Answer
In fluorescence, red and green colors, when close together, combine to form yellow color. The bcr gene is labelled green via FISH and the abl gene red. When a translocation creates what is named a "Philadelphia chromosome," these two genes are brought together to make yellow fluorescence. Because the genome shown in the figure has two copies of each gene, we can see that there are two normal chromosomes with one copy of each gene plus one translocated chromosome in which the second copy of each gene is close to the other, leading to that yellow fluorescence. Each of the two copies is seen as a pair of spots due to the fact that interphase pre-mitotic (duplicated) chromsomes are being imaged.
Work Step by Step
The key here is to recall that a normal cell has two copies of each chromosome and, thus, of each gene. One copy of each of bcr and abl can be seen by their different colors. The second copies must be present and shown in the yellow fluorescence. Pairs of spots are seen due to DNA duplication in preparation for mitosis (FISH looks at condensed chromosomes).
