Nonparental ditype

Spore Genotype Phenotype

A genl gen2 mutant

B genl gen2 mutant

C GEN1 GEN2 wild-type

D GEN1 GEN2 wild-type

The frequency of each type of tetrad will depend on the frequency of recombination. If the two mutant genes are completely unlinked, that is 50% recombination, then the frequency of PD: TT: NPD tetrads will be 1:4:1. If there is any linkage, that is the frequency of recombination is less than 50%, then the relative number of PD tetrads will increase to greater than the expected 1/6 of the total number of tetrads analyzed and the number of PD tetrads will exceed the number of NPD

tetrads. Ultimately, for crosses between two alleles, 100% of the tetrads will be PD, as is shown in Cross 2.

It is important to note that the phenotype of the double mutant may be unique. This occurs most often when the two genes encode functions involved in the same process. If mutant strains containing alterations in unrelated gene functions, such as ade2 and suc2, are crossed, then the double mutant is expected to exhibit both phenotypes, adenine requiring and unable to utilize sucrose. If mutant strains containing alterations in two related gene functions are constructed, such as MCM2 and MCM7 encoding different components of the origin recognition complex (ORC), then the double mutant could exhibit an unexpected phenotype. For example, the double mutant combination could be lethal even though each single mutant strain is viable. Often mutant genes are crossed for the purpose of determining the double mutant phenotype. As will be discussed in detail in the chapters on epistasis, suppression, and enhancement, a great deal of insight into the function and relationship between gene products can be obtained from observing the phenotype of the double mutant.

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