13.2 Connection between dna and phenotype  (Page 3/3)

Homozygous Hb ^A Hb ^A individuals have erythrocytes which retain their normal shape in the body and which retain normal shape even when blood samples are subjected to greatly reduced oxygen tension in labo­ratory tests.

Heterozygous individuals ( Hb ^A Hb ^S ) are said to be carriers for sickle-cell anemia. Note: this is a specific term and is not the same thing as sickle cell anemia—heterozygotes do not have the disease themselves but their children may inherit the condition. Carriers have no anemia, do have good health (as do Hb ^A Hb ^A individuals) and their erythrocytes maintain normal shape in the blood. In other words, they are phenotypically normal under most conditions, and probably do not know that they “carry” the Hb ^S allele. However, if heterozygotes are exposed to condi­tions of low oxygen levels (such as strenuous activity at high altitudes) some of their erythrocytes do sickle. Red blood cells in blood samples of heterozygotes subjected to greatly reduced oxygen tension in the laboratory also sickle.

Why is sickle cell anemia most prevalent in people with origins in Central Africa and the Mediterranean? If you look at the figure below ( [link] ) , you will see the occurrence of sickle cell anemia overlaps with the pervasiveness of malaria. This seems odd, but those individuals how are heterozygous ( Hb ^A Hb ^S ) for the sickle cell allele are less likely to contract and die from malaria then those who are homozygous ( Hb ^A Hb ^A ). The Hb ^S polypeptide that is produced by the heterozygous individual stops the organism ( Plasmodium ) that causes malaria from invading the red blood cells. So, in areas where malaria is common there is selection pressure for the Hb ^S allele, and the Hb ^S allele occurs in a higher frequency because the those who have one copy of the Hb ^S allele will live longer and have more children. In areas where malaria is not common, there is selection pressure against the Hb ^S allele, and the Hb ^S allele occurs in a lower frequency. As you will learn in a later chapter, there is an 25% chance that two carriers will have a child who is homozygous Hb ^S Hb ^S ), and this child will pay the evolutionary price for the protection from malaria that the parents were afforded. Hopefully, you will now understand how evolution favors the presence of such a potentially detrimental allele in a population. The sickle cell example is only one of what is called heterozygous advantage, we have provided a number of other examples in [link] .

Distribution of malaria and the frequency of sickle cell allele