Thomas ClineOffice hours:Monday 5-6pm (door closes @ 5:15p)Friday 4-5 pm11 Koshland Hallpracticing developmental geneticist (Drosophila melanogaster)former Berkeley undergrad (‘64-68)Topics for today:recombination (genetic maps)complementation(bacterio)phage as an experimental systemfamous geneticistsReading for today and Monday:Chapter 7 (and skim Chapter 8)Problems: Chp 7, 16-24genetics:epigenetics:From your textbook’s glossary:the science of heredity 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.genetics: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:the science of heredityXgenesgenetics: the science of genesgene:From your textbook’s glossary:specific segment of DNA in a discrete region of a chromosome that serves as a unit of function by encoding a particular RNA or protein.(only in cis)what about aspects relevant to when? where? how much?basic unit of biological information;encodes whatgene:specific segment of DNA in a discrete region of a chromosome that serves as a unit of function by encoding a particular RNA or protein.(only in cis)basic unit of biological information;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:specific segment of DNA in a discrete region of a chromosome that serves as a unit of function by encoding a particular RNA or protein.(only in cis)basic unit of biological information;a difference that makes a differencew/ respect to biological functiongene DNA sequence(encodes particular RNA or protein)information:“nonsense” DNA sequence(encodes nothing……such as DNA between genes?)VS.gene DNA sequence(encodes particular RNA or protein)“nonsense” DNA sequence(encodes nothing……such as DNA between genes?)VS.This is NOT the difference that really concerns geneticists. (although we will touch on it today)genetic mapThe difference that really concerns geneticists. alleles:alternative forms of a single geneDIFFERENT forms of a single geneIf you understand the concept of allelism, you will understand what genetics is all aboutand what geneticists mean by genesMendel defined alleles (but it is not a particularly useful definition):given: Heritable character difference (e.g. seed color)hybrid (yellow) hybrid (yellow)XIF:pure-breeding yellowpure-breeding greenXpure-breeding yellow pure-breeding greenhybrid yellowThen:This particular seed color differenceis due to a differencein (the alleles of) a single geneY & y are alleles; hybrid yellow is Y/y(genotype)pure-breeding yellow(phenotype)AND NOTHING ELSE(with respect to seed color)(1:2:1)hybrid (yellow) hybrid (yellow)Xpure-breeding yellowpure-breeding greenXpure-breeding yellow pure-breeding greenhybrid yellowAND NOTHING ELSE(with respect to seed color)(1:2:1)…and by extension the allele responsible for the difference between the recessive and dominant charactershybrid (yellow) hybrid (yellow)Xpure-breeding yellowpure-breeding greenXpure-breeding yellow pure-breeding greenhybrid yellowAND NOTHING ELSE(with respect to seed color)(1:2:1)Y vs. y are alleles as defined by a segregation test(1) unchanged in the hybrid(2) segregate during meiosisThe 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?almost all: AB/AB AB/ab ab/abbut occasionally rare recombinants (“non-parentals”) such as: Ab/abAB/AB X ab/abAB/abselfHow hard do we haveto look before decidingwe must be dealing witha single gene difference?Problem with the segregational definition of allelism (= definition of the gene as the unit of segregation) arose in the case of multiple allelesdifference between pb. whiteeosin vs. white+ = one gene diff. = allelesdifference between pb. white-1 vs. white+ = one gene diff. = allelesdifference between pb. white-1 vs. whiteeosin = one gene diff. = allelesHence, two different alternative mutant forms(we = orange, & w-1 = white) of one wildtype gene: white+ (= red)(all based on Mendel’s test for segregation)What’s the problem with multiple alleles?w-1/weosin X w-1/Yvery rarely: w+/Y (hardly “unchanged in the hybrid”)Isn’t this just a revertant?1) only from heterozygous moms2) always associated with recombination of flanking genetic markers and a specific nonparental arrangement of those markers!y- w-1 sn-/y+ we sn+y+ w+ sn-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” allelesbut rather“pseudoalleles”genes as “beads on a string”Didn’t pass the smell testtwo genes?genetic map: yellow w-1 we singed white-A white-Blz(BS)lz(g)wtwtGene BGene AAa BbThe first example of “pseudoallelism” was lozenge:And many more cases followed in flies,and even more in micro-organisms where one had much greater mapping resolution powerThe cis-trans (complementation)test, 1949: lozenge (M. Greene)Two different recessive mutants, both with the samephenotype (small eyes and fused facets).Are they mutations in the same gene?Make two different fly lines and compare their phenotypes.Cis: Trans:lz(BS) lz(g)wt wtlz(BS)lz(g)wtwtlz(BS) lz(g)wt wtlz(BS)lz(g)wtwtIf flies are normal, thenmutations are in different genes.If the phenotype is still mutant,then BS and g must be in the same gene!!!This is a control experiment.The flies will be wild-typeregardless of whetherBS and g are in the same gene or not.Cis: Trans:A test for allelism (a definition of the gene) based on the PHENOTYPE OF THE HYBRID(function, not segregation & NOT MENDEL!)genetic map based on segregation frequenciesmeiotic recombination can separate the parts of a “gene” like white or lozengeHow does a genetic map ofregions within a gene (as defined by the complementation test)compare with that forregions between genes (as defined by the
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