Announcements

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Announcements

• You should be working on chapter 6 problems 10,
  14, 15, 28.
• 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.
• September is sickle cell awareness month!

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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

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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

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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

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Outline of Lecture 11

• I. Genes linked on the same chromosome segregate
  together
• II. Crossing-over
• III. 3-Point Mapping

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I. Linkage and Crossing Over

• Linkage happens when genes dont 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).

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chiasma
Chiasmata Between SynapsedHomologs During
Meiosis Prophase I
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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.

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No Linkage Independent Assortment
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Linkage without Recombination
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Linkage with Recombination
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Complete Linkage P1 Cross
In complete linkage only parental gametes form
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Complete Linkage F1 Cross
Not 1111 testcross ratio! 11 testcross ratio
w/linkage
Not 9331 phenotypic ratio! 121 ? (complete)
linkage ratio
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Linkage Ratio

• The F2 phenotypic ratio unique to two linked
  genes in cross of double heterozygotes.
• If completely linked, should be
• 121 for F1 X F1
• 11 for F1 X test cross parent
• Linkage group - group of genes which show
  linkage in theory N (the haploid number).

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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.

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Crosses of Two X-linked genes
Expect only parental types if no crossing over
occurs - Confirm this for yourself with a
Punnett square.
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Morgans 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.

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Sturtevant and Mapping

• Sturtevant, Morgans undergraduate student,
  discovered frequency of crossing over between
  each pair of the 3 genes
• yellow, white 0.5
• white, miniature 34.5
• yellow, miniature 35.4

Do you see a pattern?
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Sturtevants Interpretation

• Sturtevant reasoned that recombination
  frequencies were additive, so order of genes on
  chromosome was yellow-white-miniature.

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You only see recombination when it occurs between
the genes you are watching!
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Single Crossovers Non-crossover (Parental) and
Crossover (Recombinant) Gametes
What is the maximum recombination?
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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.

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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.

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Double Crossovers
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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

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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 4685
• 4759 94.44
• SCO
• y 80
• ec w 70 1.50
• y w 193
• ec 207 4.00
• DCO
• y ec 3
• w 3 0.06
• Total 1000 100

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3-point Mapping Explanation
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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.
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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.

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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.

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To calculate recombination

• Total crossovers between y and w (SCO1 DCO)
• Total crossovers between w and ec (SCO2 DCO)

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Types of Double ExchangesNot All are Detectable
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Genetic Map of Drosophila melanogaster
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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.

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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.