genetics 4 - Chapter 4: Extensions of Mendelian Genetics...

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Chapter 4: Extensions of Mendelian Genetics (Klug) -gene interaction: a situation in which a single phenotype is affected by more than one gene. -X-linkage: cases where genes are present on the X chromosome: sex-influenced inheritance, where the sex of the individual influences phenotype I. Alleles alter Phenotypes in different ways Neo-mendelian genetics:focus on the many ways in which genes can influence an individual’s phenotype. -wild type allele: the allele that occurs most frequently in nature, designated as normal, but not always dominant. They serve as standards for comparison against other mutations occurring at a particular locus. Mutations are the source of alleles. -loss of function mutation: a mutation in a gene that changes the conformation of a specific enzyme and reduces or eliminates the enzyme’s affinity for the substrate. Dehances wild type allele -null allele: mutation when the loss is complete -gain of function mutations: enhances the function of the wild type product. It increases the quantity of the gene product. The mutation affects the regulation of transcription of the gene under consideration. -one mutation might not affect phenotype, but can tell when examine gene sequence. Usually traits are influenced by many gene products. II. Geneticists use a variety of symbols for alleles Another way to show recessive and dominant traits: example: ebony is recessive and normal is gray. e + is dominant/wild but e is recessive/mutant. Trait can be same but use slash: e + / e + . The slash indicates that the two allele designations represent the same locus on two homologous chromosomes. Also can use when considering a mutant allele that is dominant to the normal wild allele, then use upper case. Example: wrinkled is dominant mutant: Wr/Wr is wrinkled, Wr/Wr + is wrinkled and Wr + /Wr + is normal, straight. Finally, can also just do: +/+, +/e or e/e. Other times these are used are in blood types. III. Incomplete Dominance, neither allele is dominant -incomplete dominance: neither dominant nor recessive trait is shown. Since neither is dominant, the phenotypic ratio is identical to the genotypic ratio in F1. -loss of function is most likely with the white flowers, and the gene product of the wild type allele is an enzyme that participates in a rxn leading to the synthesis of a red pigment and the mutant allele produces an enzyme that can’t catalyze the reaction leading to pigment and so middle. -Tay-Sachs disease is also incomplete dominance but subtle in phenotype. In this, homozygous recessive individuals are severely affected with a fatal lipid storage disorder and neonates die during their first one to three years of life. They have no hexosaminidase A which digests lipids. Heterozygotes can still live, just less enzymes. This shows the threshold effect (normal
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This note was uploaded on 09/20/2008 for the course BIO 315H taught by Professor Payne during the Spring '08 term at University of Texas.

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genetics 4 - Chapter 4: Extensions of Mendelian Genetics...

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