Transcript Linkage II
Announcements 1. You should be working on chapter 6 problems: 10, 14, 15, 28. 3. Reminder- papers on “Monk in the garden” due in lab section 10/1, 10/2. I encourage you to see me to discuss your outlines. I will look over first drafts also. 4. September is sickle cell awareness month! Sickle Cell Resources • Sickle Cell Society: http://www.sicklecellsociety.org/index.htm • Sickle Cell Anemia Foundation: • http://www.4sicklecellanemia.org/ • Harvard Medical School Joint Center for Sickle Cell and Thalassemic Disorders: http://sickle.bwh.harvard.edu/ – See Management of Patients section Sickle-cell Trait and Disease • HbA/HbA – Normal; RBCs never sickle. • HbS/HbS – Sickle cell disease: severe, often fatal anemia; abnormal hemoglobin causes RBCs to sickle. • HbA/HbS – No anemia; RBCs sickle only under low [ O2 ] Levels of Dominance in Sickle-cell Trait • Anemia (organismal) – HbA completely dominant to HbS • RBC shape (cellular) – Incomplete dominance since normal at high [ O2 ], sickle at low [ O2 ] • Hemoglobin (molecular) – Codominance since both A and S hemoglobin can be detected electrophoretically Outline of Lecture 11 I. Genes linked on the same chromosome segregate together II. Crossing-over III. 3-Point Mapping I. Linkage and Crossing Over • Linkage happens when genes don’t assort independently. • Genes on the same chromosome are linked. • Genes linked on the same chromosome segregate together. • Crossing Over involves reciprocal exchange of chromosome segments between homologs; increases genetic variation (recombination). chiasma Chiasmata Between Synapsed Homologs During Meiosis Prophase I Crossing Over and Mapping • Linkage without crossing over creates only parental (noncrossover) gametes. • Linkage with crossing over creates parental gametes and recombinant (crossover) gametes. • Interlocus distance is proportional to the degree of crossing over between. – Little or no crossing over in close genes. – Frequent, even multiple crossovers between distant genes. • Chromosome map, determined from recombination rates, indicates relative locations of genes on a chromosome. No Linkage: Independent Assortment Linkage without Recombination Linkage with Recombination Complete Linkage: P1 Cross bwhv bw hv P1 : bwhv bw hv In complete linkage only parental gametes form bwhv F1: bw hv Complete Linkage: F1 Cross Not 9:3:3:1 phenotypic ratio! 1:2:1 (complete) linkage ratio Not 1:1:1:1 testcross ratio! 1:1 testcross ratio w/linkage Linkage Ratio • The F2 phenotypic ratio unique to two linked genes in cross of double heterozygotes. • If completely linked, should be – 1:2:1 for F1 X F1 – 1:1 for F1 X test cross parent • Linkage group - group of genes which show linkage; in theory = N (the haploid number). II. Morgan and Crossing Over • Morgan discovered crossing over when studying two genes on X chromosome in Drosophila. • Morgan proposed that the chiasmata visible on chromosomes were regions of crossing over. • Occurs between nonsister chromatids. Crosses of Two X-linked genes Expect only parental types if no crossing over occurs - Confirm this for yourself with a Punnett square. Morgan’s Interpretation • Recombination was caused by linear arrangement of genes and crossing over. • Frequency of recombination was determined by distance between genes: – y and w recombination rate = 1.3% – w an m recombination rate = 37.2% – Therefore y and w were closer together on the chromosome, while w an m are farther apart. Sturtevant and Mapping • Sturtevant, Morgan’s undergraduate student, discovered frequency of crossing over between each pair of the 3 genes: – yellow, white – white, miniature – yellow, miniature 0.5% 34.5% 35.4% Do you see a pattern? Sturtevant’s Interpretation • Sturtevant reasoned that recombination frequencies were additive, so order of genes on chromosome was yellow-white-miniature. You only see recombination when it occurs between the genes you are watching! Single Crossovers: Non-crossover (Parental) and Crossover (Recombinant) Gametes What is the maximum % recombination? Map Units • One map unit (centimorgan, cM) = 1% recombination between two genes – yellow and white are 0.5 cM apart – yellow and miniature are 35.4 cM apart – white and miniature are (35.4-0.5) = 34.9 cM apart • In Drosophila, crossing over occurs only in females, never in males. III. Three-Point Mapping • You can add % recombination between two genes to find the order of genes pretty well. • But the only way to be sure of the order of three genes is by Three-Point Mapping, which considers 3 genes at once. • You look for rare double-crossover events, and that is the clue to the gene order. Double Crossovers Probability of Double Crossovers • Equals product of each of their individual probabilities: – if PAxB = 0.20 and PBxC = 0.30 then – PAxBxC = (0.20)(0.30) = 0.06 = 6 % • Criteria for 3-point mapping cross: – Crossover gametes heterozygous at all loci – Genotypes can be determined from phenotypes – Sufficient numbers for representative sample 3-Point Mapping in Drosophila • Cross a y ec w female with wildtype male to get triply heterozygous mutant female and triply hemizygous mutant male. • Cross the F1 and examine the F2 phenotypes: – NCO: noncrossover – SCO: single crossover (2 types) – DCO: double crossover • NCO: y ec w + + + 4759 • SCO: y + + 80 + ec w70 y + w 193 + ec + 207 • DCO: y ec + 3 ++w 3 • Total: 100% 4685 94.44% 1.50% 4.00% 0.06% 1000 3-point Mapping Explanation To Deduce the Order from a 3-Point Cross: Method 1 1. Group the 8 phenotypic groups into 4 reciprocal pairs. 2. The Non-crossover (NCO) pair is the largest group. The Double crossover (DCO) pair is the smallest group. 3a. Note which gene “switches” from the parental arrangement in DCO (present on its own) - that one is in the middle. Possible Orders of 3 Genes • If yellow were in the middle, yellow phenotype would show up in DCO. • If echinus were in the middle, echinus phenotype would show up in DCO. • white is actually in the middle since white phenotype shows up in actual DCO data. To Deduce the Order from a 3-Point Cross: Method 2 3b. Assume one of the 3 possible gene orders and work the problem. If you later find a contradiction, try one of the other orders. 4b. Determine whether a DCO with your arrangement will produce the observed DCO phenotypes. – You will encounter a contradiction unless you have chosen the correct gene order. Keep trying until you get the right one. To calculate recombination %: • Total crossovers between y and w (SCO1 + DCO) : (80 70 3 3) 1.56% 1000 • Total crossovers between w and ec (SCO2 + DCO) : (193 207 3 3) 4.06% 1000 Types of Double Exchanges: Not All are Detectable Genetic Map of Drosophila melanogaster Creighton and McClintock Experiment Proved Crossing Over was a Physical Event • In maize, colorless (c)/colored (C), starchy (Wx)/waxy (wx) linked on chromosome 9. • Cytological markers on one parental homolog (knob on one end and translocated segment on the other end) allowed direct observation. Crossovers Between Sister Chromatids (SCEs) • Revealed by “Harlequin” chromosomes labeled during DNA replication • Occurs between mitotic sister chromatids. • No recombination • Significance unknown, but increased incidence correlated with some human diseases.