Unformatted text preview: ptide chains Alpha globin Beta globin The Beta globin gene Normal wildtype allele called "A" One type of common mutant allele called "S" because it causes sickling of RBC 2. PLEIOTROPY The mutant "S" allele affects more than one trait SS individuals RBCs clog after releasing oxygen Low oxygen causes tissues to cramp Anemia also results However, these individuals resistant to malaria SA individuals (consider later slide) AA individuals (consider later slide) 3. RECESSIVE LETHALITY SS Individuals Develop heart failure b/c of stress on circulatory system Lethality of "S" allele obvious here when homozygous Normal circulatory system Normal circulatory system SA individuals AA individuals 4. DIFFERENT DOMINANCE RELATIONS Codominance SS individuals SA individuals AA individuals Complete dominance & recessiveness SS individuals SA individuals AA individuals 4. DIFFERENT DOMINANCE RELATIONS (cont'd.) Complete dominance & recessiveness SS individuals SA individuals AA individuals Incomplete dominance PLEIOTROPY SUMMARY OF SICKLE CELL ANEMIA (know this table...) Part Deux: 0 Extensions to Mendel for Multifactorial Inheritance Two Genes can Interact to Determine One Trait
0 Novel phenotypes can emerge from the combined action of alleles of two genes Let's consider the traditional dihybrid cross in pea plants for seed color Start with two purebreeding parental lines Aabb x aaBB (tan x gray) All F1 offspring are AaBb (brown) F2 offspring generate 9:3:3:1 of A_B_ : A_bb : aaB_ : aabb (brown : tan : gray : green) 0 Two Genes can Interact to Determine One Trait (cont'd.)
0 The twogene hypothesis explains why there is: Only one green phenotype: purebreeding aabb Two types of tans: purebreeding Aabb; tan and green producing Aabb Two types of grays: purebreeding aaBB; gray and green producing aaBb Four types of brown: purebreeding AABB; brown and tan producing AABb; brown and gray producing AaBB; and AaBb dihybrids that give rise to all four colors Four color phenotypes arise from four genotypic classes (9:3:3:1 indicates dihybrid cross of this Two Genes can Interact to Determine One Trait (cont'd.)
0 Also the case of complementary gene action A modified 9:3:3:1 ratio is observed Instead of four phenotypic classes, only two emerge. WHY??? The 9:3:3:1 ratio gets "lumped" 9:3:3:1 = 9:7 0 So, both genes must be turned "ON" in order for purple to be expressed In Epista...
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- Fall '03