Recombination between to genes

Part A: Recombination between to genes

In Drosophila, unlike most organisms, it is important to realize that crossing over occurs during gamete formation in female flies only. Because crossing over does not occur in male flies, recombination frequencies will differ when comparing female flies with male flies. Perform the following experiments to help you understand how recombination frequencies can be used to develop genetic maps.

Part 1: recombination between two genes

To understand how recombination frequencies can be used to determine an approximate map distance between closely linked genes, cross a female fly with the eyeless mutation for eye shape with a male fly with shaven bristles. Both of these genes are located on chromosome IV in Drosophila.

What are the phenotypes of the F1 flies? What do you think their genotype is? Perform a testcross by selecting one of the F1 females; we will be monitoring recombination during meiosis during her production of eggs for mating. Select the F1 female for mating but perform the testcross by designing her mate in the mating tab; design a male with both the eyeless and shaven bristle traits. The male will be homozygous recessive for both traits.

Click the Mate flies button. Most of the progeny have either the shaven or the eyeless mutation. The testcross progeny which have either both mutations or neither mutation (wild-type) are produced by crossing over in the double heterozygous F₁ female. The percentage of these recombinant phenotypes is an estimate of the map distance between these two genes.

Draw a map that shows the map distance (in map units or centimorgans) between the locus for the shaven bristle allele and the locus for the eyeless allele.

Part B: Recombination mapping of three genes

To understand how recombination frequencies can be used to determine a genetic map for three alleles, mate a female fly with a black body, purple eyes, and vestigial wing size (OR you could use a female with black body, purple eyes, and apterous wing) to a wild-type male. These loci are all located on chromosome II. What phenotypes do you see? Are the mutations in the female dominant or recessive?

Testcross one of the F1 females to a male with all three of the mutations you used in the original female above. After you see the progeny from the mating, you can look at the numbers on the Analyze tab and since we know the mutations are on chromosome II, you can check the “Ignore sex of flies” box. The most abundant types of flies should be nonrecombinant (parental) types. Does this make sense based on how mating that produced the F1 female was set up?

The flies with the least frequent phenotypes represent double crossovers. If you compare these mutants with the nonrecombinant types, which gene moved? The is the gene in the middle of the three genes. Now that you know which gene is in the middle, find the classes of progeny that you would expect from crossing over between the middle gene and with each gene on either side. Find the recombination frequency by adding the number of single crossovers between the two genes, then add the double crossover number to this and divide by the total progeny to determine the recombination frequency (map distance) between the two genes. Now do the same for the gene on the other side of the middle and construct a genetic map with all three genes in order.

What is the coefficient of coincidence and interference for this cross? What does is mean if you see interference (or the coefficient of coincidence less than 1)?