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.