Now consider that BB and Bb have the same phenotype

Now consider that BB and Bb have the same phenotype -...

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Now consider that BB and Bb have the same phenotype (i.e., there is dominance): BB = 3cm, Bb = 3cm and bb = 1cm. A cross between BB and bb would produce Bb F1's all with 3cm beaks. An F1 cross Bb x Bb would produce F2's with a 3:1 ratio of 3cm:1cm. In these the mean of the two parents would be 2, the mean of the F1's would be 3 and the mean of the F2s would be 2.5. Thus, dominance would affect the variation in phenotypes. There is a dominance component to the variance. Thus the genetic variance can be partitioned into additive and dominance components (and an interaction component which we will ignore, thank you): V G = V A +V D +V I . The total phenotypic variance is thus partitioned : V P = V A + V D + V I + V E . The point of this is that we want to know the additive genetic component of the total phenotypic variance since this is what makes parent and offspring look alike and is what selection can act upon . We can thus refine our description of
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Unformatted text preview: heritability to mean the proportion of the total phenotypic variance that is due to additive genetic effects: h 2 N = V A /V P where h 2 N means heritability in the "narrow" sense. Returning to our regression of offspring values vs. parent values (see figure 9.4 page 227), the slope of this regression = h 2 N = V A /V P ( known as a midparent-offspring regression). This allows us to define what kind of response to selection we would get if we imposed a specific intensity of selection on a phenotypic trait. The selection differential (S) is the difference between the mean of the parents selected to produce the next generation and the mean of all individuals in the population. The response to selection (R) is the change in the mean phenotype after selection. The response to selection depends of the heritability of the trait: R = h 2 N S (see figure 9.6 pg. 237)....
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This note was uploaded on 11/05/2011 for the course BIOLOGY MCB2010 taught by Professor Jessicadigirolamo during the Fall '10 term at Broward College.

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