Example of data involving 3 linked genes in corn
v = virescent seedling all
gl = glossy seedling are
va = variable sterile recessive
Testcross results:
phenotype # Genotype of gamete from hybrid parent normal 235 + + +
gl, va 62 gl va +
va 40 + va +
va, v 4 + va v
gl, va, v 270 gl va v
gl 7 gl + +
gl, v 48 gl + v
v 60 + + v
These data tells us that the parental types are + + + and gl va v.
What is the gene order?
Compare parental to doubles:
doubles are
+ va v
gl + +
. . . gl is in the middle
Parental double crossover
+ + + + gl + v gl va v + va
Now that we know the order, we can look at all the data.
Assign (for convenience)
v gl region I
gl va region II
A single crossover in region I gives:
v + + 60
+ gl va 62
in region II gives:
+ + va 40
v gl + 48
parentals:
+ + + 235
v gl va 270
doubles:
+ gl + 7
v + va 4
726
From these data, you can calculate map distance:
What is the crossover frequency in Region I?
v + + 60
+ gl va 62
+ gl + 7
v + va 4
133 = l8.3% of 726
For Region II?
+ + va 40
v gl + 48
+ gl + 7
v + va 4
99 = 13.6% of 726
The map is
l8.3 13.6 v gl va
From this map you can predict the frequency of double crossovers:
.183 x .136 = 0.025 = 2.5%
The observed doubles = 11 = l.5% of 726
This seems to say that once a crossover occurs in one region, it interferes with one occurring in an adjacent region. This phenomenon is called interference.
The coefficient of coincidence is defined as:
observed doubles = l.5 = 0.6
expected doubles 2.5
varies from 0--1
1 = no interference
0 = complete interference
if >1 we call it negative interference
Trihybrid cross
triple heterozygote (XxYyZz) x xxyyzz
results:
+ + + 30
+ + z 32
+ y + 441
+ y z 1
x + + 0
x + z 430
x y + 27
x y z 39
Parentals are:
+ y + and x + z
Doubles are:
+ y z and x + +
Which gene is in the middle? z
x z + x + +
+ + y + z y