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Unformatted text preview: on resistance gene
[Graph] bell-shaped curve shifted to the right
o Disruptive selection Natural selection favours organism with two extreme forms of a characteristic
Balanced polymorphism: equilibrium of non-carriers and carriers of a characteristic
caused by natural selection
o Abnormal Hb makes red blood cells sickle-shaped / stick in capillaries
o People homozygous for this recessive allele die before reproducing
o People heterozygous for the allele should be at a disadvantage / red blood cells
can sickle during exercise / allele should be selected against and rare
o Where malaria is found, people heterozygous for sickle-cell have an advantage
(resistant) and are likely to survive, reproduce and pass on the allele; people
without the allele also have an advantage, because their red cells behave normally
o Balanced polymorphism is produced / carrier is heterozygous for sickle cell
[GRAPH] Acts against the mode in a range of variation producing a bimodal distribution
(two new modes)...
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This document was uploaded on 02/28/2014 for the course BLG 144 at Ryerson.
- Summer '10