PD NPD T AB aB Ab ab Random assortment 1 : 4 Linkage >1 : <1 ? Centromere linkage <4

Distances between genes can be calculated by counting the different tetrad types; Formula: ½ T + NPD (recombinants) X 100 Total tetrads Distance is expressed as % recombination; Recombination frequencies can never be > 50% (= random assortment; genes behave unlinked)

Segregation of two unlinked genes Example: ADE3, ADE2 ADE2 ADE3 ade3 ade2 Haploid, Ade+, white Haploid, Ade-, white ADE2 ADE3 ade3 ade2 Diploid, Ade+, white

Resulting tetrads after sporulation Possible distribution of chromosomes during meiosis ADE2 ade2 ade2 ADE2 ADE3 ade3 ade3 ADE3 Resulting tetrads after sporulation nonparental ditype parental ditype ade3 ADE2 ade2 ade3 ADE3 ade2 ADE3 ade2 ade3 ade3 ADE2 ADE2 ADE3 ADE3 ade2 ADE2

Parental Ditype – not informative (two spores carry both mutations) ade2 ADE2 ade3 ADE3 URA3 Parental Ditype – not informative (two spores carry both mutations) ADE3 ADE2 ADE3 ADE2 ade3 ade2 ade3 ade2

2 x Ade+, white 2 x Ade-, white ADE3 ADE3 ADE2 ADE2 ade3 ade2 ade3

ade2 ADE2 ADE3 URA3 ade3 Non-parental ditype –informative (two spores carry one allele, two spores the other mutant allele) ADE3 ade2 ADE3 ade2 ade3 ADE2 ade3 ADE2

All Ade-, two white, two red!

or Tetratype ade2 ADE2 ADE2 ade2 ade3 ade3 ADe3 ADE3 ADE3 ade3 ade2

Tetratype Ade+, white Ade-, red Ade-, white Ade-, white ADE3 ADE3 ADE2