in homozygotes. One example is sickle-cell anemia. Indi-viduals with this disorder are homozygous for the allele that codes for hemoglobin-S instead of the normal hemoglobin. They die from a severe anemia because hemoglobin-S crystallizes at low oxygen concentrations, causing the red blood cells to become sickle-shaped and then to rupture (Figure 19-10). As we saw in Chapter 6, another example in hu-mans is phenylketonuria, where tissue degenerates as the result of the accumulation of a toxic intermediate in the pathway of tyrosine metabolism. This case also illus-trates how ﬁtness is altered by changes in the environ-ment. People born with PKU will survive if they observe a strict diet that contains no tyrosine. How selection works Selection acts by altering allele frequencies in a popula-tion. The simplest way to see the effect of selection is to consider an allele a that is completely lethal before re-productive age in homozygous condition, such as the al-lele that leads to Tay-Sachs disease. Suppose that in
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.