question: Are the genes linked? map distance = (# NPD asci) + (1/2)(# TT asci) *

Question

question: Are the genes linked?

map distance = (# NPD asci) + (1/2)(# TT asci) * 100/total # asci NOTE the progeny/ascus distinction. You can get the map distance from counting all progeny too, but realize there are 8 progeny in each ascus in Neurospora, but only 4/ascus in Chlamydomonas and other critters with unordered tetrads. Example problem: Cross ab x ++ Ascus type (PD, NPD, or TT) ____ ___ _____ __ __ __ ___ ____ Div. Segreg A? (FDS or SDS) ____ _____ _____ __ __ ___ ___ Div. Segreg B? (FDS or SDS) __ _______ _____ ______ ___ ______ _____ ____ (1) Are the genes linked?

Explanation / Answer

If we cross cells that are ab x ++, there are three possible types of 4-spore asci:

ab ab ++ ++ parental ditype PD – two genotypes which match the two parental genotypes

a+ a+ +b +b nonparental ditype NPD – two genotypes which interchange the parental alleles

ab a+ +b ++ tetratype TT – all four possible genotypes

If TT= 0, NPD = 0)-      Linked gene pairs

PD = NPD, and TT = 0 No linkage

Thus, 3rd, 4th, and 8th are not linked while rests are linked

If we cross cells that are AB x ab, there are three possible types of 4-spore asci:

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