Analysis of unordered yeast tetrads from the cross ade ura x

Analysis of unordered yeast tetrads from the cross ade ura^+ x ade^+ ura yielded the following data: What is the map distance between the genes? The genes are not linked 20.4% 42.9% 40.8% 22.4%

Solution

Tetrads can be as:

PD = Parental Ditype

NPD = Non-Parental Ditype

T = Tetratype.

If the PD=NPD the genes are unlinked and no map distance can be calculated, PD>T>NPD the genes are linked and map distance can be calculated.

In the given problem,

tetrad class 1 is a PD

ade ura+ is a PD and ade+ ura is also a PD.

tetrad class 2 is a Tetratype

ade ura+ is a PD, ade ura is recombinant, ade+ ura+ is recombinant, ade+ ura is PD.

tetrad class 3 is a NPD

ade ura is recombinant and ade+ ura+ is also recombinant.

As the parental ditype is equal to non-parental ditype, the answer for this problem is a. The genes are not linked.

Total parental is equal to total recombinant (RF = 50%), the locus is unlinked.

RF = 1/2 T + NPD