When two mutant strains of Ascobolus 1 and, 2 are crossed with wild type strains, gave unordered tetrads. The progeny are:
Spore pair 1 black spore pair 3 fawn
Spore pair 2 black spore pair 4 fawn
a) The details of the crosses are:
m x m+
The resulting tetrads show 1:1 segregation. This ratio indicates that each mutant is the result of a mutation in a single gene.
The two mutant strains were crossed. The unordered tetrads had the following types:
Spore pair 1 fawn spore pair 3 fawn
Spore pair 2 fawn spore pair 4 fawn
b) The details of the crosses are:
m 1 and m2
The results indicate that either both strains are mutant for the same gene or that they are mutant in different but closely linked genes:
m 1 m 2 + x m1+m2
When large numbers of unordered tetrads were screened, some rare ones that contained black spores were found. The four cases are:
| Case A | Case B | Case C | Case D | |
| Spore pair 1 | Black | Black | Black | Black |
| Spore pair 2 | Black | Fawn | Black | Abort |
| Spore pair 3 | Fawn | Fawn | Abort | Fawn |
| Spore pair 4 | Fawn | Fawn | Abort | fawn |
c) The four cases can be explained in the following way:
Case A:
This is a nonparental ditype tetrad and would be the result of a four-strand double crossover.
m 1 + m 2 + black
m 1 + m 2 + black
m 1 m 2 fawn
m 1 m 2 fawn
Case B:
This is a tetratype and would be the result of a single crossover between one of the genes and the centromere.
m 1 + m 2 + black
m 1 + m 2 fawn
m 1 m 2 fawn
m 1 m 2 fawn
Case C:
This is a result of nondisjunction during meiosis I.
m 1 + m 2 ; m1m2+ black
m 1 + m 2 ; m1m2+ black
No chromosome abort
No chromosome abort
Case D:
This is a result of recombination between one of the genes and the centromere followed by nondisjunction during the process of meiosis II.
m 1 + m 2 ; m1m2+ black
No chromosome abort
m 1 m 2 + fawn
m 1 + m 2 fawn
d) The mutations in the two original mutant strains were not in a single gene but closely linked genes. The cross is therefore:
m 1 m 2 + x m1+m2