3 - Biol 241 2010 Lectures 4-6: Extensions of Mendelian...

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Biol 241 - 2010 Lectures 4-6: Extensions of Mendelian Genetics I) Characterization of alleles A) Loss-of-function – Mutations that decrease gene functions. They are often recessive mutations (not always, see haploid-insufficient), because it takes two loss-of-function mutations to knock out both alleles. Alleles that show a complete loss of function are called null (or amorphic ) alleles. Alleles that show a partial loss of function are called hypomorphic alleles. B) Gain-of-function - Mutations that increase gene functions. Gain-of-function alleles are called hypermorphs . They are often, but not always, dominant. When they are dominant, they are also called dominant-active alleles. C) Antimorphs are mutations that, in heterozygous state, can interfere with the normal function of the other (wild-type) allele. They are also called dominant- negative alleles. D) Neomorphs are a special class of gain-of-function alleles, in which the mutations generate novel functions. They are usually dominant. E) Haploid-insufficient refers to loss-of-function mutations that, when heterozygous, exhibit phenotype. This is because one copy of wild-type allele is not sufficient to sustain normal gene function. F) Conditional mutations - Mutant phenotype manifested under certain conditions (the restrictive condition). Under the permissive condition, wild-type phenotype is exhibited. Example: temperature-sensitive mutations. G) Lethal mutations – Recessive lethal mutations cause homozygous mutant to die. They disrupt essential genes. In some cases, recessive lethal mutations have dominant phenotypes. Example, agouti/yellow mice. Another important point here is that the same mutation can be both dominant and recessive, depending on the context. II) Nomenclature (Drosophila convention, from Flybase ) A) The name begins with a lowercase letter when the gene is named for a mutant phenotype recessive to the wild-type in a normal diploid. B) The name begins with an uppercase letter when the gene is named for a mutant phenotype that is dominant to the wild-type in a normal diploid. C) Genes named after a protein product or other molecular feature begin with an 1
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Biol 241 - 2010 uppercase letter. III) Multiple alleles of the same gene A) Multiple alleles can exist for one gene. Many of these alleles disrupt the gene function to different degrees (allele series ). For instance, the Drosophila white mutation. B) How do you know whether any new mutation is an additional allele of an old gene or else an allele of a completely newly discovered gene? This can be determined using Complementation (or cis-trans) test. If two mutations are alleles of the same gene, trans-heterozygotes will be mutant (no wild-type copy of the
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This note was uploaded on 02/09/2010 for the course BIOL 241 taught by Professor Henrychang during the Spring '10 term at Purdue University-West Lafayette.

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3 - Biol 241 2010 Lectures 4-6: Extensions of Mendelian...

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