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Genetics & Gregor Mendel Gregor Mendel • Modern genetics began in the mid-1800’s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas – Used experimental method – Used quantitative analysis • Collected & counted data – Excellent example of scientific method Mendel’s work • Bred pea plants – Cross-pollinated true breeding parents (P) Pollen transferred from white flower to stigma of purple flower P • P = parental – Raised seeds & observed traits in (F1) anthers removed offspring all purple flowers result • F = filial – Allowed offspring to self-pollinate & observed traits in next generation (F2) F1 self-pollinate F2 Mendel collected data for 7 traits Taking a closer look … true-breeding purple-flower peas P F1 X true-breeding white-flower peas 100% purple-flower peas Where did the white flowers go? 100% generation (hybrids) self-pollinate F2 generation 75% purple-flower peas White flowers came back! 25% white-flower peas 3:1 What did Mendel’s findings mean? • Traits come in alternative forms – Purple vs white flower color – Alleles • Different alleles vary in sequence of nucleotides at the specific locus of a gene – Some difference in sequence of A, T, C, or G purple-flower allele & white-flower allele are two DNA variations at flower-color locus different versions of gene at same location on homologous chromosomes Traits are inherited as discrete units • For each characteristic, an organism inherits 2 alleles, 1 from each parent – Diploid organism • Inherits 2 sets of chromosomes, 1 from each parent • Homologous chromosomes • Like having 2 editions of a Biology book – Campbell “Biology” – Raven & Johnson “Biology” What are the advantages of being diploid? What did Mendel’s findings mean? • Some traits mask others – Purple & white flower colors are separate traits that do not blend • Purple X white ≠ lavender • Purple masked white I’ll speak for – Dominant allele both of us! • Functional protein • Masks other allele – Recessive allele wild type allele producing functional protein mutant allele producing malfunctioning protein • Allele makes a malfunctioning protein homologous chromosomes Genotype vs phenotype • Difference between how an organism “looks” & its genetic code – Phenotype • Description of an organism’s trait • Physical expression – Genotype • Organism’s genetic makeup X P Explain Mendel’s results using …dominant & recessive …phenotype & genotype purple F1 all purple white Making crosses • Can represent alleles as letters – Flower color alleles P or p – True-breeding purple flower peas PP – True-breeding white flower peas pp PP x pp X P purple white Pp F1 all purple Looking closer at Mendel’s work true-breeding purple-flower peas P X true-breeding white-flower peas PP pp 100% purple-flower peas F1 Pp Pp genotype 100% generation (hybrids) Pp phenotype Pp self-pollinate F2 75% purple-flower peas 25% white-flower peas generation ? ? ? ? 3:1 Punnett squares Pp x Pp F1 Aaaaah, phenotype & genotype can have different ratios generation (hybrids) % genotype male / sperm P p PP 25% 75% female / eggs Pp P PP % phenotype 50% Pp Pp p Pp pp pp 25% 1:2:1 25% 3:1 Genotypes • Homozygous = same alleles = PP or pp • Heterozygous = different alleles = Pp homozygous dominant heterozygous homozygous recessive Phenotypes vs genotypes • 2 organism can have the same phenotype but different genotypes purple PP homozygous dominant purple Pp heterozygous How do you determine the genotype of an individual with with a dominant phenotype? Can’t tell by lookin’ at ya! Test cross • Breed the dominant phenotype – the unknown genotype – with a homozygous recessive (pp) to determine the identity of the unknown allele x is it PP or Pp? pp How does that work? How does a test cross work? Am I this? Or am I this? x PP p pp x Pp p p pp p P Pp Pp P Pp Pp P Pp Pp p pp pp 100% purple 50% purple:50% white or 1:1 Mendel’s 2nd law of heredity • Law of Segregation – During meiosis, alleles segregate P PP P • Homologous chromosomes separate – Each allele for a trait is packaged into a separate p gamete pp p P Pp p Law of Segregation • Which stage of meiosis is responsible for the Law of Segregation? Metaphase 1 Whoa! And Mendel didn’t even know DNA or genes existed! Monohybrid cross • Inheritance of a single characteristic – Flower color – Seed color • Both parents are heterozygous (Pp x Pp) X Dihybrid cross • Inheritance of two different characteristics – Seed color AND seed shape • Both parents are heterozygous for both traits Dihybrid cross true-breeding yellow, round peas P Y = yellow R = round x YYRR generation (hybrids) 100% YyRr Dihybrid X generation y = green r = wrinkled yellow, round peas F1 F2 yyrr true-breeding green, wrinkled peas self-pollinate 9:3:3:1 9/16 yellow round peas 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas What’s going on here? • If genes are on different chromosomes … – How do they assort into the gametes? – Together or independently?? Is it this? YyRr YR yr Or this? YR YyRr Yr Which system explains the data? yR yr Is this the way it works? YyRr YR YyRr x YyRr yr or YR YyRr Yr yR yr 9/16 yellow round YR YR yr yr YYRR YyRr YyRr yyrr Well, that’s NOT right! 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled Or this way? YyRr YR YyRr x YyRr YR YR YYRR Yr YYRr yR YyRR yr YYrr YyRr Yyrr yR YyRR YyRr yyRR yyRr Yyrr yyRr yyrr Yr yR yr 9/16 yellow round 3/16 green round YyRr YYRr YyRr YR yr Yr yr or YyRr BINGO! 3/16 yellow wrinkled 1/16 green wrinkled We will find an exception to this? Mendel’s 3rd law of heredity • Law of Independent Assortment – Different loci (genes) separate into gametes independently • Non-homologous chromosomes align independently • Gamete types produced in equal amounts yellow – YR = Yr = yR = yr green • Only true for genes on separate chromosomes or on same chromosome, but far enough apart for crossing-over to occur frequently round wrinkled YyRr Yr Yr 1 yR : yR 1 YR : YR 1 yr : yr 1 Law of Independent Assortment • Which stage of meiosis is responsible for the Law of Independent Assortment? Again Mendel didn’t even know DNA —or genes— existed! Metaphase 1 EXCEPTION If genes are on same chromosome & close together will usually be inherited together rarely crossover separately “linked” • The chromosomal basis of Mendel’s Laws Review: Mendel’s laws of heredity • Law of Dominance – One allele will be dominant over the other • Law of Segregation – Each allele segregates into separate gametes • Established in Meiosis I • Law of Independent Assortment – Genes on separate chromosomes assort into gametes independently • Established in Meiosis I EXCEPTION linked genes metaphase1 Mendel was wise to chose peas • Peas plants are good for genetic research – Available in many varieties with distinct heritable features with different variations • Flower color, seed color, seed shape, etc – Mendel had strict control over which plants mated with which • Each pea plant has male & female structures • Pea plants can self-fertilize • Mendel could also cross-pollinate plants: moving pollen from one plant to another Mendel was wise & lucky to chose peas • Peas plants are good for genetic research – Relatively simple genetically – most characteristics are controlled by a single gene with each gene having only 2 alleles • One completely dominant over the other Any questions?