Consider two genes: GHI1 and GHI2 . GHI1 is almost precisely at the centromere o
ID: 73095 • Letter: C
Question
Consider two genes: GHI1 and GHI2. GHI1 is almost precisely at the centromere of a chromosome while GHI2 is 60 cM. away from the centromere on one arm of the same chromosome. When a wild-type (GHI1 and GHI2) strain is crossed to a doubly mutant strain (ghi1 and ghi2) and sporulated, what is the expected number of tetrads of each type if 1,000 tetrads are examined? Neglect interference. In other words, per 1,000 tetrads, how many of each of the following types do you expect?
tetrads with two GHI1 GHI2 spores and two ghi1 ghi2 spores;
tetrads with two GHI1 ghi2 spores and two ghi1 GHI2 spores; and
tetrads with one GHI1 GHI2 spore, one ghi1 ghi2 spore, one GHI1 ghi2 spore and one ghi1 GHI2 spore.
Explanation / Answer
distance between two is 60cm. if no recombination, den parental type, GH1 GH2 gh1 gh2 in a ratio (1:1),
Tetrad Types ,900 parental type,0 non parental,100 tetratype.
Linkage is measured as the number of recombinants divided by the total progeny. Because there are four progeny per tetrad, here we have 400 spores, and 2 of the 4 in each TT tetrad are recombinants. Therefore we have 20 recombinants out of 400 = 5% recombination or 5% linkage. This is called 5 centiMorgans (cM). 1 cM is defined as 1% recombination.
In terms of tetrads, ½T/(T+N+P) × 100 = number of cM. Yeast, however, have so much recombination that you often see double crossovers (DCOs).