Fly eye facet formation responds to environmental

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Fly eye facet formation responds to environmental temperature (Figure 4.6c). The Norm of Reaction is the effect of environmental variation on a phenotype.
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4 Overdominance occurs when heterozygotes have superior traits. Overdominance (or heterozygote advantage) occurs when the heterozygous genotype possesses characteristics that are more beneficial to survival or reproduction than either homozygous genotype. An example in humans is sickle-cell anemia. Hb A allele encodes normal hemoglobin. Hb S allele encodes an abnormal hemoglobin that causes red blood cells to assume a sickle shape under a low oxygen environment (Figure 4.7). Individuals who are Hb S Hb S have a shortened life span due to sickle-cell anemia, and Hb A Hb A individuals are highly susceptible to malaria (malaria is caused by a parasite, Plasmodium , which uses both humans and Anopheles mosquitoes as hosts). Heterozygous individuals ( Hb A Hb S ) have an advantage because they do not suffer the deleterious effects of sickle-cell anemia and they are resistant to malaria because their red blood cells are likely to rupture when exposed to Plasmodium , thus halting the spread of the infection. Other examples: Cystic fibrosis – heterozygote is resistant to diarrheal disease. PKU – heterozygote is resistant to fungal toxins. Tay-Sachs disease – heterozygote is resistant to tuberculosis. Overdominance is usually the result of two alleles that produce proteins with slightly different amino acid sequences. There are three common explanations for how the two protein variants in the heterozygote produce a more favorable phenotype (Figure 4.8): Functional differences between the proteins affect the ability of a pathogen to propagate in the cells. A change in the subunit composition of the proteins. For example, a gene for a homodimer that exists as two alleles that encode polypeptides designated A1 and A2. Homozygotes can produce only one type of homodimer (A1A1 or A2A2), while a heterozygote can produce three (A1A1, A2A2, and A1A2). This can allow greater function under a wider range of conditions. Proteins encoded by each allele exhibit differences in their functional activity such that a heterozygote can function under a wider range of environmental conditions. A similar phenomenon in modern agriculture is heterosis, or hybrid vigor. This is when the hybrid displays characteristics that are superior to the parental strains.
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5 Many genes exist as three or more different alleles. Previous examples have worked with a two-allele system, yet it is possible to have a gene in a population that exists in more that two alleles. An example is coat color in rabbits (Figure 4.9), which has a four-allele system. C designates normal coat color c ch designates a chinchilla coat color c h designates a himalayan coat color c designates the albino coat color At the molecular level: tyrosine eumelanin (black) phaecomelanin (orange/yellow) C is the normal allele that allows production of both eumelanin and phaecomelanin.
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