Linkage
100% crossover = 50% recombination, 2r = percent of crossover events.
In the absence of a crossover the ratio would be 2AB:2ab gametes. If
a crossover occurs in 10% of meiotic events, then the 1AB:1Ab:1aB:1ab
ratio is 10% of the total. The non-crossover event will occur in 90%
of the meiotic events, with the resultant 2AB:2ab ratio. The overall
result expected from ten meiotic events is:
| Number
of gametes |
Type
of gamete |
Event |
Type
of gamete |
| 1 |
AB |
C.O. |
parental |
| 1 |
Ab |
C.O. |
non-parental |
| 1 |
aB |
C.O. |
non-parental |
| 1 |
ab |
C.O. |
parental |
| 9 |
AB |
N.C.O. |
parental |
| 9 |
AB |
N.C.O. |
parental |
| 9 |
ab |
N.C.O. |
parental |
| 9 |
ab |
N.C.O. |
parental |
The ratio is then 19AB:19ab:1Ab:1aB. When the two loci were greater
than 50 recombination units apart, the expected result of ten meiotic
events would be 10AB:10ab:10aB:10Ab. When crossing over occurs in a
heterozygous plant, new combination, of genes are created. The total
number of gametes that are parental combinations is 38 and there are
2 non-parental types of gametes.
38 = x
40 100
40x = 3800
40x = 95 = 95% parental
95% of the gametes are parental types. 5% of the gametes are non-parental
types.
In this example, a crossover meiotic event occurred 10% of the time.
Thre was 5% recombinant types. Therefore, each crossover event produces
50% observed recombination. In other words, we double the proportion
of observed recombination proportion to obtain the relative frequency
of crossover events.
5% observed recombination equals 10% occurrence of crossovers. A 5
recombination unit distance equates to 10% crossovers. 50 recombination
units equates to 100% crossover, which means that on average there is
one crossover between the two loci for each meiotic event.
2 = 0.05 = r
40
When r = 5%, then crossover events have occurred 10% of the time.
If we testcross an AaBb heterozygous plant to an aabb plant and the
two loci are independent, the result will be:
| |
AB |
ab |
Ab |
aB |
| ab |
AaBb |
aabb |
Aabb |
aaBb |
This can be summarized as AaBb x aabb 
1AaBb:1aabb:1Aabb:1aaBb
If we cross an AaBb heterozygous plant to an aabb plant
and the two loci are linked 10 cross-over units apart,
the result will be:
| |
1-r
x
AB
2 |
1-r
x
ab
2 |
r
x
Ab
2 |
r
x
aB
2 |
| ab |
1-r
x
AaBb
2 |
1-r
x
aabb
2 |
r
x
Aabb
2 |
r
x
aaBb
2 |
In this example r = 0.10. The resulting progeny will have the testcross
ratio:
0.45AaBb:0.45aabb:0.05Aabb:0.05aaBb
r/2 = 0.05
r = 0.10
Now we have crossover events occurring 20% of the time!
