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Lecture 13

Biology Lecture 13.docx

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Department
Biology
Course
Biology 1001A
Professor
Tom Haffie
Semester
Fall

Description
Biology Lecture 13 Before: Extensions of Mendel’s fundamental principles: In incomplete dominance, dominant alleles do not completely compensate for recessive alleles Incomplete dominance  Occurs when the effects of recessive alleles can be detected to some extent in heterozygotes (Rr)  Example: When a red and white snapdragon plant are crossed they create a pink snapdragon (F1 generation) o When two F1 generations are crossed they create 1 white:2 Pink:1 Red snapdragon plants (F2 Generation) o **When one allele is not completely dominant we use a subscript to indicate the character (C ,C ) o This cross is the same ratio as when two heterozygotes are crossed  Real life Example: Sickle cell disease is an example of incomplete dominance o An alteration in the hemoglobin molecule that changes to shape of red blood cells when low oxygen available o Individuals with homozygous recessive alleles have this defective polypeptide that creates the hemoglobin molecule o Individuals who are heterozygous with the recessive allele and normal allele both have a condition known as sickle cell trait  Milder form of the disease because they create normal and abnormal polypeptides from the allele  Real Life Example 2: Familial hypercholesterolemia o Individuals who are homozygous  They have a defective LDL receptor gene (produces no LDL receptors)  These individuals usually have a heart attack as children due to the build up of cholesterol in ateries o Individuals who are heterozygous  These individuals have half the number of receptors  Although it takes longer these individuals usually have heart attacks by the age of 35  Thus it is seen that many alleles appear to be completely dominant but infact are incomplete dominant o To determine if incomplete or complete it depends on the level at which on is examinging to decide between the two o Eg. A heterozygous may produce enough pigment to look the same as a homozygous. But at the biochemical level the levels could infact be different  Real Life Example 3: Tay-Sachs Disease o Individuals who are homozygous recessive  Do not have a functional version of the enzyme that breaks down gangliosides (membrane lipid) Biology Lecture 13  this leads to mental impairment and death o heterozygotes  have no symptoms of the disease  however when looked at the biochemical level it is seen that there is a reduction in the amount of gangliosides that are broken down Conclusion:  NOT ALL traits are simple or complete dominance as Medel’s law may lead us to believe** In codominance, the
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