Gene Linkage and
Crossing Over
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
Linked Genes
• 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
Gene Mapping
•
•
A chromosome map  a linear map of
genes on the chromosome. It contains:
1. The order in which specific genes
occur on a chromosome
2. The distances between the genes
A Map is derived from crossover
frequencies.
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
Cross over frequency
=
Map distance
Chromosome Map
96
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?
• 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
FV/C
B/C
FV/S
C/S
= 2.5%
= 3%
= 5.5%
= 8%
= 11%
Use the crossover frequencies to plot a gene map
How did Mendel calculate crossover
frequencies?
Crossover % = number of recombinations
total number of offspring
X
100
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
• Phenotype
EeNn
normal/straight
100%
100%
But let’s say an actual cross
resulted in:
• 282 wild/straight
E_N_
• 9 black/straight
eeN_
• 9 wild/curled
E_nn
Crossover % = new combinations
Total number
= 18
300
Crossover frequency= 6%
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.