If two mutations are required to contribute to one phenotype GENE X gene x GENE Y gene y Haploid mutant phenotype Wild type gene x GENE X gene y GENE Y Diploid, wild type phenotype

Possible distribution of chromosomes during meiosis gene x gene x GENE X GENE X GENE Y gene y GENE Y gene y Resulting tetrads after sporulation parental ditype ( Two wild type, two mutant) 2:2 nonparental ditype (all wild type) 4:0 GENEX1 GENEY geneX GENEY GENEX1 geneX geney geneX GENEX1 GENEY GENEY geney GENEX1 geneX geney geney

or Tetratype Three wild type, one mutant 3:1 gene y GENE Y GÉNEY geney genex tgenex GENX GNEX GENEX GENEX genex Tetratype Three wild type, one mutant 3:1 GENEY genex geney GENEX GENEY genex geneX geney GENEX1 GENEY geneX geney geney GENEX GENEX1 GENEY

PD:NPD:TT is 1:1:4 1x (2:2) +1x(4:0) +4x (3:1) (wild type : mutant phenotype)  Wild type: 2+4+(4x3) = 18 Mutant phenotype 1x2 + 4x1 = 6 RATIO: 18 : 6 = 3:1 (WT phenotype : Mutant phenotype)