04_Genetic_Equilibrium - BI SC 002 LECTURE 4GENETIC...

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BI SC 002 LECTURE 4—GENETIC EQUILIBRIUM Draft: October 27, 2010 Many traits that dramatically appear in the population disappear almost as dramatically. Why is that? It is because individuals with these traits fail to reproduce and leave descendants with those traits. In other words, the trait does not confer any kind of evolutionary “fitness”. What if that “fitness” of a newly arising trait was equal to that of an existing trait—will that trait be eliminated through natural processes? In other words, if there are no selective pressures on a trait, will a trait disappear from a population just because it is recessive? Let’s go back to our example of eye color. Remember that brown eyes (B) are dominant over blue eyes (b). Our question then is, because the blue eye allele is recessive, won’t blue-eyed individuals eventually go extinct? Remember, when we have these alleles, there are six possible matings of individuals (you should remember this concept from the corn genetics activity): BB x BB—100% BB (100% Brown eyes) BB x Bb—50 % BB, 50% Bb (100% Brown eyes) BB x bb—100% Bb (100% Brown eyes) Bb x Bb—25% BB, 50% Bb, 25% bb (75% Brown eyes, 25% Blue eyes) Bb x bb—50% Bb, 50% bb (50% Brown eyes, 50% Blue eyes) bb x bb—100% bb (100% Blue eyes) Now, the question becomes this: who gets to breed with whom? Many populations exhibit what we call random mating . In random mating, any one individual in a population has an equal chance of paring with any other individual (of the opposite sex) in the population. So, let’s say we have a population of 96 individuals. 48 of the individuals are males, 48 are females. Let’s also say that in the population ¼ of each sex (12) are BB, ½ (24) are Bb, and ¼
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04_Genetic_Equilibrium - BI SC 002 LECTURE 4GENETIC...

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