1. 4.1Linkage: basic diploid eukaryotic chromosome mapping

2. Linkage discovery Bateson and Punnet (early XX c.):
Purple flowers and long pollen PPLL
Red flowers and round pollen ppll F2
Purple long (P_L_)
Purple round (P_ll ) 390
Red long (ppL_ ) 393
Red round (ppll ) 1338
TOTAL
Expected
9 3991
3 1303
1 435

3. Linkage in Drosophila T. H. Morgan pr/pr +: purple/red
vg/vg+ : vestigial winds/normal
pr pr vg vg X pr+ pr+ vg+ vg+
Test cross pr pr vg vg X pr+ pr vg+ vg
Expected porportion1:1:1:1
pr + vg 151
pr vg+ 154
pr vg
pr + vg
TOTAL 2839
Repulsion
Coupling

4. Linkage in Drosophila (II)
Crossing with alternated alleles:
pr+ pr+ vg vg X pr pr vg+ vg+
F1 pr+ pr vg+ vg X pr pr vg vg
pr + vg 965
pr vg
pr vg 146
pr + vg
TOTAL 2335

5. Linked alleles tend to be inherited together

6. Crossing over produces new allelic combinations

7. Linkage detection

8. For linked genes, recombinant frequencies are
less than 50 percent

9. Linked genes notation
Genotype and allele location in chromosomes can be displayed
Double heterozigote Aa Bb (linked)
AB / ab (coupling)
Ab / aB (repulsion)
A B
a b
Coupling
A b
a B
Repulsion

10. Recombination frequency
Morgan y Sturtevant (1911): recombination frequencies can be used for making maps, since they reflect the genes separation distances.
pr pr ct ct X pr+ pr+ ct+ ct+
Test cross pr ct/ pr ct X pr+ ct+ / ct pr
pr + ct/ ct pr 21
pr ct + / ct pr 23
pr ct / ct pr 165
pr + ct + / ct pr 191
TOTAL 400
Recombinants
Parentals
RF=recombinants/total

11. Linkage maps pr ct 11 cM pr + ct/ ct pr 21 Recombinants
TOTAL 400
Recombinants
Parentals
RF=recombinants/total
FR= (21+23)/400=0.11
X100: 11 maps units or centimorgans (cM) pr ct
11 cM

12. Three point cross vermillion: vermillion eyes, v
crossveinless: no veins in wings, cv
cut: cut end wings, ct
P: v+ cv ct / v+ cv ct X v cv+ ct+ / v cv+ ct+
Test: v cv ct / v cv ct X v+ cv ct / v cv+ ct+
v cv+ ct
v+ cv ct
v cv ct
v+ cv+ ct
v cv ct
v+ cv+ ct
v cv+ ct
v+ cv ct
TOTAL
Parentals
Rec. cv-v
Rec. cv-ct
Rec. v-ct

13. Recombination frequencies
FR v-cv=( )/1448=18.5 cM
FR v-ct=( )/1448=13.2 cM
FR ct-cv=( )/1448= 6.4 cM
Linked genes (FR<50 cM) ct cv v
13.2 cM
6.4 cM
18.5 cM

14. Coefficient of coincidence and interference
cv-ct = 0.064
ct-v = 0.132 ct cv v
13.2 cM
6.4 cM
Positive interference: when the occurrence of one crossover reduces the probability that a second one will occur in the same region.

15. Steps to perform a three point maps
Determine phenotypes and numbers of progeny
Determine parental genotypes
Determine the no recombinant phenotypic classes no recombinants and double ones (more and less frequent)
Determine the central locus
Determine the order
Determine the location of crossovers that led to phenotypic classes
Determine the recombination frequencies
Construct the map
Determine the coefficient of coincidence and the interference