Imagine that you set up a threepoint mapping cross to determ

Imagine that you set up a three-point mapping cross to determine the order of three X-linked genes. You have recessive mutations available for all three genes: yellow (y), white (w) and echinus (ec)*. A heterozygous F_1 Drosophila female is crossed to a hemizygous mutant male. The observed numbers and phenotypes of F_2 are as follows: 4685 yellow, white, echinus 4759 wild type 80 yellow 70 white, echinus 193 yellow, white 207 echinus 3 yellow, echinus 3 white Based on this data answer the questions in the following order (each answer yields a clue useful in following question). (a) Determine which of the above phenotypic classes represent the parental types and the recombinant types (specify SCO, DCO). (b) Identify the reciprocal classes of phenotypes. (c) Determine which gene is in the middle. (d) Calculate the recombination frequency (% RF) based on the observed F_2 numbers of each pair of reciprocal classes. (e) Calculate the gene distances. (f) draw a genetic map of the 3 genes (include the gene location, symbol and m. u. between neighboring genes). The frequency of crossover between genes A and B it 7.5%, between genes B and Cis 34.5%, and between genes A and C is 41.4%. What is the most likely order of these three genes? (a) The gene order cannot be determined from these data. (b) B C A (c) A B C (d) C A B Genes X. Y. and Z are linked. Crossover gametes between genes X and Y are observed with a frequency of 25%, and crossover gametes between genes Y and Z are observed with a frequency of 5%. What is the expected frequency of double crossover gametes among these genes? (a) 50% (b) 5% (c) 1.25% (d) 30% *Keep in mind the fact discussed in class. Drosophila mutant phenotypes are often reflected directly in the name of the gene. eg. while the wild type body color is gray, the mutation in gene called yellow makes body color yellow. The presence of the wild type allele for body color in yellow locus makes the body color gray.

Solution

9. c ABC.

Since the distance between A and C is furthest we know those genes will be on the outside. B must be further from C than it is from A, therefore the correct order is ABC.