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Modifications of dominance relationships – Incomplete dominance Modifications of dominance relationships – Incomplete domina Complete dominance in terms of gene product 1. Threshold effect – the phenomenon that “50% of the protein is enough” is fairly common among many genes Example: Tay- Sachs disease At the molecular level, the mutation that causes TSD is in a gene that encodes for the enzyme hexaminidase A (hex A). Enzyme is responsible for the breakdown of gangliosides. Modifications of dominance relationships Co-dominance MN blood group Genotype LMLM Phenotype M LMLN MN LNLN N Which phenotypes may be produced in a mating between two heterozygotes? Multiple alleles – allelic forms of a gene Table 4.1 Biochemical basis of ABO blood groups The Bombay phenotype Lethal alleles Other variations: Essential genes and lethal alleles Dihybrid cross Two modes of inheritance Modification of 9:3:3:1 ratio Phenotypes are often affected by more than one ge Gene Interaction – Novel phenotypes rrP_ R_pp R_P_ rrpp Genetic Interaction Epistasis Recessive epistasis Genetic Interaction – Recessive epistasis Black lab = B_E_ Yellow lab = __ e/e Chocolate lab = bbE_ Genetic Interaction – Dominant epistasis Dominant epistasis – hypothetical explanation Genetic Interaction – Duplicate recessive epistas Duplicate recessive epistasis – hypothetical explana In some plants a red pigment, cyanidin, is synthesized from colorless precursor. The addition of a hydroxyl group (OH) to cyanidin molecules causes it to become purple. In a cros between two randomly selected purple plants the following results were obtained: 94 purple 31 red 43 white 1. Construct a pathway to represent the synthesis of the pu pigment. How many enzymes? How many genes? 2. Are the results of the cross in agreement with your pathw Explain. 3. What are the genotypes of the purple parental plants? Complementation test Two different genes affecting the same trait Complementation test Phenotypic Expression Pleiotropy Penetrance and Expressivity Sex-limited and sex-influenced traits Genetic background – suppression and position effects Conditional mutations – temperature Other variations: penetrance and expressivity Variable expressivity - neurofibromatosis Variable expressivity Sex-limited traits Sex influenced traits Other variations: Sex influenced traits Temperature sensitive genes Position effect Imprinting: The Igf2 gene in mice X-linked inheritance Pedigree analysis – sex linked traits what to look for: Recessive trait: 1. More males that females affected 2. Affected sons are usually born to unaffected mothers, zig-zag pattern – from grandfather to grandson through an unaffected female. 3. Approximately 50% of the progeny of a carrier female are affected 4. It is never passed from father to son 5. All daughters of affected fathers are carriers Pedigree analysis – sex linked traits what to look for: Dominant trait: 1. Both males and females are affected; often more females than males 2. Does not skip a generation. Affected sons must have an affected mother; affected daughters must have either an affected mother or an affected father. 3. Affected fathers will pass the trait on to all their daughters. 4. Affected mothers ( if heterozygous) will pass on the trait to approximately 50% of their daughters and 50% of their sons.