Linkage II

1. You should be working on chapter 6 problems: 10, 14, 15,
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:
• Sickle Cell Anemia Foundation:
• Harvard Medical School Joint Center for Sickle Cell
and Thalassemic Disorders:
– See Management of Patients section
Sickle-cell Trait and Disease
• HbA/HbA
– Normal; RBCs never
• 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
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
• Crossing Over involves reciprocal exchange of
chromosome segments between homologs; increases
genetic variation (recombination).
Chiasmata Between Synapsed
Homologs During Meiosis Prophase I
Crossing Over and Mapping
• Linkage without crossing over creates only parental (noncrossover)
• Linkage with crossing over creates parental gametes and recombinant
(crossover) gametes.
• Interlocus distance is proportional to the degree of crossing over
– 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
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
– yellow, white
– white, miniature
– yellow, miniature
Do you see a pattern?
Sturtevant’s Interpretation
• Sturtevant reasoned that recombination frequencies
were additive, so order of genes on chromosome was
You only see recombination when it occurs
between the genes you are watching!
Single Crossovers: Non-crossover
(Parental) and Crossover (Recombinant)
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
• Cross the F1 and
examine the F2
– NCO: noncrossover
– SCO: single
crossover (2 types)
– DCO: double
• NCO:
y ec w
+ + + 4759
• SCO:
y + + 80
+ ec w70
y + w 193
+ ec + 207
• DCO:
y ec + 3
++w 3
• Total:
3-point Mapping Explanation
To Deduce the Order from a 3-Point
Cross: Method 1
1. Group the 8 phenotypic groups into 4 reciprocal
2. The Non-crossover (NCO) pair is the largest group.
The Double crossover (DCO) pair is the smallest
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%
• Total crossovers between w and ec (SCO2 + DCO) :
(193  207  3  3)
 4.06%
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