Bio. 2203 Genetics Lecture 5 and 6 RSA 1-30-2009

Bio. 2203 Genetics Lecture 5 and 6 RSA 1-30-2009 -...

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Extensions of Mendelian Genetics -- Chapter 4 Since Mendel’s work was rediscovered in the early 1900’s , Researcher focused on the ways genes influence an organism’s phenotype. These studies examine types of inheritance observed by researchers that did not conform to the expected Mendelian ratios. Gene and Alleles: Alleles are alternate forms of the a gene Wild Type Allele: The allele occurring most frequently in a population (normal allele) is called the wild-type (wt) allele Wt allele is usually dominant and is expressed as the wild- type phenotype, Wt allele used as “standard” for comparison of all mutations (alternative alleles) of the gene/locus
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How do the new alleles arise ? Mutations - Ultimate source of new alleles Genetic information is modified - often (but not always) produces altered gene product New phenotypes result from changes in functional activity of gene product Changing relative enzyme efficiency Changing overall enzyme function E.g. enzyme specificity Eliminating enzyme function
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Systems of Genetic Nomenclature and Gene Symbols For simple Mendelian traits: 1 st letter of recessive form Lowercase Italicized = recessive allele - dwarf - d / D Tall dominant Drosophila melanogaster Initial letter ; Combination of two or three letters of mutant If trait is recessive - lowercase: dominant - uppercase A mutant allele can be dominant e.g. wrinkled wing use initial uppercase letter to designate dominance Wrinkled wing Wr / Wr - Wrinkled wing Wr / Wr + - Normal strait wings Wr + / Wr + Body color gray / ebony Gray homozygote e + /e + (wild type) , gray heterozygote (wild type) e + /e Ebony homozygote (mutant) e /e
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Systems of Genetic Nomenclature More Conventions The Drosophila system can be applied to other organisms as well When no allelic dominance, upper case letters and superscripts designate alternate alleles R 1 and R 2 ; L M and L N There are other systems of genetic nomenclature: leu - for bacteria with mutation blocking leucine biosynthesis BRCA1 for a human gene associated with inherited risk of breast cancer
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How Alleles Affect Phenotype? Not always simple dominant / recessive issue Gene interaction Phenotype controlled by more than one gene Sex-linked genes (X-linkage in X/Y organisms) Phenotype can depend on genotype and environmental effects.
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Incomplete Dominance Offspring with an intermediate phenotype incomplete or partial dominance Example: red snapdragon crossed with white snapdragon, each genotype has its own phenotype Heterozygotes show a distinct intermediate phenotype- different from homozygous genotypes Neither trait is dominant How this works? Are these traits blended? NO … the alleles are particulate (they
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Bio. 2203 Genetics Lecture 5 and 6 RSA 1-30-2009 -...

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