IB31Lecture+7+outline+Mendelian+Genetics+II+Sp08 - IB 31,...

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IB 31, Spring, 2008 2/13/2008 XII. Why Dominance? There are several reasons. A. If the expression of a gene is determined by the presence or absence of a chemical product, one copy of an allele may be sufficient to produce all that is needed to turn on or off other genes or produce material for construction, enzymes, etc. With skin pigment that we discussed earlier, one normal allele of a gene producing an enzyme important for the production of melanin was sufficient to create skin pigment. B. Any allele that contributes to fitness more than another allele ultimately will be favored. In the heterozygous condition, anything that causes the favored allele to be expressed will also be favored - for example, other genes that promote its effect. This is called Epistasis. XIII. But if dominant alleles are better (greater fitness), why aren’t recessives eliminated? Several possibilities: A. Environment may change periodically reversing selection. ABO blood; Salt and Pepper moths. B. Environmental Heterogeneity and dispersal. Lizard skin color. C. Frequency dependent selection. An allele when rare is favored, but not when common. Batesian Mimicry of a poisonous model (Monarch and Viceroy butterflies), rare male effect in Drosophila . D. Recessives are hidden from selection. If a recessive allele occurs at a frequency of 1%, slightly fewer than 1 in 100 individuals have it. The chance of two individuals with the recessive allele mating is 1/10,000 and only 1/4 of their offspring will express the trait (homozygous recessive; 1/40,000). Selection can only act on the expressed phenotype, so only the homozygous recessive is selected against. Even with strong selection, it will take a very long time to get rid of it. E. It also depends on when and how the trait is expressed. For example, Huntington’s Chorea in humans is caused by a dominant allele. However, most individuals with the allele don’t show symptoms until they are 25 or
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older. By then, they have reproduced and have passed on the allele to their children. F. Heterozygous Advantage. Sickle-Cell Anemia is a good example. Out of an 1800 unit long DNA code for the manufacture of hemoglobin protein, 17th base for the B chain is different (adenine instead of uracil). Thus the 6th amino acid is valine instead of glutamic acid. This changes the structural properties of the hemoglobin and causes the red blood cells with two copies of the allele to be sickle shaped and inefficient carrying oxygen. Homozygous recessive can be fatal without good medical care.
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IB31Lecture+7+outline+Mendelian+Genetics+II+Sp08 - IB 31,...

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