<p>Linkage and mapping</p><p>I. Linkage A. Ind. Assortment = Ind. Alignment of non-homologous chromosome pairs at MI B. What if genes are on same chromosome? 1. they may show linkage (tendency of genes to maintain parental configuration) 2. Sturtevant proposed that strength of association (Parental combination = P) was a measure of how close genes are 3. Non-P (recombinants = R) is a measure of separation i. distance (map units or centiMorgans) = 100*R/(P+R) ii. distances are additive (chromosomes are linear) 4. Placement of genes along a line in accordance with the frequencies with which they recombine is a genetic map C. Are two genes linked? (Are P > R? Use Chi2 test; critical value for 1 d.f. = 3.8) D. Determination of gene order 1. Can determine with multiple pairwise combinations 2. Much easier with 3 factor (or 3 point) testcross</p><p>II. Fundamental properties of meiotic exchange A. x-over at 4-strand stage (after replication, before MI segregation) and any single exchange involves two (of four) non-sister chromatids B. reciprocal recombinants are made with equal frequency C. no chromatid interference D. maximum directly-measurable frequency of recombination between any genes is 50% (consequence of IIA and IIC) E. there is chiasma interference</p><p>III. 3 pt. mapping procedure A. Group 8 phenotypic classes into complementary pairs B. P most frequent C. dco least frequent (sometimes may be zero) D. order given by which of three possible orders correctly predicts the observed dco E. distances given by 100*(sco + dco)/total for each interval •note – dco have one of their exchanges in the interval, ideal map accounts for every exchange within an interval</p>