Tetrad (genetics)

The tetrad is the four spores of a yeast, that separate after mating. If the two parents have a mutation in two different genes, the tetrad can segregate as the parental ditype, the non-parental ditype or as the tetratype.

Parental ditype
A tetrad type containing two different genotypes, both of which are parental. A spore arrangement in Ascomycetes that contains only the two non-recombinant-type ascospores.

Non-parental ditype
A spore arrangement in Ascomycetes that contains only the two recombinant-type ascospores (assuming two segregating loci). A tetrad type containing two different genotypes, both of which are recombinant.

Tetratype
A Tetratype is a tetrad containing four different genotypes, two parental and two recombinant. A spore arrangement in Ascomycetes that consists of two parental and two recombinant spores indicating a single crossover between two linked loci.

Linkage analysis
The ratio between the different types arrising after the sporulation is a measure of the linkage between the two genes.