Testcross Data
Testcross Coupling Data
Probability of crossover event is 2p. "p" is percent of crossover
progeny. Because the crossover occurs at the four-strand stage, only
50% of gametes from a crossover are recombinant gametes. Therefore:
(Percent of crossover events)/2 = 2p/2 = p
= percent recombinant gametes
| Genotypes |
Expected |
Observed
Count |
Type |
| AaBb |
1/2(1-p) |
f11 |
parental |
| Aabb |
1/2p |
f12 |
recombinant |
| aaBb |
1/2p |
f21 |
recombinant |
| aabb |
1/2
(1-p) |
f22 |
parental |
because with coupling data Aabb and aaBb are recombinant types.
* See statistical Genomics: Linkage, Mapping, and QTL Analysis. B.H.
Liu pgs. 171-172.
Punnett square
| |
Parental |
Recombinant |
| |
AB |
ab |
Ab |
aB |
| 100%
ab |
AaBb |
aabb |
Aabb |
aaBb |
| probability |
1-p
2 |
1-p
2 |
p
2 |
p
2 |
Probability of A allele, averaged across B and b in the F1.
| Type |
Probability |
| AB |
1-p
2 |
| Ab |
p
2 |
| Sum |
(1-p+p)
= 1
2 2 |
Probability of B allele averaged across A and a in the F1.
| Type |
Probability |
| BA |
1-p
2 |
| Ba |
p
2 |
| Sum |
(1-p+p)
= 1
2 2 |
Conclusion
The probability of the A allele equals 1/2, regardless of how closely
A and B are linked. Likewise, the probability of the B allele equals
1/2, regardless of linkage to the A locus. Also, p/2 +p/2 = p.
P(Ab gamete) + P(aBgamete) = p/2 + p/2 = p.
Assume A and B are independent (are not linked).
Then p = 0.5, A and B are at least 50 cm apart.
| Testcross
Data |
Expected
Frequency |
p=1/2 |
| Genotypes |
Coupling |
Repulsion |
|
| AaBb |
1/2(1-p) |
1/2(p) |
1/2(1/2)=1/4 |
| Aabb |
1/2p |
1/2(1-p) |
1/2(1/2)=1/4 |
| aaBb |
1/2p |
1/2(1-p) |
1/2(1/2)=1/4 |
| aabb |
1/2(1-p) |
1/2(p) |
1/2(1/2)=1/4 |
