The Heterozygote Advantage
Evolution through nat
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Evolution through natural selection generally involves alteration of gene frequencies. This process requires large amounts of genetic variability. One source of variability is mutation. In order for mutant genes to persist though, they must be passed on to offspring. Since, on average, mutations tend to be harmful, selective pressures tend to eliminate them from any given population's gene pool. One mechanism, however, which serves to maintain genetic variability is heterozygote advantage. This phenomenon, which can be defined in mathematical terms, occurs when a heterozygote genotype possesses greater fitness than either of its two homozygote forms. Typically, this results in a balanced polymorphism which is responsive to the external environment. Heterozygote advantage has been identified in a number of natural systems: these may involve both human diseases such as sickle cell anemia, as well as animal gene pools. In fact, the mechanism is so profound, that its primary flaw may be over application. Variability is of major importance to biological systems. It may result through various mechanism including mutation, meiosis, and crossing over. The myriad of genotypes thus produced are then subjected to the forces of natural selection. Mechanisms of selection can include such environmental fluctuations as changes in temperature or humidity. They may also involve differences in sexual attraction (10:758 759). Selective pressur
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requency among zygotes p2 2pq q2
Viability a b c
It can be seen that p' = ap2 + bpq = p ap + bq = p ap/q + b
q' bpq + cq2 q bp + cq q bp/q + c
With u=p/q and u'=p'/q', then u' = u au + b
bu + c
Similarly, ue is defined as follows: ue = b c
b a
Then, with A,a as the most viable of the three possible genotypes, b is greater than both a and c. Therefore ue is positive and a possible value for u.
Now, u' = u au + b = u (au + c) + (b c)
bu + c (au + c) + u(b a)
= u (au + c) + ue(b a)
(au + c) + u (b a)
Subtracting ue from both sides of this equation gives the following:
u' ue = (u ue) au + c
bu + c
Hence, if u equalled ue, (u ue) = 0, u' would equal ue, and the allele frequencies would remain constant. In addition, because of heterozygote advantage, a is less than b and (au + c)/(bu + c) is less than 1. Therefore, if u does not equal ue, (u' ue), is smaller than (u ue). Hence it can be seen that regardless o
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Some common words found in the essay are:
AA Aa, Advantage Evolution, According Ford, ABO Rhesus, Drosophilia Drosophila, Normally Peppered, Furthermore Manchester, heterozygote advantage, Britain's Peppered, Literature Cited, Wiley Sons, au +, genetic variability, york ny, = , ue, balanced polymorphism, = au +, = au, + +, aa aa, au + +, random mating alleles, example heterozygote advantage, john wiley sons,
Approximate Word count = 2042
Approximate Pages = 8 (250 words per page)
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