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Lecture 15

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Biology 1001A
Tom Haffie

Lecture 15:  How heterozygote advantage and heterozygote disadvantage affect genetic variation:  The heterozygotes advantage : --simUtext: heterozygotes have higher fitness than the other homozygotes.: Make the heterozygotes beneficial more than the other homozygotes. What happens is the freqencies of allele stabilize 50-50 ratio In genotypes we see that there are of heterozygots and a few of homozygotes. When a heterozygotes advantage at some allete, does the population experience selection? A- yes B- no C- at first yes, but only until allele frequencies have stabilized. -SOLUTION: yes, selection happen constantly all the way through. Selection happen when ever there is a fitness difference between the various genotypes. Heterozygotes advantage is a type of selection.  2- is it a HWE proportions? A- yes B- no SOLUTION: no, b.c. every time we have x and y alleles this must happen x=.5 and y=.5 But in this situation in pig there were more heterozygotes than we would expect under HWE in absence of selection. Then heterozygotes advantage is a kind of selection that affecy HWE  3- heterozygotes advantage, does the population evolve? A-yes B- no C- at first yes ,but only until allele frequencies have stabilized. SOLUTION: only at first and then stop stabilize. Once these alleles reach HWE, the allele frequencies stop changing, so evolution s defined an allele frequencies change from generation to the next. Selection does not always result in evolution b.c. selection is still happening but evolution is not happening and we can have evolution without being the result of selection! -------------  If heterozygotes (RS) have the highest fitness, but RR homozygotes are fitter than SS, what will eventually happen to alleles R and S? A-f(R) will reach 1, f(S) will reach 0 B-allele frequencies will stabilize at f(R)=f(S)=0.5 C-allele frequencies will stabilize at f(R > f(S) > 0 SOLUTION: C, it stabilize but it doesn‟t reach a 50-50 ratio 2- What if selection favour both homozygous genotypes (RR,SS) equally (heterozygote disadvantage)? A-the allele with the higher starting frequency will reach a frequency of 1, the other will reach 0 B-allele frequencies will stabilize at f(R)=f(S)=0.5 C-allele frequencies will fluctuate randomly -SOLUTION: it is A, the more starting allele went to fixation, so where as heterozygotes advantage is a type of selection that is fundamentally stabilizing type of selection that tries to maintain more than one allele in a population for long period of time, heterozygotes disadvantage is exactly the opposite, it is a fundamentally unstable situation and very short lived in which any allele begins in low frequency in a population and in which any high frequency at starting have huge advantage over the other one . The reason of that is that the common allele is less likely to find itself in the body of a heterozygote This is not a very common situation in nature b.c. it is very short lived period.  Whether selection always results in evolution:  Selection can vary over time & space… Adaptations are environment-specific: -Selection vary overtime(temperature… wet dry …) -We end up in an adaptive trait that is always going to be environment specific. (ex. White and black mouse the color was selectively favoured because of environment “the different colored onse might be killed by preys ….” -Selection pressure could change as we move from place to plcace or from time to time -How do you fluctuating selection pressure affect genetic variation? The more that environment change over time and space, the more genetic variation tend to see maintained by selection  How positive and negative frequency-dependent selection affect genetic variation:  Negative frequency-dependent selection  e.g. predators prey disproportionately on most common prey  effect on genetic variation?  phenotype; other examples? -There is a selective advantage of being rare ( we see this as a predictive prey interaction: if there is a predator it is easier to focus on the common type of preys (make a search image) “ black an grey squirrels which ever type of squirrel is common the predator is going to look for the common one” -the effect is that the allele and phenotype is maintained in the population b.c. there is a fitness advantage to being rare the rare will increase in frequency but as soon it increases to 50% of the population it no longer enjoys that rare phenotype advantage and the selective advantage flips to favour the other types, so it preserves high level of genetic vari
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