Chapter 5 - 2:22:00 AM Chapter 5 Extensions and Exceptions...

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27/09/2007 01:22:00 Chapter 5: Extensions and Exceptions to Mendel’s Laws Alkaptonuria – A pleiotropic, meaning a single mutation causes multiple symptoms that  reflect the varied distribution of the protein. 5.1 When Gene Expression Appears to After Mendelian Ratios  In some cases, offspring do not result as Punnett squares would predict. Lethal Allele Combination A lethal genotype causes death before the individual can reproduce, thus  preventing the passage of hi or her genes to the next generation. In humans, early-acting lethal alleles causes spontaneous abortion  When a man and woman each carry a recessive lethal allele, there is a 25%  chance of spontaneous abortion. Sometimes, a double dose of dominant alleles is lethal, as is the case of the  Mexican Hairless Dog. Multiple Alleles A gene can exists in more than two alleles in a population because of ways it  mutates, different allele combinations can make new variations in  phenotypes. Testing the phenotype can rarely show the course of an illness because it  cannot take in to account the effects of other genes and the environment.  Multiple Alleles are taken in to consideration for CF, researchers found out  that different symptoms raised from different allele combination of the same  gene. Mutant alleles caused about 70% of CF.
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Different Dominance Relationships In incomplete dominance, the heterozygous phenotype is the intermediate of  the homozygous phenotypes.  A snap dragon plant is a good example, RR = Red, rr = White, Rr = Pink. Different alleles that are both expressed in the phenotype are known as  codominant, for example the ABO blood type.  Blood types are determined by the patterns of cell surface molecules on red  blood cells. Most molecules are proteins embedded in the plasma membrane.
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