1. Chapter 5 Genetic Linkage and Chromosome Mapping
Jones and Bartlett Publishers © 2005

2. Use of cytologically marked chromosomes shows that crossing over involves breakage and reunion of chromosomes

3. Recombination rates in males and females
In Drosophila, males do not have any recombination, so all syntenic genes (those on one chromosome) always are completely linked.
How and why are not known.
Human males have recombination rates about 60% of that seen in females.

4. Unusual inheritance of X-linked genes in crosses involving female Drosophila with attached X-chromosomes

5. The gametes generated by the 3 kinds of double crossovers

6. Consequences of a 2-strand double crossover in a cross involving 3 genes

7. Chromatid interference
Sometimes crossing over at one point on a chromosome interferes with other crossing over events on the same chromosome:
Chromatid interference means that there will be fewer double, triple, etc. crossing over events.
Interference is greatest over short distances.

8. Three-point testcross (p. 195)
Parental
Single
Double

9. Three-point cross in corn
Crossovers between lz and su:
Lz su gl 40
Lz Su Gl 33
Lz su Gl 4
lz Su gl 2
79 => 79/740=10.7%
Between su and gl:
Lz Su gl 59
lz su Gl 44
Lz su Gl 4
lz Su gl 2
109 => 109/740=14.7%

10. Coefficient of Coincidence
Chromosome interference is much more common than chromatid interference.
i=interference; 1-Coefficient of Coincidence
CC=observed # double crossovers/predicted
Predicted: P(single crossover between lz and su)*P(single crossover between su and gl).

11. Coefficient of Coincidence
For the previous corn data,
R1=0.107 for lz and su
R2=0.147 for su and gl.
If independent, double crossovers would occur (R1 x R2)x # of progeny:
0.107 x X 740=11.6.
Only 6 double crossovers were observed.
CC=6/11.6=0.51, i=interference=1-0.51=0.49.

12. Coefficient of Coincidence
With greater distance between genes, interference usually disappears.
In Drosophila, i=0 at about 10 cM
For most organisms, interference disappears at about 30 cM (CC=1).

13. A mapping function corrects for the loss of detectable recombinants due to multiple crossovers

14. Three-Point Testcross Another example (Morgan’s data)
Progeny Progeny
Phenotype Genotype Number
Scute echinus crossveinless sc ec cv /sc ec cv
Wild type / sc ec cv
Scute sc + + / sc ec cv
Echinus crossveinless + ec cv /sc ec cv 1002
Scute echinus sc ec + / sc ec cv
Crossveinless cv / sc ec cv
Scute crossveinless sc + cv / sc ec cv 8808
Echinus + ec + / sc ec cv 8576
Total ,785

15. Morgan’s three-point results
Distance between sc and ec:
( )/20,785 x 100 = 6.74 cM
Distance between cv and ec:
( )/20,785 x 100 = 9.65 cM
Predicted DC=( x ) x 20,785=135.2
CC=5/135.2 = ; i =

16. Another example: Three linked loci in tomato:
Mottled (m) vs. normal (M) leaf
Smooth (P) vs. pubescent (p) epidermis
Purple (Aw) vs. green (aw) stems

17. Three Linked Tomato Loci
Progeny phenotype Number
Normal smooth purple (M P Aw) 18
Mottled pubescent green (m p aw) 15
Normal smooth green (M P Aw) 180
Mottled pubescent purple (m p Aw) 187
Normal pubescent purple (M p Aw) 1880
Mottled smooth green (m P aw) 1903
Mottled smooth purple (m P Aw) 400
Normal pubescent green (M p aw) 417
Total
Questions: What are the genotypes of original parents, the gene order, and map distance between these genes?

18. Genetic maps are based on % recombination.
There is much less recombination in heterochromatin compared to euchromatin
Genetic maps are based on % recombination.
Physical maps are based on other methods
such as gel electrophoresis or DNA sequencing