Without medical attention sufferers may die we can

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rupture, causing severe anemia, great pain, and serious tissue damage. Without medical attention, sufferers may die. We can analyze the inheritance of sickle cell anemia by using the same approach that Mendel used for garden peas. We cannot, however, engineer crosses among human beings. Instead, we must rely on family pedigrees to establish inheritance patterns. We will consider two alleles for hemoglobin:The allele for the normal hemoglobin protein, called HbA , and the allele for sickle cell anemia, called HbS . Individuals homozygous for normal hemoglobin, designated HbA HbA , have no disease. Homozygotes for the sickle cell gene, HbS HbS , are afflicted with the disease. In heterozygotes, HbA HbS , about 1% of red blood cells exhibit the sickling trait.These individuals, called sickle cell carriers ,show no symptoms of the disease under normal circumstances.Carriers lead nor- mal lives as long as they avoid strenuous exercise, high altitudes, and other situations in which oxygen levels in their blood might get very low. What would happen if a woman who carries two normal alleles for hemoglobin and a man who suffered from sickle cell anemia had children? The children born to this couple would be analogous to the hybrids from Mendel’s first experimental crosses. Each child would receive one HbA allele from its mother and one HbS allele from its father. Each child would be heterozygous for the sickle cell gene. Each would be a sickle cell carrier. Punnett Squares Predict Possible Genotypes A clever way of envisioning how alleles are distributed during reproduction, the Punnett square was developed in the early 20th century by the British geneticist Reginald C. Punnett. For a cross involving a single trait such as sickle cell anemia, the alleles of the parents are arranged on the top and sides of a matrix as shown in Figure 3-6. Each block of the square represents a possible combination of alleles, a genotype, that could occur in the generation. Each egg produced by the mother contains one of the maternal alleles. With regard to the gene for hemoglobin, all the possible eggs F 1 F 1 > > > Figure 3-5 Red blood cells. (a) Cells containing normal hemoglobin are disk shaped with depressions on both sides. (b) Cells from persons suffering from sickle cell anemia become sickle shaped when the level of oxygen in the blood decreases. (magnified 9,620 times) (a) (b)
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3-1 How Are Traits Passed from Generation to Generation? 71 HbA/HbS HbA/HbS HbA/HbS HbA/HbS Mother's Alleles Father's Alleles HbA HbA HbS HbS Figure 3-6 Punnett square illustrating possible genotypes of the offspring of a father suffering from sickle cell anemia and a normal mother.All children born to this couple are sickle cell carriers. that this mother can produce with regard to the Hb gene are listed at the far left of the rows. Because this mother is homozygous and has only HbA alleles to contribute, all of her eggs contain the HbA allele. Likewise, the father is homozygous for the HbS allele, so all of his sperm contain the HbS allele. At the heads of the columns of the
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