Normal red blood cells Sickled red blood cells le in africa malania in arnica 5-10% 口10-20% FIGURE 20.1 Frequency of sickle cell allele and distribution of Plasmodium falciparum malaria.(a)The red blood cells of people homozygous for the sickle cell allele collapse into sickled shapes when the oxygen level in the blood is low. (b)The distribution of the sickle cell allele in Africa coincides closely with that of P falciparum malaria Malaria and Heterozygote Advantage Consequently, even though most homozygous recessive The average incidence of the S allele in the Central African individuals die before they have children, the sickle cell al population is about 0.12, far higher than that found among lected for) because of its association with resistance to African Americans From the Hardy-Weinberg principle, malaria in heterozygotes and also, for reasons not yet fully heterozygous at the S allele, and 1 in 100 develops the fatal understood, with increased fertility in female heterozygotes form of the disorder. People who are homozygous for the having the sickle cell allele in the heterozygous condition ally die before they reach reproductive age. Why is the s has adaptive value (figure 20. 12). Among African Amer allele not eliminated from the Central African population by selection, rather than being maintained at such high ley- some 15 generations in a country where malaria has been els?People who are heterozygous for the sickle cell allele relatively rare and is now essentially absent, the environ causes of illness and death in Central Africa, especially Consequently, no adaptive value counterbalances the ill: among young children--in the areas where the allele is tion is acting to eliminate the s allele. Only 1 in 375 common. The reason is that when the parasite that causes malaria, Plasmodium falciparum, enters a red blood cell,it African Americans develop sickle cell anemia, far less than leads to cell sickling even in heterozygotes. Such cells.g causes extremely low oxygen tension in the cell, wl in Central africa quickly filtered out of the bloodstream by the spleen, thus eliminating the parasite(the spleens filtering effect is what anemia in homozygotes, is maintained by heter ell he hemoglobin allele S, responsible for sic leads to anemia in homozygotes as large numbers of red dvantage in Central Africa, where heterozygotes for blood cells are removed). he s allele are resistant to malaria Chapter 20 Genes within PopulMalaria and Heterozygote Advantage The average incidence of the S allele in the Central African population is about 0.12, far higher than that found among African Americans. From the Hardy–Weinberg principle, you can calculate that 1 in 5 Central African individuals are heterozygous at the S allele, and 1 in 100 develops the fatal form of the disorder. People who are homozygous for the sickle cell allele almost never reproduce because they usu￾ally die before they reach reproductive age. Why is the S allele not eliminated from the Central African population by selection, rather than being maintained at such high lev￾els? People who are heterozygous for the sickle cell allele are much less susceptible to malaria—one of the leading causes of illness and death in Central Africa, especially among young children—in the areas where the allele is common. The reason is that when the parasite that causes malaria, Plasmodium falciparum, enters a red blood cell, it causes extremely low oxygen tension in the cell, which leads to cell sickling even in heterozygotes. Such cells are quickly filtered out of the bloodstream by the spleen, thus eliminating the parasite (the spleen’s filtering effect is what leads to anemia in homozygotes as large numbers of red blood cells are removed). Consequently, even though most homozygous recessive individuals die before they have children, the sickle cell al￾lele is maintained at high levels in these populations (it is se￾lected for) because of its association with resistance to malaria in heterozygotes and also, for reasons not yet fully understood, with increased fertility in female heterozygotes. For people living in areas where malaria is common, having the sickle cell allele in the heterozygous condition has adaptive value (figure 20.12). Among African Ameri￾cans, however, many of whose ancestors have lived for some 15 generations in a country where malaria has been relatively rare and is now essentially absent, the environ￾ment does not place a premium on resistance to malaria. Consequently, no adaptive value counterbalances the ill ef￾fects of the disease; in this nonmalarial environment, selec￾tion is acting to eliminate the S allele. Only 1 in 375 African Americans develop sickle cell anemia, far less than in Central Africa. The hemoglobin allele S, responsible for sickle cell anemia in homozygotes, is maintained by heterozygote advantage in Central Africa, where heterozygotes for the S allele are resistant to malaria. Chapter 20 Genes within Populations 433 Sickle cell allele in Africa 1-5% 5-10% 10-20% P.falciparum malaria in Africa Malaria (b) (a) Normal red blood cells Sickled red blood cells FIGURE 20.12 Frequency of sickle cell allele and distribution of Plasmodium falciparum malaria. (a)The red blood cells of people homozygous for the sickle cell allele collapse into sickled shapes when the oxygen level in the blood is low. (b) The distribution of the sickle cell allele in Africa coincides closely with that of P. falciparum malaria