1. Gene Linkage and Crossing Over

2. Gene Linkage & Crossing Over
Genes that are found on the same chromosome are said to be LINKED genes
Linked genes are inherited TOGETHER.
Genes will NOT separate during meiosis…
unless...CROSSING OVER occurs

3. Linked Genes from the two homologues One cell gets a,b,c,d
Chromosome “13” (first copy):
------a----b c d
Chromosome “13” (second copy):
------A----B C D
When separated during meiosis:
One cell gets a,b,c,d
One cell gets A,B,C,D
What happens if cross over occurs between B and C?
One cell gets a,b,C,D (recombinant)
One cell gets A,B,c,d (recombinant)
Finding freq of recombinants indicates amount of crossing over
from the two
homologues
during Anaphase 1

4. Gene Mapping
A chromosome map  a linear map of genes on the chromosome. It contains:
The order in which specific genes occur on a chromosome
The distances between the genes
A Map is derived from crossover frequencies.

5. Mapping Chromosomes
The closer genes are together on a chromosome the fewer the crossovers that occur.
Genes farther apart (more map units apart)  affected by more crossovers (higher crossover frequency)
For eg. Crossover value of
1% = 1 map unit apart - genes are close together;
12% = 12 map units apart - genes are further apart

6. Cross over frequency =
Map distance

7. Chromosome Map 96

8. Mapping Chromosomes: An Analogy
The Houses on Main Street
Arnold lives 12 doors away from Beth
Carlos lives 11 doors away from Deanna
Beth lives 3 doors away from Carlos
Arnold lives 4 doors away from Deanna and 15 away from Carlos
In what order are the houses on the street?

9. Hint: start with the ones
T. H. Morgan collected the following COF’s from data with Dros. Fruit flies
B = bar shaped eyes.
C = carnation eyes.
FV = fused veins.
S = scalloped winged.
They are all on the same chromosome
FV/B = 2.5%
FV/C = 3%
B/C = 5.5%
FV/S = 8%
C/S = 11%
for % - think
map units apart
Use the crossover frequencies to plot a gene map
Hint: start with the ones
furthest apart

10. How did Mendel calculate crossover frequencies?
Crossing-over
(which occurs during synapsis in meiosis)
provides some ‘new’ combinations.
The frequency of crossing over
can be stated as a percentage.
Crossover % = number of recombinations X
total number of offspring

11. EXAMPLE: Drosophila melanogaster (fruit fly)
E = normal body color e = ebony
N = straight wings n = curved wings
In a cross between EENN and eenn, what genotype and phenotype would you expect?
F1:
Genotype EeNn 100%
Phenotype normal/straight 100%

12. Mendel attributed this to
But let’s say an actual cross resulted in:
282 wild/straight E_N_
9 black/straight eeN_
9 wild/curled E_nn
Mendel attributed this to
crossing over.
Crossover % = new combinations
Total number
= 18
300
Crossover frequency= 6%

13. Example:
Assume the crossover frequency between gene A and B is 12%, between B and C is 7%, and between A and C is 5%. Draw the chromosome.