Single gene segregation

Spore Genotype Phenotype

genl-33 genl-62 genl-33 genl-62

mutant mutant mutant mutant


If strains carrying mutations in two different genes are crossed, then the genes will recombine producing recombinant meiotic products with a wild-type and double mutant genotype. The frequency of recombination will depend on whether the genes are linked (map close to one another on the same chromosome) and, if linked, how tightly they are linked. When the genotypes of the spores in different tetrads from such a diploid are determined, three classes of tetrads are obtained as follows.

Parental ditype (PD) tetrads result when no recombination occurs between the mutant genes during meiosis of the diploid cells. These will contain four spores, two of each parental genotype. When recombination occurs during meiosis of the diploid cell, either tetratype (TT) or nonparental ditype (NPD) tetrads are obtained. A tetratype tetrad contains four spores each with a different genotype, including the two parental genotypes and the two recombinant genotypes, which are wild-type and the double mutant. A nonparental ditype tetrad contains two types of spores neither of which is the parental genotype, i.e. both are recombinant types, including two wild-type spores and two double mutant spores. This is shown below in Cross 3.

Cross 3: genl GEN2 x GEN1 gen2 (genotypes of parental strains) (mutant) (mutant) (phenotype of parental strains)

Diploid: genl gen2

GENI GEN2 (wild-type)

(genotype) (phenotype)

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