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Lecture 5

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University of Toronto St. George
Human Biology
Stephen Wright

Lecture 5 61 Interactions between the alleles of Single Gene Variations on Dominance121 ratioinheritance pattern is based on TWO alleles of a single gene but the heterozygotesintermediate in phenotypeINCOMPLETE DOMINANCEINCOMPLETE DOMINANCE situation in which a heterozygote shows a phenotype quantitatively but not exactly intermediate between two homozygous phenotypes Exact intermediacy means NO dominanceEach wild type allele generally produces a set dose of its protein productNumber of doses of wild type allele determines the of a chemical made by the protein ie pigmentCodominanceCodominance the expression of both alleles of a heterozygoteIe human bloodcodominance in antigen alleles ABO blood groups are determined by three alleles of one geneinteract in several waysproduce 4 different blood typesBAMajor allelesiI I person can only have TWO of the three alleles or two copies of one of them combinations result in 6 different genotypes3 homozygotes and 3 different types of heterozygotes AAAAIIi IAIis dominant over i BBB BiII IBIis dominant over iABIIABeach allele produces its own form of the antigenare codominant iiO Alleles determine the presence and form of a complex sugar molecule present on the surface of red blood cells Sugarantigen cell surface molecule that can be B A recognized by immune systemIItwo different types of the antigen ino antigen because is a NULL alleleSickle cell anemiagene concerned encodes for hemoglobin which is responsible for transporting oxygen in blood vessel and is the major constituent of RBCsASTwo main alleles Hb dominantif have thisno sickle celland Hbthree different genotypesAA Hb Hbnormal No sickle cellSSHbHbsevere often fatal anemia abnormal hemoglobin causes RBCs to have abnormal shapeAS HbHb no anemia RBC sickle only under low Ohaplosufficient 2
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