Class Notes (808,298)
Canada (493,122)
HMB265H1 (242)
Lecture 20

Lecture 20- Linkage and recombination

11 Pages
Unlock Document

University of Toronto St. George
Human Biology
Christian Campbell

Thursday, March 19, 2009 -Lets take stock: -Recall: We started off with Mendel and we were looking at his traits and the simple model he came up with for the explanation of how these traits were inherited from generation to generation. And of course, just by looking around the room and observing the diversity of people that we see, Mendels simple model obviously cant explain all. We have a lot of variation that exists out there that must be explained and accommodated within this basic set of laws that Mendel established. -A pt. that needs to be made today is that Mendel wasnt wrong. What Mendel observed was what he could observed and what he observed actually, the basic laws hold true for everything that were going to look at. But what he looked at was a fairly unelaborated story, what were going to do now is elaborate Mendels simple laws so that we can accommodate this variation. -Now, amongst those people who started to bring together what was known about cell biology with Mendels laws, establishing a little bit of a wrinkle in that now we understand the cellular basis for the inheritance mechanism that Mendel referred to are these 4 individuals who established the chromosomal theory of inheritance: Nettie Stevens, E.B. Wilson, T.H Morgan, and Calvin Blackman Bridges. -Today well build upon their findings & move beyond a simple explanation for sex-linked traits that we looked at at the end of the last lecture & we want to move on from here. Well move on from an important observation thats based on the pedigree & indeed, its based on the observations that Thomas Hunt Morgan made w regards to what one could view as being the co-segregation of sex determination, so looking at sex determination of a trait, the co-segregation of that trait w some sort of visible trait such as haemophilia or eye colour in the case of Morgans flies. It wasnt just that the trait accompanied the sexuality of the organism its sex but rather, if you view the determination of the sex of the individual as being a trait, it means that 2 traits were moving along together. -So far, weve acted like genes function as individual entities in the genome but theyre joined up on chromosomes & one might expect that the genes would move together just like weve seen: sex determination and something like eye colour in flies segregating together. get started on giving this consideration to traits moving together and genes moving together as chromosomes segregation. -The story begins w an unusual observation made by Bateson and Punnett. They were working w plants and looking at the co-segregation or what should be the independent assortment of 2 different traits: flower colour and pollen shape. -In the parental generation, the individual on the left is homozy g os u dominant and is a true-breeding individual. On the right, its a homozygous recessive individual for the opposing traits. These traits are similar to what Mendel looked at so basically, Bateson & Punnett were just trying to re-iterate the independent assortment of alleles & the independent segregation of traits win the genome. So the 2 individuals were crossed & gave rise to the F1 hybrid individual thats heterozygous at both loci. -Then they asked, What would happen if we took this individual & selfed it to create the F2 generation? They predicted that they would find what Mendel had observed & what they themselves had observed many times before in the past & that is, one should see a phenotypic segregation ratio of 9:3:3:1. And, as they looked at a large number of plants, they came up with the predicted numbers associated w that. In actuality, what they observed was different than what they predicted. A departure from the stereotypical 9:3:3:1 ratio; so they observed ratios that based on statistical analysis showed a significant departure from the predicted ratios. This creates a problem, so whats going on? -Bateson and Punnett just ignored this problem so they just set it aside and didnt really consider it further. It actually took a long time for Bateson to be won over and Punnett was never won over to what were going to be presented today which we know now is obviously true. -Morgan actually conducted similar experiments. He looked at 2 traits at the same time: eye colour (red vs. purple red is the dominant trait to purple) and wing shape (wild type vs. vestigial wings the wild type are elongated wings that were dominant to the vestigial wings). -He also had unusual results. -So he started w true-breeding parental individuals and the designations (AA BB) are a means by which you can think of the traits as being the same as that but the proper designations are shown above that. So he has the homozygous dominant parent & the homozygous recessive parent for both of the loci in question & he creates an F1 that is a di- hybrid individual. Now, he crosses the F1 w a tester strain which is homozygous recessive at both of the loci in question & generates an F2 population which he predicts should segregate in a stereotypical Mendelian pattern in the following phenotypic ratio: 1:1:1:1 for each of the types of segregants that he would expect. -What he finds is something curious. Instead of finding the 1:1:1:1 ratio, he finds something entirely different which can be seen in the far right side. He observes that he has a very large number of parental types
More Less

Related notes for HMB265H1

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.