Genes Located 50 Or More Recombination Units Apart Segregate Independently
Although two genes may be located on the same chromosome, the probability
of each genetic combination may be the same as independent events.
Let r = 0.5, then the following gametes will result from 10 meiotic
events:
| Number
of gametes |
Type
of gamete |
Event |
Type
of gamete |
| 10 |
AB |
C.O. |
parental |
| 10 |
ab |
C.O. |
parental |
| 10 |
Ab |
C.O. |
non-parental |
| 10 |
aB |
C.O. |
non-parental |
Each crossover event results in recombination between two of four sister
chromatids. The resulting testcross ratio would be:
| |
1-r
x
AB
2 |
1-r
x
ab
2 |
r
x
Ab
2 |
r
x
aB
2 |
| ab |
AaBb
|
aabb |
Aabb
|
aaBb
|
The resulting ratio of progeny is the same as that of genes located
on seperate chromosomes.
1-r = 1-0.5 =
0.25
2 2
r = 0.5 = 0.25
2 2
There are 20 gametes with the parental combinations and 20 gametes
with non-parental combinations of the two genes. There are a total of
40 gametes that result from 10 meiotic events.
20 = P(parental type gamete) = 0.5
40
20 = P(non-parental gametic combinations)
= 0.5
40
This result shows that when genes are located 50 or more recombination
units apart, the expectation is one crossover event for each meiotic
event. Each crossover event produces two parental and two non-parental
combinations for a ĽAB:Ľab:ĽAb:ĽaB segregation ratio.
**This is the same segregation ratio for two genes located on seperate
chromosomes.
