Thomas Cline practicing developmental geneticist Drosophila melanogaster former Berkeley undergrad 64 68 Office hours Monday 5 6pm door closes 5 15p Friday 4 5 pm 11 Koshland Hall Topics for today recombination genetic maps complementation bacterio phage as an experimental system famous geneticists Reading for today and Monday Chapter 7 and skim Chapter 8 Problems Chp 7 16 24 From your textbook s glossary genetics the science of heredity epigenetics study of states of gene functionality that are not encoded within the DNA sequence but that are still heritable from one generation to the next From your textbook s glossary genetics the science of heredity X epigenetics genes study of states of gene functionality that are not encoded within the DNA sequence but that are still heritable from one generation to the next From your textbook s glossary genetics the science of genes gene basic unit of biological information specific segment of DNA in a discrete region of a chromosome that serves only in cis as a unit of function by encoding a particular RNA or protein encodes what what about aspects relevant to when where how much gene basic unit of biological information specific segment of DNA in a discrete region of a chromosome that serves only in cis as a unit of function by encoding a particular RNA or protein What do we do with the Drosophila Dscam gene Down syndrome cell adhesion molecule encodes 38 000 different proteins via alternative pre mRNA splicing Drosophila has only 14 000 genes gene basic unit of biological information specific segment of DNA in a discrete region of a chromosome that serves only in cis as a unit of function by encoding a particular RNA or protein information a difference that makes a difference w respect to biological function gene DNA sequence encodes particular RNA or protein nonsense VS DNA sequence encodes nothing such as DNA between genes nonsense DNA sequence gene DNA sequence VS encodes particular RNA or protein encodes nothing such as DNA between genes This is NOT the difference that really concerns geneticists although we will touch on it today genetic map The difference that really concerns geneticists alleles alternative forms of a single gene DIFFERENT forms of a single gene If you understand the concept of allelism you will understand what genetics is all about and what geneticists mean by genes Mendel defined alleles but it is not a particularly useful definition given Heritable character difference e g seed color IF pure breeding yellow X pure breeding green hybrid yellow pure breeding yellow yellow hybrid yellow genotype phenotype Then X hybrid yellow 1 2 1 pure breeding green AND NOTHING ELSE with respect to seed color This particular seed color difference is due to a difference in the alleles of a single gene Y y are alleles hybrid yellow is Y y pure breeding yellow X pure breeding green hybrid yellow pure breeding yellow hybrid yellow X hybrid yellow 1 2 1 pure breeding green AND NOTHING ELSE with respect to seed color and by extension the allele responsible for the difference between the recessive and dominant characters pure breeding yellow X pure breeding green hybrid yellow X hybrid yellow 1 2 1 pure breeding yellow hybrid yellow pure breeding green AND NOTHING ELSE with respect to seed color Y vs y are alleles as defined by a segregation test 1 unchanged in the hybrid 2 segregate during meiosis The gene defined as the unit of segregation during meiosis not with respect to function Problem What if there are really TWO genes involved that are very closely linked AB AB X ab ab AB ab self How hard do we have to look before deciding we must be dealing with a single gene difference almost all AB AB AB ab ab ab but occasionally rare recombinants non parentals such as Ab ab Problem with the segregational definition of allelism definition of the gene as the unit of segregation arose in the case of multiple alleles difference between pb whiteeosin vs white one gene diff alleles difference between pb white 1 vs white one gene diff alleles difference between pb white 1 vs whiteeosin one gene diff alleles all based on Mendel s test for segregation Hence two different alternative mutant forms we orange w 1 white of one wildtype gene white red What s the problem with multiple alleles w 1 weosin X w 1 Y very rarely w Y hardly unchanged in the hybrid Isn t this just a revertant y w 1 sn y we sn 1 only from heterozygous moms 2 always associated y w sn with recombination of flanking genetic markers and a specific nonparental arrangement of those markers w w 1 we Must not be true alleles true alternative forms of the same gene but rather pseudoalleles genetic map yellow w 1 we singed white A white B genes as beads on a string w w 1 we Must not be true alleles but rather pseudoalleles genetic map Didn t pass the smell test two genes yellow w 1 we singed white A white B genes as beads on a string The first example of pseudoallelism was lozenge Gene A Gene B wt lz g lz BS wt Aa Bb And many more cases followed in flies and even more in micro organisms where one had much greater mapping resolution power The cis trans complementation test 1949 lozenge M Greene Two different recessive mutants both with the same phenotype small eyes and fused facets Are they mutations in the same gene Make two different fly lines and compare their phenotypes Cis Trans wt wt wt lz g lz BS lz g lz BS wt Cis Trans wt wt wt lz g lz BS lz g lz BS wt This is a control experiment The flies will be wild type regardless of whether BS and g are in the same gene or not If flies are normal then mutations are in different genes If the phenotype is still mutant then BS and g must be in the same gene A test for allelism a definition of the gene based on the PHENOTYPE OF THE HYBRID function not segregation NOT MENDEL 1e genetic map based on segregation frequencies meiotic recombination can separate the parts of a gene like white or lozenge How does a genetic map of regions within a gene as defined by the complementation test compare with that for regions between genes as defined by the complementation test do true segregational alleles exist and if so how do they compare with functional alleles Who said it 1e genetic map based on segregation frequencies meiotic recombination can separate the parts of a gene like white or lozenge How does a genetic map of regions within a gene as defined by the complementation test compare with that for regions between genes as defined by the complementation
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