Genetics

Fine spines (s), smooth fruit (tu), and uniform fruit color (u) are three recessive traits in cucumbers whose genes are linked on the same chromosome. A cucumber plant heterozygous for all three traits is used in a testcross and the progeny at the top of the following page are produced from this testcross. S U Tu 2 s u Tu 70 S u Tu 21 s u tu 4 S U tu 82 s U tu 21 s U Tu 13 S u tu 17 total 230 (a) Determine the order of these genes on the chromosome (1 pt). Nonrecombinants are s u Tu and S U tu. Double crossovers are s u tu and S U Tu. Because Tu differs between the nonrecombinants and the double crossovers, Tu is the middle gene. (b) Calculate the map distances between the genes (2 pts). S-Tu distance: recombinants are S Tu and s tu. RF = (2 + 4 + 21 + 21)/230 = 48/230 = 20.9% or 20.9 m.u. U-Tu distance: recombinants are u tu and U Tu. RF = (2 + 4 + 13 + 17)/230 = 36/230 = 15.6% or 15.6 m.u. (c) Determine the coefficient of coincidence and the interference among these genes (1 pt). Expected dco frequency = (20.9%)(15.6%)=(.209)(.156) =0.0326=3.26% Observed dco frequency = 6/230 = 2.6% C.o.C. = observed dcos/expected dcos = 2.6/3.26 = 0.8 I = 1 – C.o.C. = 0.2 (d) List the genes, in the correct order, found on each chromosome in the parents used in the testcross (1 pt). In the correct gene order for the heterozygous parent: s Tu u and S tu U For the testcross parent: s tu u and s tu u