Gene mapping question.

Gene mapping question?

• when working out genetic distances in a 3 point cross how do you know which single crosss over to use to work out the distance between the first two genes. eg: if you given a table with this info 3 recesssive genes produce black body(b) cinnabar eyes(cn) and vestigal wings (vg).. testcrossing F1 produced 664 wildtype; 652 black, cinnabar, vestigal, 72 black, cinnabar, 68 vestigial, 70 black, 61 cinnabar, vistigial, 4 black, vestigial, 8 cinnabar. so how do you know which single crossover to use when finding distance between b and cn for example... will you add the double cross over percentage to either the 70b and 61 single cross over or to the 72 b cn 68 vg single cross over. pls explain thanx

• Answer:

  From the results of your test cross, you know that the heterozygous parent had all three wild-type alleles on one chromosome and all three recessive alleles on the other. You can tell this because those are the "parental" or "non-recombinant" offspring (those with the largest numbers). You also know that the black, vestigial and the cinnabar offspring are those which resulted from a double crossover, because those two groups are present in the smallest numbers. So, from those results you know that the order of the genes is black, cinnabar, vestigial. Now, to determine the distance between the black and cinnabar genes, you need to recognize that a single crossover between those two genes will produce black offspring and cinnabar vestigial offspring. To determine the map distance between those two, you add together those two groups of offspring and add the double-crossover offspring to that, and divide by the total number of offspring. Distance b-cn = (70+61+4+8 / 1599) X 100 = 8.9 map units In calculating the distance between cn and vg, you do the same thing with the black cinnabar and vestigial offspring, also adding the double crossover offspring: Distance cn-vg = (72+68+8+4)/1599 X 100 = 9.5 map units. You add the double crossovers to both groups because the double crossover offspring did have a crossover between the two genes you are looking at in each case. To calculate the map distance between the two outer genes, you include all of the recombinant offspring except the double crossover ones in the calculation. You exclude the double crossovers, because in just looking at the outer two genes, the double crossovers don't appear to have recombined (even though you know they did). map distance b-vg = (72+68+61+70) / 1599 X 100 = 16.9 map units Hope this helps....