on your coat unless you want their pee.
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